CALIFORNIA GEOLOGICAL SURVEY FAULT EVALUATION REPORT FER-254

FAULTS OF THE YUHA DESERT

and the southeastern portion of the Elsinore Fault Zone Imperial County, California

by

Jerome A. Treiman

February 22, 2012

INTRODUCTION

The April 4, 2010 El Mayor-Cucapah Mw 7.2 earthquake caused extensive fault rupture in Baja California on a series of faults that extend northward toward the California border (Figure 1; Fletcher et al, 2010). The earthquake and its aftershocks caused triggered slip and primary rupture on a number of faults within the Yuha Desert, north of the U.S.-Mexico border, extending to the southeastern Elsinore Fault Zone (Treiman et al, 2010a; Treiman et al, 2010b; Rymer et al, 2011). These faults, none of which were previously zoned under the Alquist-Priolo Earthquake Fault Zoning Act (APEFZ Act) include two branches of the active Laguna Salada Fault Zone, the southeastern tip of the Elsinore Fault Zone and

Figure 1 - Index map showing location of the April 4, 2010 El Mayor-Cucapah earthquake and associated ground rupture (red) against a background of previously mapped faults (orange). (base image from Google Earth)

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numerous north and northeast-trending faults (Figure 2). The latter include the Yuha Fault, the Yuha Well Fault Zone and several other unnamed faults near the U.S.-Mexico border. It is the purpose of this evaluation to document those faults in California that produced surface fault rupture and to identify related faults that, by corollary, may also be considered active and capable of surface displacement in the event of an earthquake within this border region. Those faults considered active and capable of surface fault rupture (sufficiently active) and which are well-defined as surface faults are recommended for Earthquake Fault Zones (EFZs) under the APEFZ Act (Bryant and Hart, 2007). Faults being evaluated fall into two general groups: northwest-trending right-lateral faults of the Laguna Salada and Elsinore fault zones, and a broad array of north to northeast-trending, left-lateral to normal faults within the Yuha Desert and southeast of the Coyote Mountains. These faults lie within the Carrizo Mtn., Painted Gorge, Plaster City, Coyote Wells, Yuha Basin and Mount Signal 7.5-minute USGS topographic quadrangles (Figure 2).

Figure 2 - Surface rupture and faults evaluated in the Yuha Desert, southern Imperial Valley, California, and key to 7.5-minute topographic quadrangles affected.

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RUPTURE SYNOPSIS Primary fault rupture occurred on a series of faults in Baja California (Fletcher et al, 2010; Hauksson et al, 2010). The rupture was initiated by a Mw 6.25 earthquake on an unnamed normal fault, rapidly followed by the Mw 7.2 main shock, with strike-slip and thrust displacements that progressed up the Pescadores, Borrego, Laguna Salada and Paso Superior faults in northern Baja California. The earthquake triggered slip along the east and west branches of the Laguna Salada Fault Zone into the Yuha Desert area of California (Figure 2). Significant concurrent rupture occurred on the newly named Yuha Fault and probably other related faults within the Yuha Desert, including portions of the Yuha Well Fault Zone (revised name) and a number of faults north and west of Cerro Centinela (Mount Signal and Yuha Basin quadrangles). A vigorous sequence of aftershocks, particularly in the northern portion of the rupture zone (next section), accompanied continued slip on the Laguna Salada and Yuha faults. It is not known how much of the rupture on other faults was co-seismic with the main shock and how much occurred later, but radar interferometry shows that there was significant afterslip on many of the faults (Treiman et al, 2010a). The strongest aftershock (Mw 5.7 on June 14, 2010) was accompanied by new rupture of faults to the northwest, including faults in the Ocotillo Fault Zone (new name), the southeastern Elsinore Fault and the southernmost Painted Gorge Wash Fault. Creepmeter data showed slip concurrent with the June 14 aftershock on one strand of the Laguna Salada Fault Zone, south of the aftershock (Rymer et al, 2011). As with the rupture following the mainshock sequence it was not possible, aside from those faults noted, to distinguish co-seismic rupture from afterslip. Additional slip on other faults could not be differentiated from initial slip except in a few instances where we had repeated observations at the same location.

SEISMICITY Because of the density of aftershock activity it is difficult to assign seismic events to specific faults. In general, a very active aftershock sequence was largely confined southeast of the Yuha Well Fault Zone (Figure 3a) until a large aftershock occurred just north of this fault zone on June 14, 2010. Commencing with that aftershock there was a flurry of aftershocks further northwest (Figure 3b) that appeared to accompany displacement on several surface faults, and likely also along a northwest extension of the Laguna Salada Fault Zone. A discussion of the El Mayor-Cucapah earthquake and its aftershocks has been presented by Hauksson et al (2010).

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Figure 3 – Aftershocks of the April 4, 2010 earthquake. M2.0-M3.9 indicated with pluses and M ≥4 indicated by solid dots. (Gray line work indicates faults mapped and/or interpreted). 3a) aftershocks from April 1 to April 30, 2010 in magenta and from May 1 to June 13, 2010 in green. 3b) aftershocks from June 14-June 30, 2010 in red.

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PREVIOUS MAPPING

Prior geologic mapping in the Yuha Desert goes back to some of the earliest geologic maps of California, with the southern extension of the San Jacinto Fault Zone being generally recognized (Jennings, 1985). The first detailed map that was useful for fault evaluation was by Dibblee (1954; also 1996). Detailed fault mapping by Clark (1982 and earlier unpublished maps) provided the greatest detail on the Elsinore and Laguna Salada fault zones and a number of related geomorphic features. Smith (1979a,b & c), Kahle et al (1984) and Kahle (1988) incorporated Clark’s earlier mapping with their own observations to identify a broader spectrum of mapped and inferred faults in the region. Thesis mapping by Christensen (1957), Barnard (1968) and Isaac (1987) was also useful. Ongoing mapping by Morgan and Morgan (2006 and personal communication) has provided greater detail along the southeast margin of the Coyote Mountains. Prior mapping of individual faults is detailed later in this report. Figure 4 provides an overview of the previous fault mapping, color coded to source. These traces are also repeated for reference on Plates 1a-1f.

Figure 4 (FER-254) – Overview of previous fault mapping in the Yuha Desert area.

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AERIAL PHOTO INTERPRETATION Aerial photo interpretation was done utilizing several image sources. These included standard black and white and color print film imagery (see photo references at the end of this report) but also included imagery on Google Earth (various dates) and NAIP digital imagery (2005). Because the area has not been heavily developed, recent imagery on Google Earth was generally more useful than the older lower-resolution print film formats. Faults identified in available imagery guided much of our initial field response, prior to availability of the UAVSAR interferograms (next section). Interpreted lineaments are shown on Plates 1a-1f. Annotations are included for faults not otherwise characterized by observed fault displacement in the recent earthquake.

SYNTHETIC APERTURE RADAR INTERFEROMETRY Synthetic Aperture Radar (SAR) has been available for many years from satellite based instruments, and interferometric comparison of SAR data to detect changes in the ground surface between successive satellite passes has also been available for some time. Interferometric Synthetic Aperture Radar (InSAR) data were very useful in identifying remote areas of ground rupture after the 1999 Hector Mine Earthquake (Simons et al, 2002; Treiman et al, 2002). NASA/JPL has recently developed and deployed an aircraft-borne instrument capable of obtaining SAR data from lower altitudes and at a much higher resolution than previously available [ http://uavsar.jpl.nasa.gov/ ]. The instrument, termed UAVSAR (Unoccupied Aerial Vehicle Synthetic Aperture Radar), had been flown over the Yuha Desert (as well as other areas) in October 2009. A subsequent UAVSAR flight on April 13, 2010 allowed the processing of a very detailed and sensitive interferometric image of the ground north of the border that showed ground changes during that ~ six month interval (Figures 5a & 5b). The majority of the change is assumed to have occurred on April 4, 2010 in response to the El Mayor-Cucapah earthquake. Processed images were made available to us in early June by the JPL team and immediately produced two revelations: 1) almost all of the ground rupture mapped up to that point was depicted in the interferometric image and 2) there were many other probable faults that had not yet been mapped. Surface rupture was indicated by linear offsets of ground displacement contours (colored bands in Figures 5a-c). These images were relied upon heavily for interpretation of surface rupture and, since this is a relatively recent advance and capability, it is worth some justification (Treiman et al, 2010a). First, as noted above, already observed surface rupture was clearly and accurately identified in the interferometric image (Figure 5a & 5b). The interferograms were opened in the Google Earth browser and all visible lineaments were plotted and then transferred to another GIS platform (MapInfo). Lineaments that had not already been verified in the field were targeted for further mapping. It was not possible to walk out, or even spot-check, all lineaments, but it was found that, to a remarkable degree, the identified lineaments corresponded to verifiable surface rupture. This gave us tremendous confidence in the fidelity of the rupture picture provided by the UAVSAR data and encouraged us to infer rupture in areas that were not verified.

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Figure 5b – Detail of UAVSAR image (Figure 5a, box). Left side is bare image; right side shows field-verified rupture. Additional displacement across unmapped faults is also evident in the image.

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Additional interferometric imagery, using a later flight of the UAVSAR instrument on July 1, 2010, captured subsequent deformation, including that associated with the June 14, 2010 aftershock (Figure 5c). Much of this deformation was field-verified either as new brittle surface rupture (Ocotillo Fault Zone, Elsinore Fault Zone) or is inferred to be distributed deformation associated with faulting at depth (northwest extension of the Laguna Salada Fault Zone). In the latter case, although new rupture at the surface was not clearly demonstrated in this event, subtle evidence of the presence of a pre-existing surface fault was observed. Eric Fielding (NASA/Jet Propulsion Laboratory) interprets the data as indicating that fault rupture at a depth of 4-10 km accompanied the June 14 aftershock (Treiman et al, 2012). The second interferogram (Figure 5c) along with other InSAR data (Treiman et al, 2010a) also demonstrated continued slip on the Yuha Fault that preceded the June aftershock.

Interpreted lineaments were characterized with respect to the sharpness of the lineament and the apparent magnitude of displacement across the lineament (Figures 6a-6f). The sharpness of the lineament indicates whether the displacement was focused in a narrow zone (usually discrete surface rupture) or distributed across a broader zone of distributed shear or warping. A diffuse lineament may also indicate rupture at depth that has not reached the surface. The magnitude of displacement is approximate as the indicated displacement is only a line-of-sight measurement from the instrument and combines both lateral and vertical components of displacement. In some cases one component may partially cancel the other in the line-of-sight direction. Nevertheless, those lineaments with the sharpest expression and the largest magnitude were universally verified where visited, as were all but the most diffuse or those with the smallest magnitude of offset. Based on the success of those features that were field verified, we judged the UAVSAR-derived images to be a reliable indicator of surface rupture and used these data to map

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other, un-verified, surface faults. More weakly indicated faults (diffuse expression and small displacement) were generally not recommended for zoning unless there was additional field or aerial photo evidence for the fault, or if there was a reasonable association with better defined faults. The character of expression indicated on Figures 6a-f is based on interpretation of the 12-cm wrapped interferogram, as shown in Figure 5. A later rendering of the differential interferometry, portrayed with a 5-cm wrap cycle, showed some of the lineaments more strongly, or with greater continuity and was also a factor in our zoning decisions.

Figures 6a through 6f (above and on following pages) show fault displacement interpreted from interferograms derived from UAVSAR data. These interpreted faults include features from the initial interferogram (October 2009 – April 2010 – Figure 5a) as well as the April-June 2010 interferogram (Figure 5c). Some features, evident in both interferograms, will show two lines. Interpretation is based on 12-cm wrap, as shown in Figure 5.

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POST-EARTHQUAKE FIELD OBSERVATIONS Field mapping to document recent surface rupture commenced on April 5 and continued through April 11. Additional mapping took place on May 11-12, May 27, June 16-19, August 2-4, 2010 and February 22-24, 2011. Mapping and observations were by the author as well as the following geologists: Michael Rymer (USGS), Katherine Kendrick (USGS), Janis Hernandez (CGS), Pamela Irvine (CGS), Robert Sickler (USGS), George and J.R. Morgan (independents), and Martin Siem (Construction Testing & Engineering). Portable GPS receivers were used to provide location of specific observations as well as to track field traverses. Mapping included a combination of more rapid reconnaissance (slowly driving dirt roads and trails) and focused foot traverses. Accessibility, limited personnel and time constraints provided limits to what could be mapped and verified. Mapping in June and August was guided by lineaments detected in InSAR images (see discussion later in this report). Figure 7 documents (based on GPS track data) which areas were field checked and also provides a record of what areas were not visited. Some additional field observations, aside from surface rupture, are also noted on Plates 1a-1f.

For ease of discussion the faults are divided into twelve general zones, A-L, based on shared orientation and proximal association (refer to Figure 8). A through E are roughly northwest trending right-lateral faults and F through L are roughly northeast to north trending left-lateral and normal faults.

–

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SUMMARY OF FAULTS EVALUATED

As mentioned earlier, the faults being evaluated herein comprise a broad zone of both northwest-trending and northeast-trending faults. To assist in an orderly discussion of these faults they are further divided into twelve more-or-less distinct groups of faults that share orientation, proximity and style of faulting. These groups are defined on Figure 8 and additionally labeled on the Plates 1a through 1f. NORTHWEST-TRENDING FAULTS (FAULT GROUPS A - E) A. Laguna Salada Fault Zone (Coyote Wells & Yuha Basin quadrangles – Plates 1d & 1e) The Laguna Salada Fault Zone comprises a broad zone of right-lateral faults, originating in Baja California and named for the Laguna Salada, a structurally controlled playa and intermittent lake south of the border. From approximately six kilometers south of the international border the Laguna Salada Fault Zone splits into two branches that extend northward into southern California. The fault zone ruptured in Baja California in 1892 (Mueller and Rockwell, 1995) but historic rupture had not been traced into California. In the study area, the Laguna Salada Fault Zone consists of the two primary branches (herein designated the west and east branches) and several sub-parallel splays (Figures 2 & 8). Aa. Laguna Salada Fault, east branch Previous mapping The east branch of the Laguna Salada Fault Zone has been previously mapped in California by Dibblee (1954, 1996), Clark (1982 and unpublished), Isaac (1987) and Kahle (1988). Isaac (1987) called it the Sunrise Butte strand, but the others merely labeled it the Laguna Salada fault. It has also been called the Yuha Buttes Fault (Mueller and Rockwell, 1991, their figure 3) but this name has not been otherwise established in the literature. It is primarily expressed as a west-facing scarp (Clark, 1982) in late Quaternary fan deposits. Much of the detail at the northern end of the fault is from Isaac (1987). This fault strand was previously evaluated by Smith (1979c) for possible zoning under the APEFZ Act but, although considered to exhibit evidence of Holocene displacement, was not recommended for zoning due to its isolated location. Post -earthquake observations This branch of the fault was field checked on April 6-8 and May 11-12, 2010. Portions of the fault adjacent to Sunrise Butte, immediately north of Highway 98, were inaccessible due to private land holdings. Maximum right-lateral displacements of up to 4 cm were observed on the northern part of the rupture, accompanied by up to 3 cm vertical offset (down to the west). Displacements diminished southward. A prominent left-step occurs in the fault at Highway 98 and appears responsible for the topographic relief of Sunrise Butte, which lies within the stepover. Rupture was nearly continuous from Highway 98 to about 650 meters north of Pinto Wash, from which point southward the fracturing became increasingly obscure within soft aeolian sand deposits. Continuous or closely spaced left-stepping ruptures followed the base of the pre-existing scarp very closely and exhibited primarily right-lateral displacement of up to 20 mm. Vertical offsets were generally minimal along most of the trace south of the highway, but locally were as much as 15 mm, west side down. Rupture was not traceable on trend

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across or south of Pinto Wash and appeared to step right approximately 400 m to another strand of the fault that extended to the border. This southernmost fault trace, near the border, was first mapped on April 8. It was revisited on May 12, after there had been some local rainfall, and the rain-modified surface was found to be freshly ruptured, indicating continued minor slip since the initial rupture in April. Rupture to the south (in Mexico) was apparently discontinuous and was not observed to connect to the primary rupture (Sinan Akciz, personal communication). The northern portion of this fault zone (as indicated on prior mapping and InSAR) was not visited. Photo analysis Much of the eroded west-facing scarp of the east branch of the Laguna Salada Fault Zone was evident in aerial imagery. The scarp has a notable narrow bajada extending to the west. A concentration of vegetation indicated a ground water barrier in Pinto Wash. The northern extent of the fault was visible as fault line morphology (linear base of slope), occasionally highlighted by truncated geologic structures. Some of the traces mapped by Isaac (1987) were visible as geomorphic or tonal lineaments. UAVSAR interpretation UAVSAR data indicate that most of this fault had displacement in April 2010 with some subsequent displacement along the southern part of the fault and a few of the northernmost splays. UAVSAR did not detect movement on the stepover near the border nor on most of a series of splays fanning out at the north end of the fault. The northernmost strand of the fault showed later (April-July) movement, perhaps in response to the July 14 aftershock. Ab. Laguna Salada Fault, west branch Previous mapping This appears to be the more complex and continuous (from Mexico) trace of the Laguna Salada Fault Zone, although that name was not applied to it in the published literature except south of the border by Barnard (1968) and Mueller and Rockwell (1991). Dibblee (1954, 1996) and Isaac (1987), each, showed stylized traces of this fault but did not include much detail. Clark (1982 and unpublished, cited by Jennings, 1985) mapped numerous smaller pieces of a broad complex of faults, including a number of short traces to the west, but did not portray the continuity of the structure (Figure 9 – compare with Plate 1d). The continuity of the zone was implied by Jennings (1975). The fault zone is geomorphically expressed as a group of eroded and discontinuous east-facing scarp segments. Clark’s mapping was the source for most of the traces depicted by Smith (1979c) and many of those by Kahle (1986).

Figure 9 - part of map from Clark (1982)

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Post -earthquake observations The west branch of the Laguna Salada Fault Zone was field checked on April 6-8 and May 12, 2010. This branch consists of two primary faults that converge northward, becoming a single trace near Highway 98. The two faults appear to be transtensional, flanking a shallow graben-like area. They were characterized by relatively continuous zones of left-stepping en echelon fractures. In many localities the recent rupture was along the base of, or slightly outboard (up to a few meters) of, an older eroded scarp. In places the old scarp was highly eroded with prominent alluvial recesses. In other areas the rupture coincided with preexisting vegetation lineaments. Right-lateral displacement was typically 1-2 cm measured on individual strands (maximum up to 4 cm), with at least 4 cm cumulatively measured across the entire zone; vertical offset, down toward the graben center, was generally minor (~7-8 mm) to undetectable but reached 2 cm at one locality. Surface rupture could be seen continuing southward across the border along both strands, but was not completely mapped. To the north, rupture died out just before crossing Highway 98. No rejuvenation of surface fractures in response to aftershocks was evident in May, but Roger Bilham (personal communication; also cited reported in Rymer et al, 2011) reported 2 mm of coseismic triggered slip at his creepmeter just north of Pinto Wash on June 14, 2010 (locality A1, Plate 1d). Photo analysis Very dissected and intermittent scarps can be interpreted in the aerial imagery, most clearly in the areas north and south of Pinto Wash. The western splay (east-facing) is more clearly expressed, with a couple of more prominent washes abruptly fanning out as they cross the fault. These fans are dammed by the west-facing eastern splay. A prominent vegetation lineament marks the fault location across a small playa near the northern end of the fault. UAVSAR interpretation Almost all of the strands of this fault showed evidence of displacement, even across Pinto Wash where surface rupture was difficult to detect on the ground. Displacement was imaged very sharply along most of the western strand. Some of the greatest displacements were indicated approaching the border. Ac. intermediate faults Previous mapping Several short parallel faults were mapped between the two principal strands of the Laguna Salada Fault Zone by Smith (1979c), Clark (1982) and Isaac (1987) based on several discontinuous east or west-facing highly eroded and subdued scarps (Figure 4; Plates 1d &e). Post -earthquake observations The most prominent of these faults was investigated on April 6 and others were inspected at their highway crossings. Only the dissected east-facing scarp north of Highway 98 (A2, Plate 1e) had minor fractures along the base of the scarp. These did not show an en echelon pattern or other indications of directed strain and may have been merely a result of trace-fracturing due to shaking. The other faults are

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apparently inferred to control a very subtle set of parallel ridges that were difficult to discern at ground level. Photo analysis These faults appear in some imagery as contrasting tones due to locally elevated gravel surfaces. The sometimes subtle relief of the features is more visible in the 1972 photography (BLM_C_M000). UAVSAR interpretation None of these faults had evident surface displacement in the UAVSAR imagery. Ad. west of west branch Previous mapping This westernmost splay of the Laguna Salada Fault Zone, and a few shorter faults to the east, were partially mapped by Clark (1982), Isaac (1987) and Kahle (1988) (Figure 4 and Plate 1d). Kahle (unpublished notes) observed faceted scarps suggestive of multiple offsets. Topographic displacement is generally down to the east. Post -earthquake This fault was inspected on May 11 and June 16, 2010. Minor rupture was found at the northern end (A3) along a very subtle east-facing scarp within an otherwise relatively featureless fan surface. Displacements could not be measured but are interpreted to be right-lateral based on the left-stepping fracture pattern. Although the fault to the south was prominently expressed by east-facing scarps, no recent surface rupture could be detected (green lines on Plate 1d). Active aeolian sand deposition may have obscured any evidence of faulting along its southernmost portion. Photo analysis

Locally prominent scarps bounding older fan surfaces show up in most of the imagery. UAVSAR interpretation Only the southernmost extent of this fault had any expression in the UAVSAR data. Its expression there was somewhat sharp but indicated only small displacement. B. Laguna Salada Fault Zone, northwest extension (Painted Gorge & Coyote Wells quadrangles – Plates 1b & 1d) Previous mapping A general connection between the Elsinore Fault Zone and the Laguna Salada Fault Zone (east branch) was suggested by Elders et al (1972) on a small-scale map (1:1,000,000). A possible connection was also indicated, but to the west branch, by Mueller and Rockwell (1991) on a similarly small-scale map. These two suggested faults traces are shown on Figure 4. A connection was also shown by Jennings (1975 & 1994) based on these earlier sources. However, no specific fault traces have been mapped on the ground along these trends other than a few isolated lineaments by Kahle (1986).

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The overlap of the southwest end of the “Yuha Wells Fault” (Jennings, 1994) with minor faulting in 2010 (Be) is considered coincidental, as the previously mapped “Yuha Wells Fault” is very likely mis-located. Ba northwestern extension of the Laguna Salada Fault Zone Post -earthquake observations A northwestern extension of the Laguna Salada Fault zone (Ba) was suggested by InSAR data following the June 14 aftershock (see discussion of InSAR data, below). This trend, including numerous air photo lineaments, was inspected in the field on August 3-4, 2010 and February 24, 2011. Limited evidence of prior faulting was observed but only very minor and inconclusive fracturing (short orange traces on Plate 1d) was found along this trend. Some very degraded scarp-like morphology (more of a southwest-facing escarpment) was fairly general but a single, more convincing, east-facing scarp was mapped at locality B1 (Plate 1d). Photo Analysis This lineament is very weakly expressed, with a few minor features marked by vegetation lineaments, truncated topographic remnants and one back-facing scarp. A number of lineaments were identified that did not correspond with any mapped rupture, principally in a subparallel zone closer to Highway 98. These may be related to paleodrainage patterns. UAVSAR interpretation Most of this lineament had a weak and diffuse signature in the UAVSAR data for the April-July time period (Figure 5c). It did not appear in the earlier interferogram. Eric Fielding (NASA/JPL, personal communication) interprets the data to indicate slip at a depth of greater than 2 km. Bb Post-earthquake observations Minor fracturing was observed along the trend of an inferred fault strand (Bb) at locality B2 (Plate 1d). Photo analysis This fault is indicated by a moderately embayed and eroded southwest-facing paleoscarp. UAVSAR interpretation Displacement on this fault trace was indicated in the April-July interferogram. Bc Post-earthquake observations A linear zone of left-stepping fractures was observed along the base of a paleoscarp (B3, Plate 1d). Photo analysis This southwest-facing scarp is somewhat less embayed than Bb.

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UAVSAR interpretation Displacement on this fault trace was indicated in the April-July interferogram. Bd Post-earthquake observations Discontinuous fracturing was observed along the base of this northeast-facing scarp (B4, Plate 1d). Photo analysis This fault has a clearer and more youthful looking scarp than the other faults in this group. UAVSAR interpretation Displacement on this fault trace was mostly indicated in the April-July interferogram, but some earlier movement is also indicated on a splay fault. Be Post-earthquake observations A small cluster of fractures (Be) was observed on June 16, 2010 about 3 km to the southwest of the principal zone of displacements (Ba). This rupture looked fairly fresh for its relatively minor appearance and apparently occurred in response to the June 14 aftershock. Displacements were too small to measure, but en echelon patterns, locally, were left-stepping and suggested a northwest-trending right-lateral fault. The fracture zone is in an area of shallowing alluvium adjacent to a local bedrock high and is only 1.5 km west of the M5.7 aftershock. Photo analysis This area had no suggestive features in the aerial imagery that coincided with the observed rupture. Conversely, some interpreted features did not have any rupture expressed in the recent earthquake. UAVSAR interpretation This zone of faulting coincides with a poorly defined color boundary in the UAVSAR interferograms which was not readily identified as a fault displacement prior to the field observations. C. Elsinore Fault Zone (Carrizo Mountain & Painted Gorge quadrangles – Plates 1a & 1b) Previous mapping The Elsinore Fault Zone is one of the better known fault zones in southern California, and is a major element of the San Andreas system of plate boundary faults. The southernmost mapped section of this fault zone is called the Coyote Mountain section (WGCEP, 1995) and has been previously identified along the southern margin of the Coyote Mountains in Imperial County. Detailed mapping of this portion of the fault has been published by Smith (1979a), Clark (1982), Kahle (1986) and Rockwell (1990). Parts of the fault were also mapped by Christensen (1957), whose traces extend further southeast than traces mapped by others (Figure 4). The Elsinore fault is principally a right-lateral fault, based on offset drainages and other geomorphic features (Clark, 1992; Rockwell, 1990). Historic surface rupture has not

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been previously reported on this section of the Elsinore fault zone. An EFZ was established for portions of the fault within the Carrizo Mtn. Quadrangle based on an evaluation by Smith (1979a) (Figures 2, 4). Smith’s compilation was based on earlier unpublished mapping by Clark. Post -earthquake observations -- Initial reconnaissance during the first week after the earthquake consisted of spot reconnaissance along the main trace of the fault where it was mapped along the southwest flank of Coyote Mountain. Three localities visited on April 11, 2010 showed no clear evidence of rupture (green lines, Plate 1a). The western locality had no fractures observable although geomorphic features indicative of prior rupture (sidehill benches, saddles, tectonically stirred rubble) were visible. Some fine fractures along a road bank at locality C1, although close to the mapped trace, could not be distinguished from shaking-related phenomena. Inspection of several splay faults in the southeast corner of the Carrizo Mtn. quadrangle (Ca) was made on February 22, 2011. Low subtle scarps were observed along one of these splays (C2), although no surface fractures were noted. One possible extension of this zone (C3), mapped by Christensen (1957) displayed only fault line geomorphology with no indication of Holocene displacement, whereas an alternate trace just to the northeast was indicated by a low scarp and local drainage impacts. George Morgan (personal communication, 2011) stated that Christensen’s splay fault does not offset older surfaces to the northwest. Reconnaissance on August 3, 2010, guided by interpretation of UAVSAR imagery (Figure 5c), verified discontinuous surface fracturing along an eastern extension of the mapped fault zone (Plate 1b). Principal observations were linear zones of fractures along the inferred geomorphic expression of the fault. Tectonic displacement was suggested by the linear continuity of the fractures, indication of about 1 cm of right-lateral offset at one locality (C4) and the discontinuities visible in the UAVSAR imagery. A traverse across several northwest-trending faults (Cd) mapped by Christensen (1977) and Kahle (1988) did not identify any recent rupture on those faults, nor along a weak UAVSAR lineament. Photo analysis Ca - A couple of splays to the Elsinore Fault are suggested by aligned saddles, linear drainages tonal

contrasts and truncated talus slopes. One of these was previously mapped by Christensen (1957). Cb - This segment of the Elsinore Fault is not specifically visible in the aerial images, but a general

truncation of eroded geologic structure is suggested. Cc - A weakly defined linear valley with a locally linear southern margin is the only suggestion of

faulting in the aerial images. Some lineations within the hills to the north could not be associated with any active faulting.

Cd - Most of the previously mapped faults were not evident in aerial imagery. A few other structural discontinuities were interpreted in the Google Earth image, but were not field checked.

UAVSAR interpretation There was no indication in the initial interferogram of displacement on the Elsinore Fault. However, in the April-July interferogram (Figure 5c) there appeared moderate to weakly defined lineaments along an extension of the fault (Cb & Cc). Only one of the northwest-trending structures (Cd) had a signal in the later interferogram.

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D . Other northwest trending faults (Coyote Wells quadrangle – Plate 1d) Da. Davies Valley Fault Previous mapping Kahle (1986 and unpublished notes) mapped a fault along the eastern margin of Davies Valley (Figure 4) and identified it as possibly having Holocene displacement. The fault was characterized by well-defined west-facing scarps along the valley margin and dipped 80° to the east at one locality (Kahle, unpublished notes). The fault is herein named for the valley to which it forms the eastern margin. Post –earthquake observations No field observations were made due to accessibility constraints. Photo analysis The Davies Valley Fault is clearly marked by well-defined scarps and right-deflected drainages. A set of subparallel faults with apparent normal displacements is visible across an isolated knob of resistant metasedimentary rock in the middle of the valley, however these faults do not project across the younger alluvium or older dissected fans to the north and south. UAVSAR interpretation Displacement in Davies Valley was only vaguely suggested by UAVSAR data, but not with sufficient definition to plot a line. The west margin of the valley showed a little more definition but is outside the evaluation area. Db. unnamed minor traces Previous mapping The irregular rangefront west of the Laguna Salada Fault Zone is locally marked by prominent northwest-trending lineations, one of which was mapped by Kahle (1988 and unpublished notes)(Figure 4 and Plate 1d). The inferred fault was expressed by a few low southwest-facing scarps. Post –earthquake observations One segment of this linear front, north of the feature mapped by Kahle (1988), was inspected in the field on June 16, 2010. A prominent paleoscarp was observed but no fracturing was found anywhere along this scarp (D1, Plate 1d).

Photo analysis The fault strands appear as prominent scarps in older dissected fan surfaces but do not appear in young or intermediate age fans (relative ages only). The faulted fan remnants may be pre-Holocene, based on similar morphology to late-Pleistocene fan Qf5 of Mueller and Rockwell (1991). UAVSAR interpretation UAVSAR data gave no indication of fault displacement in this area. An alignment of weak northwest-trending lineaments that lie between Davies Valley and the eastern front roughly follow a ridge crest and may represent ridgeline shaking effects.

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E . Other faults (Plaster City & Yuha Basin quadrangles – Plates 1c & 1e) Ea. unnamed fault Previous mapping This fault within the Yuha Basin badlands was not previously mapped. Post- earthquake observations Field inspection on February 24, 2011 found no evidence remaining of recent fracturing, but a subtle north-facing scarp (pre-existing) at the west end of the feature was deflecting south flowing drainage. Another local drainage is prominently deflected in the eastern portion of this fault (E1, Plate 1e). Although stream deflections appear to be right-lateral, wind gaps aligned with the upper drainage sections suggest the drainages were defeated and deflected by uplift along the fault. Photo analysis This fault did not stand out in the available imagery, being only weakly expressed as a somewhat linear hill front with a couple of right-deflected drainages as they reach the north-facing scarp. The fault was detected by its expression in the InSAR imagery. UAVSAR interpretation A moderately well-defined zone of displacement was indicated in the April-July interferogram (Figure 5c). Eb. Dixieland Fault Previous mapping A zone of fracturing was first identified by James Barclift (citation in Smith, 1979b) and was subsequently mentioned also by Sharp and Clark (1972). Further evidence of possible historic creep or fracturing was summarized by Smith (1979b) who also named the fault. The inferred fault was not zoned due to limited outcrop length and uncertainty as to its origins (Smith, 1979b). A zone of north-northwest trending lineaments, including the Dixieland Fault, was further depicted by Kahle (1988) who cited a creep rate of 3.2 mm/yr (Figure 4). The basis for this creep rate is not known. Kahle’s fault location, published on a smaller-scale map (1:100,000), was slightly different from that of Smith (1:24,000) and may be mislocated. Post- earthquake observations This inferred fault has two locations shown in the literature (Smith, 1979b and Kahle, 1988). The western location was inspected on February 23, 2011 and no sign of fractures or fissuring was observed, although a small kink in the railroad tracks to the south was observed on trend with the mapped trace (E2, Plate 1c). It is not known if this is the result of recent deformation or just a construction-related defect. The eastern location was inadvertently not inspected as the conflicting locations had not been noted. Photo analysis The Dixieland Fault, in the vicinity of the old Evan Hewes Highway, does not have any visible expression. Some possibly related structures are hinted at by a few weak vegetation lineaments to the northwest.

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UAVSAR interpretation There was no indication in the UAVSAR data of displacement on this inferred fault. NORTHEAST AND NORTH-TRENDING FAULTS (FAULT GROUPS F - L) F. Painted Gorge Wash Fault (Painted Gorge Quadrangle – Plate 1b) Previous mapping A general north-northeasterly alignment of faults flanks the southeast end of the Coyote Mountains and extends from the vicinity of the Elsinore Fault Zone for several kilometers to the northeast (Figures 2, 4). Parts of this fault were previously mapped by Clark (1982) and Kahle (1988). Several other northeast-trending faults were mapped in the area by Christensen (1957) but few of these correlate with the Painted Gorge Wash Fault as identified herein. Morgan and Morgan (2006, personal communication 2011) have done more detailed mapping of the central portion of this fault (north of Painted Gorge) and named it the Painted Gorge Wash Fault. This is to be distinguished from the Painted Gorge Fault Zone, a northwest-trending fault zone mapped by Christensen (1957) that might appear to cut across the Painted Gorge Wash Fault. Morgan (p.c., 2011), however, believes that Christensen’s fault does not cut the northeast-trending faults. Clark (1982), Kahle (1988) and Morgan and Morgan (2006) all identified scarps associated with the NNE-trending zone within late Quaternary alluvial surfaces, and the latter two references suggested the fault has had Holocene displacement. Morgan and Morgan (2006) identified mostly east-facing scarps as high as 2.4 m in the southern portion of the zone, with scarp heights diminishing northward, where they are commonly 0.3 to 0.6 m high. Morgan and Morgan (2006) propose that the fault originated as a right-lateral fault that has been partially reactivated after ~50° clockwise rotation. Fa Painted Gorge Wash Fault, southern section Post -earthquake observations Minor fracturing in a linear zone was observed August 3, 2010 along the southernmost portion of this fault (Fa). There was no sign of surface fracturing further to the north at locality F1 (Plate 1b). However, subsequent interpretation of aerial imagery suggests two additional traces just west of F1 that were not checked. Two facing scarps at F2 (mapped on February 23, 2011) define a broad shallow graben cutting across an older fan surface. Modern drainage across the eastern fault is locally ponded, suggesting recent vertical offset; there was no clear lateral offset of crude bar and swale topography. Photo analysis This portion of the fault zone is well-defined by scarps in its southernmost reach but has several gaps to the north with little or no expression. The fault is well-expressed again south of Painted Gorge wash (F2) where scarps are visible across an older fan surface. UAVSAR interpretation Displacement on the southern portion of Fa was indicated in the April-July interferogram. There was no sign of movement in the prior image.

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Fb Painted Gorge Wash Fault, middle section Post-earthquake observations The middle section (Fb) was inspected on April 11, 2010 and February 23, 2011 to assess the character of the fault features. Although no recent fractures were seen, much prominent active fault morphology was observed, including well-defined scarps, fault-line troughs and ponded alluvium. Inspection of two east-facing scarps (F3, Plate 1b), on April 11, did not find any indication of surface rupture, however this was well prior to the more northerly aftershocks. At localities F4 and F5 there are distinct east-facing scarps across all but the youngest active alluvium (Figure 10a at F5). Between F5 and F6 are one to two parallel fault-line furrows (Figure 10b). A small, local drainage that crosses the fault at F6 displays a cumulative left-lateral offset or deflection of 4-5 m. At the northern end of section Fb the fault is expressed by both prominent older scarps and more subtle scarps and ponded alluvium (F7) on younger surfaces.

Figure 10. Youthful fault morphology along the Painted Gorge Wash Fault. a) looking south along scarp across young fan and channel deposits; b) looking north along fault-line trough on hillside, with locality F6 in front of geologist. Photo analysis The middle portion of this fault zone is distinguished by well-defined scarps that are mappable across most of the older to younger fan surfaces. Faults mapped by Kahle (1988) to the west are clearly visible on older surfaces but do not appear to cut younger fans. UAVSAR interpretation There was no indication of surface displacement in the UAVSAR data for the southern portion of this fault. Data were not evaluated further north. Fc Painted Gorge Wash Fault, northeastern section Post-earthquake observations The northeast section of the fault (Fc), splays into several strands, two of which were inspected on February 23, 2011. These two splays were marked by southeast-facing scarps. No recent fractures

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were observed. The eastern of these scarps (F8, Plate 1b) was relatively degraded with equivocal evidence of younger displacement. The western splay (F9) consisted of a series of right-stepping en echelon fault strands and several apparent stream deflections. These deflections, on at least three drainages and on the order of several meters, appeared to be right-lateral. Photo analysis This set of faults, which splays off to the northeast from the more continuous southern and middle sections, is expressed as multiple parallel traces with subtle scarps and, more prominently, offsetting or truncating remnants of deformed older sediments. One prominent ridge is cut and displaced left-laterally. There are also a number of left-deflected drainages visible on most strands. UAVSAR interpretation UAVSAR data was not interpreted for this fault section. G. Ocotillo Fault Zone (Painted Gorge and Coyote Wells Quadrangles – Plates 1b & 1d) Previous mapping This zone of north and northeast-trending faults lies south of the southeastern end of the Elsinore Fault (Figures 2 and 8) and was partially mapped on the Coyote Wells quadrangle by Smith (1979c), Clark (1982) and Kahle (1988) (Figure 4). Smith (1979c) judged the principal trace to be Holocene, but did not recommend zoning at the time because of the short length of the fault and its remoteness from development. This trace shows very prominent geomorphic expression, with a slightly sinuous west-facing scarp. The fault is newly named for the nearby community of Ocotillo (Rymer et al, 2011). The name of the fault zone is taken from the fault and the community. Ga – Ocotillo Fault, northeast strand Post-earthquake observations Rupture was mapped (August 3 & 4) as a somewhat continuous to right-stepping, northeast-trending zone of fractures with left-lateral displacements of up to 1 cm. This fault segment was locally marked by subtle west-facing scarps and elevated fan remnants on the east. Photo analysis This fault was weakly expressed in imagery, probably because of the low contrast surface material and low relief of the fault morphology. The identified traces were mostly indicated by vegetation lineaments and a few truncated fan remnants at the northern end. UAVSAR interpretation Much of this fault was indicated by displacements in the April-July interferogram (Figure 5c), but was not visible in the earlier data (Figure 5a). Gb – Ocotillo Fault, main strand Post-earthquake observations The principal strand of the Ocotillo Fault was field checked on June 16, 2010 and was found to have fresh surface rupture, very likely a result of the M5.7 aftershock two days earlier. The rupture,

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which trended about 10° west of north, had one primary break within a zone of subparallel fractures that was as much as thirty meters wide and encompassed the pre-existing scarp. The principal break was commonly at the east margin of the zone but was also found in the center of the zone in some reaches. The dominant sense of displacement was down to the west (commonly around 5 cm, but up to 8.5 cm) with a small right-lateral component (up to 2 cm). Some unmeasured component of extension was also indicated. Photo analysis The Ocotillo Fault is clearly expressed by a prominent west-facing, slightly sinuous scarp. The fault scarp is also marked by a distinct vegetation lineament, with slightly denser light vegetation on the upstream side. UAVSAR interpretation The Ocotillo Fault was interpreted with some difficulty in the April-July interferogram because the line-of-sight signal from the left-lateral offset is partially cancelled by the signal from the vertical displacement. The offset was more visible in the re-processed data with a 5 cm wrap cycle. Prior displacement was not detected in the earlier interferogram but small amounts of displacement cannot be precluded due to the possibility of similar effects. Gc Post-earthquake observations Possible rupture east of the Ocotillo Fault was explored on August 3, 2010 and February 23, 2011(green lines at Gc), but fractures were not seen. Aeolian deposits are likely to have obscured evidence of recent faulting. Photo analysis This fault is expressed by a few subtle vegetation lineaments, including a contrast in vegetation where the fault crosses a modern distributary of Palm Canyon Wash. A short possible scarp just north of I-8 may alternately be an accumulation of aeolian sand anchored by vegetation along the fault. UAVSAR interpretation Displacement on this fault segment was suggested by the April-July interferogram (Figure 5c). Gd Post-earthquake observations These traces were investigated on August 2, 2010 and February 22, 2011. A pre-existing scarp was noted at locality G1 (Plate 1d) but no evidence of recent faulting was observed other than one very minor fracture of ambiguous origin. Photo analysis Faults in this section are inferred based on roughly linear margins to an erosional outlier of older fan deposits and some internal lineaments aligned with one of those margins. Several additional lineaments are identified to the west, based on truncated fan remnants and linear erosional features across some of those fan remnants.

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UAVSAR interpretation Displacement on the easternmost lineament was indicated by a weak signal in the April-July interferogram (Figure 5c). Ge Post-earthquake observations South of Highway 98 a north-south trending zone of fractures was mapped on August 2, 2010; these fractures were both right and left-stepping with no detectable lateral offset but some small sense of extension. The land surface is elevated to the east. A northeast-trending topographic escarpment (G2, Plate 1d) was explored and no fracturing was observed; this feature may be erosional. Two short scarps to the west (green lines) were inspected and no recent fractures were observed. The more prominent scarp was on the order of 1-2 m high and looked as fresh or fresher than many other scarps that did have renewed rupture from the recent earthquake. Photo analysis The principal fault was interpreted based on a strongly linear margin to one of the outlying fan remnants. Two to three parallel fault scarps were visible across fan remnants to the west. UAVSAR interpretation Displacement on the north-south fault was indicated along a moderately sharp lineament that was visible in both the original and follow-up interferograms. The splay on the east side of the ridge was equally well expressed in the April-July interferogram but not in the earlier image. Gf Post-earthquake observations This roughly linear but irregular southeast-facing escarpment did not have any indication of recent fracturing (as observed on February 22, 2011). Photo analysis This prominent wash margin was somewhat sinuous but also aligned with Gd. UAVSAR interpretation The southwest end of this lineament was quite sharply defined in the April-July interferogram (Figures 5c and 6d), but it became more diffuse to the northeast. Gg Post -earthquake observations A parallel set of northeast-trending scarps was partially inspected on February 22, 2011. No fractures were visible, although nearly a year had passed since the earthquake. A distinctly linear scarp along part of the zone (G3, Plate 1d) looked as if it may be fault-related and increased in height to the north. The area was also visited in June, 2010 to look at some northwest-trending scarps and no ground fractures were seen in the area. Other scarps in this zone may be modified by fluvial erosion if not entirely erosional.

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Photo analysis These features flank some older and younger surfaces along the southeast margin of the compound fan draining out of Davies Canyon. They are generally parallel to the drainage direction and could be erosional in nature. UAVSAR interpretation All four of these lineaments had indication of displacement in the April-July interferogram, most clearly along G3 (Figure 5c and 6d). Gh Post -earthquake observations This feature was not field checked. Photo analysis This lineament roughly follows a break in topography, between steep bedrock slopes (commonly dip-slopes) on the east to dissected older alluvium on the west. Abrupt linear erosional scarps closely parallel this transition in some locations as well as a few linear saddles or benches. UAVSAR interpretation The lineament is roughly on trend with the lineament at Ge but is rather diffuse and weakly defined in the October-April interferogram. It is not visible in the April-July interferogram. H. Yuha Well Fault Zone (Plaster City, Coyote Wells, and Yuha Basin Quadrangles – Plates 1c, 1d & 1e) Previous mapping The Yuha Well Fault Zone is a new name (Rymer et al, 2011), replacing what was called the Yuha Wells Fault (Rockwell et al, 1990; Thomas and Stinson, 1990). The prior mapping was based on interpretation of Landsat imagery with follow-up field reconnaissance mapping. The map by Thomas and Stinson (1990) was published on a poorly controlled base (grid coordinates only) and was difficult to register, but nevertheless appears to identify older, perhaps inactive structures (Figure 4). Subsequent compilation by Jennings (1994) introduced a significant error in location of the (generalized) fault. The current re-definition (Figures 2 & 8) is a correction in both fault nomenclature and definition. Not only is there just a single well named Yuha (east margin of Plate 1d, section 6), but also there is no single through-going fault as previously depicted. The Yuha Well Fault Zone is a set of sub-parallel northeast-trending, left-lateral faults that appear to truncate the principal faults of the Laguna Salada Fault Zone and merge with some of the lesser strands of that zone (Figure 8). A few shorter fault segments (Ha, Hb, Hd, He, Hh and Hi) were also mapped, at least in part, by others ( Clark, 1982; Kahle, 1988; Isaac, 1987) (Figure 4). Kahle (unpublished notes) observed a compound scarp suggestive of multiple events (H2, Plate 1d). The entire zone has a width of up to 3 km. At its southwest end (Hd and Hh) the zone partially blends into more northerly oriented faults (and zones of horst and graben), but also may extend to the border (He).

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Two of the longer faults within the Yuha Well Fault Zone are the newly named Vista de Anza Fault (Hc) located near the southwestern end of the fault zone (Figure 8) and the newly named (and restricted) Yuha Well Fault (Hf & Hg) extending to the northeastern end of the fault zone (Rymer et al, 2011). Other, shorter faults have not been named. Various strands of the Yuha Well Fault Zone were mapped on April 6 & 9, May 11, June 16-19, August 4, 2010 and February 22, 2011. Ha (Ha1 & Ha2) Post -earthquake observations No significant observations were made and no clear evidence of rupture was seen during a limited reconnaissance in August, 2010 of several faults previously mapped by Clark (1982) and Kahle (1988). No fractures were seen where Ha1 would project across Highway 98. Photo analysis

Ha1 – The most prominent part of this fault is a northeast-trending trough and scarp separating higher ground on the southeast from what appears to be the same surface on the northwest. The middle section of the fault is suggested by what appears to be slightly greater incision into the fan surface west of the fault. The southern part, also identified by Kahle (1988), is suggested by a linear southeast-facing slope break where the fan to the east is impinging on elevated ground to the west.

Ha2 - The eastern of three faults is largely defined by aligned linear drainages across an older surface, visible in several different images. It does not show any youthful geomorphic features, except perhaps at its southern end where it appears to veer into the northwestern extension of the Laguna Salada Fault Zone (Ba); at locality H1 there appears to be a slightly irregular east-facing scarp that has diverted the local surface drainage, and a similar scarp a few hundred meters to the south. The other two inferred faults to the west were suggested by somewhat irregular topographic lineaments (drainage channel or margin of higher ground) which appeared quite modified by erosion. UAVSAR interpretation Ha1 is suggested by a lineament visible in the April-July interferogram (5-cm wrap). Ha2 had no indication of displacement in the interferograms. Hb Post -earthquake observations The southwestern portion of this fault was visited several times during the first week after the earthquake. Right-stepping en echelon fractures were observed but there was no measurable displacement (estimated <1mm). The en echelon pattern and the northeast strike suggested a sense of left-lateral shear. The southern end of the central strand was visited on June 19, 2010 and no surface fractures were observed (short green lines on Plate 1d). The remainder of the central strand and the northern strand were not visited. Photo analysis The southern portion of this discontinuous fault was identified based on a prominent scarp (H2) in older fan deposits northeast of Highway 98. The remainder of the fault zone is not readily visible in aerial images.

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UAVSAR interpretation This fault had indications of surface rupture on all of its splays in the April-July interferogram (Figure 5c). Limited offset was also indicated on the central strand in the earlier UAVSAR data. Hc Vista de Anza Fault Post -earthquake observations Nearly continuous rupture was mapped on the portion of the Vista de Anza Fault that was inspected (May 11 & June 17, 2010). Slip was consistently left-lateral and decreased northward, from a high of 13 mm. The Vista de Anza Fault curves smoothly into the extensional zone (Hd) at the southern end of the Yuha Well Fault Zone. A graben, orthogonal to the fault (H3) may indicate a conjugate extensional zone to the north or accommodation for differing slip rates along the Vista de Anza Fault. This graben was visited several months after the earthquake (August 2010) and only very minor cracking was seen in a limited zone. Northern strands were not visited. Photo analysis The southern segment of the Vista de Anza Fault is locally marked by low scarps visible in imagery, and a weak vegetation contrast. The northern two segments, within the badlands, were not identified in aerial imagery. UAVSAR interpretation This entire fault showed indications of displacement with activity shifting to the more northerly portion in the April-July time period. Hd transitional horst and graben Post -earthquake observations The extensional zone (Hd) was mapped on May 11-12 and June 16-17, 2010. The zone here includes northwest-trending faults with right-lateral offsets, northeast-trending faults with left-lateral offsets and several northerly-trending faults with principally extensional displacement. Observed offsets were less than 2 cm. Most of these faults had prior expression (scarps); the extensional zone presented subtle horst and graben structures. Photo analysis Most of the faults in the western portion of this zone were indicated in various imagery by dissected scarps across slightly elevated remnants of older fan deposits. UAVSAR interpretation Almost all of these faults had weak to moderate (i.e. low color contrast), but sharply defined, indication of surface faulting in the initial interferogram (Figure 5b, 6d), which proved to be an excellent guide to further field mapping. He southern trace Post -earthquake observations

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A north-northeast oriented and possibly related fault (He) extends southwest to the border, southwest of the Laguna Salada Fault Zone. Fault He has prior but very degraded geomorphic expression and only very ambiguous indication of recently triggered slip (a couple of minor cracks observed on February 22, 2011 – H4, Plate 1d). Photo analysis This fault trace was evident in various imagery as a west-facing scarp bounding an older alluvial surface. UAVSAR interpretation Surface rupture was indicated by a relatively sharp lineament in the initial interferogram. Hf Yuha Well Fault, southern section Post -earthquake observations The southern portion of fault strand Hf was mapped on June 18, 2010 and was found to have 12-15 mm of left-lateral offset. This sense of displacement continues as the strike of the fault shifts to a north-south orientation and then to west of north at the southern end of faulting. The northern segments of this fault were not visited. Photo analysis The southern segment of the Yuha Well Fault was not readily identified in aerial imagery, although some weakly defined lineaments (dissected base of slopes and linear drainages) could be identified after the fault was known. UAVSAR interpretation Displacement on this entire fault segment was indicated by the UAVSAR data. Faulting along the southern portion was strongly indicated in the initial interferogram with displacement shifting northward in the later period. Hg Yuha Well Fault, northern section Post -earthquake observations The northern section of the Yuha Well Fault was field checked on April 9, 2010 where it projects to Interstate Highway 8 and no fracturing was found. On June 18, after the June 14 aftershock, a visit to the mid-section of this fault strand found discontinuous fracturing with a right-stepping pattern (northern margin of Plate 1e). Geomorphic indications of prior faulting were also observed (small scarps and beheaded drainages). Photo analysis The northern segment of the Yuha Well Fault was not readily identified in aerial imagery as a well-defined fault although a more general northeast-trending truncation of topography was interpreted (which guided the initial inspection at I-8, Plate 1c). Better defined are several shorter faults, just east of the main trace, which can be seen to truncate bedding and structure within the upturned beds of the Palm Spring Formation.

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UAVSAR interpretation Surface displacement was indicated along the entire length of this fault in the initial interferogram, with little indication of later displacement. Displacement was not indicated on the splay faults. Hh Post -earthquake observations Fault Hh was observed on June 18, 2010 (northern end) and February 22, 2011 (central portion). The recent rupture was expressed as a zone of right-stepping extensional fractures. A prominent scarp-like landform at the northeast end of the fault was found to be due to a resistant bed. Photo analysis Much of this fault was visible in aerial imagery as a slightly sinuous scarp. UAVSAR interpretation There was weak evidence for displacement along this fault in the initial interferogram. Hi Post -earthquake observations No field observations were made. Photo analysis This fault is strongly expressed as a prominent scarp in aerial imagery. UAVSAR interpretation This fault did not have any indication of movement in the UAVSAR data. I. Yuha Basin Faults (Yuha Basin and Plaster City quadrangles – Plates 1c & 1e) This newly identified zone of northeast-trending left-lateral faults has not been previously mapped, other than a couple of isolated strands of the Yuha Fault, south of Highway 98 (Kahle, 1988). It includes the Shell Beds Fault (named herein), the Yuha Fault (Rymer et al, 2011) and a number of other unnamed faults spread across an 8 km wide zone. The Shell Beds Fault (fault Ib) is named for an area of exposed Pliocene fossil oyster reefs through which it slices, indicated on the Yuha Basin 7-1/2 minute quadrangle as “Shell Beds”. The Yuha Fault (If, Ig & Ih) is named for the Yuha benchmark along Highway 98, very close to where the fault crosses this road (Rymer et al, 2011). Unlike the Yuha Well Fault Zone, the Yuha Basin faults appear to abut the Laguna Salada Fault Zone to the southwest. Extent to the northeast is uncertain. The zone is named for the eroded Yuha Basin in which most of the faults are exposed. Dibblee (1996) and Isaac (1987) mapped a short northerly-trending splay from the Laguna Salada Fault Zone in this area but it showed no sign of recent rejuvenation, based on InSAR data assessed for this study.

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Ia Post-earthquake observations No field observations were made. Photo Analysis There were no fault related features interpreted in older aerial photography. The terrain includes eroding badlands and younger fans, neither of which would preserve small offset surfaces. UAVSAR interpretation The April-July interferogram indicated displacement on this fault, based on a strong but slightly diffuse lineament. Ib Shell Beds Fault Post- earthquake observations Recent surface fault rupture, suggested by InSAR data, was not immediately verified on this fault, but field observations ten months after the earthquake (February 24, 2011) confirmed that the recorded deformation coincided with a previously unmapped fault which was observable in several locations. A

Figure 11 – A zone of sheared clay (white arrows) was evident across a pediment surface, indicating the location of the Shell Beds Fault (Locality I1, view to the southwest).

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clayey shear zone indicated the location of the fault across a low pediment surface across the southern section of the fault at locality I1 (Plate 1e; Figure 11) and subtle relief across the shear zone was suggestive of recent offset. Nearby, an occurrence of possibly ponded silt and clay (locality I2) was aligned with an apparent scarp along a splay fault a short distance to the southwest. To the north (locality I3), evidence of the central section of the fault was provided by a paleoscarp and a fault-line trough with active piping of surface runoff into the fault (Figure 12 a, b). The northern section of the fault was not visited.

Figure 12. a) paleo-scarp on a bedding plane surface of resistant shell beds; b) smaller scarp to the north of photo a) with piping occurring along fault zone (below geologist). Photo analysis The southern section of the fault was locally identifiable by some possible scarps and short tonal lineaments. There were also some indications of truncation of geologic structure and bedding, but the fault is often closely parallel to bedding, making such interpretaion difficult. The parallelism with bedding also made interpretation of tonal and topographic lineaments somewhat suspect. Interpreted truncation of structure in the deformed beds of the Palm Spring Formation helped refine the location of inferred faults in the central and northern portions of the Shell Beds Fault, although they are largely taken from the UAVSAR interpretation. Many of the features observed on the ground were too small to be seen in the aerial imagery. UAVSAR interpretation Most of the Shell Beds Fault was strongly indicated in the initial interferogram (Figure 5a) and portions of it showed continued displacement in the April-July interferogram. Ic Post-earthquake observations One locality along this lineament was visited on February 24, 2011 but no evidence of fracturing was visible at that date. The geomorphology at that location was vaguely suggestive of a pre-existing scarp but could also be explained by lateral stream erosion.

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Photo analysis There were no clear fault related features interpreted in older aerial photography. A few linear dissected embankments are noted that may be erosional. UAVSAR interpretation Fault Ic had weak to moderate indications of displacement in the initial interferogram. Id Post-earthquake observations No field observations were made of this cluster of short fault segments. Photo analysis Two of the northern strands of this fault set have subtle to strong expression in aerial images, primarily truncation of exposed geologic structure within the Palm Spring Formation and an eroded escarpment on that formation. Scarp-like landforms and a tonal lineament guided relocation of part of the southern fault in this group. A southeast-facing scarp was also mappable across the dissected fan surface along the east margin of this fault set. UAVSAR interpretation Most of this fault group showed moderate indications of displacement in the initial interferogram. However, no 2010 displacement could be discerned for the scarp on the east side of this group. Ie Post-earthquake observations No field observations were made. Photo analysis There are several eroded scarp-like landforms along or adjacent to parts of this inferred fault set that can be interpreted in the aerial imagery. Many of these, however, appear to be related to older high-stands of ancient Lake Cahuilla. The most prominent scarps flank a linear plateau. It is possible, but difficult to demonstrate without field confirmation, that wave action in the ancient lake may have enhanced previous fault scarps. Subtler linear saddles and scarps can be observed along the more prominent UAVSAR lineament, defining this lineament and a secondary fault just to the east. Another short but distinct splay lies to the southeast and is defined by west and east facing scarps. The southern trace in this set is partially suggested by a linear drainage aligned with a UAVSAR lineament. UAVSAR interpretation The eastern fault strand had some indication of displacement in the initial interferogram and somewhat stronger expression in the April-July interferogram. The expression was diffuse enough to be associated with the primary fault and the secondary trace. The western scarp set had a much weaker expression. The northeast-trending trace to the south showed probable offset of color fringes in the initial interferogram.

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If Yuha Fault, main section Post-earthquake observations The Yuha Fault was mapped on April 6-7, 9 & 11 and on May 11-12, 2010. Additional observations were made on May 27. The fault had subtle to moderate prior topographic expression, with low gentle scarps and some pressure ridges at stepovers. Fault rupture was relatively continuous with regular right-stepping en echelon fractures. The fault appears to bifurcate to the southwest, south of Highway 98. The maximum lateral displacement measured on any single fault strand north of the border, 4.5 cm left-lateral, was recorded just south of Highway 98 (I4, Plate 1e), and a combined displacement of 5-6 cm occurred across the zone in this same vicinity. Photo analysis Parts of this fault section are visible in aerial imagery as low scarps and a prominent truncation of upturned bedding (presumed Palm Spring Formation) at its northeast end, but the southern portion, near Highway 98, was too subdued to be very apparent. UAVSAR interpretation Surface rupture was strongly indicated on the initial interferogram and continued displacement was evident in later UAVSAR data, as well as InSAR data from Envisat (Treiman et al, 2010a). Ig Yuha Fault, southwest trace Post-earthquake observations This splay of the Yuha Fault was field checked on April 7 & 11 and May 12, 2010. Only very minor fracturing was found in widely separated locations along the fault. The pre-existing scarp is visible but somewhat subdued. Photo analysis This portion of the Yuha Fault is expressed as a moderately distinct northwest-facing scarp along its northeastern portion. The southwest part of the fault is poorly expressed, if at all, with a couple of subtle saddles suggesting its location at the southwest end, adjacent to Pinto Wash. UAVSAR interpretation UAVSAR data suggest fault displacement on the northeastern strand of this fault but the main (southwest) section of this fault did not show displacement. Data from Envisat for the period of May 2-June 6, 2010, although of lower resolution, suggests displacement on Ig, possibly at depth. Ih Yuha Fault, southern trace Post-earthquake observations The projection of this strand, just north of Pinto Wash, was inspected on June 19, 2010 but no fracturing was seen (green line, Plate 1e). However, any fracturing that may have occurred may have been obscured by blowing sand by the time it was inspected. Photo analysis There were no fault related features interpreted in older aerial photography.

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UAVSAR interpretation Fault Ih had moderately strong indications of displacement in the initial interferogram. J. “Route 247 fault” (Yuha Basin and Mount Signal Quadrangles – Plates 1e & 1f) This fault had not been previously mapped. The informal name is taken from a Bureau of Land Management route that crosses the fault. Post-earthquake observations The principal lineament was inspected in the field on May 11, 2010. A somewhat obscure and dissected scarp or lineament to the north (Plate 1f) had little, if any, fracturing evident (one ambiguous crack), but wind blown sand may have obscured any more continuous cracks. More continuous minor faulting (up to 1 cm left-lateral) was mapped on the southern portion of this fault. Geomorphic expression along the entire zone was subtle and may have resulted more from vegetation differences and sand capture across the fault than actual vertical displacement. Photo analysis This fault was initially inferred based on prominent vegetation and tonal lineaments in the aerial imagery. UAVSAR interpretation The principal strand of fault J did not have any recognizable signature in the UAVSAR data. However, a possible branch or splay off of the northern end of the fault had moderate expression in the UAVSAR data. This splay is on trend with the Yuha Fault (If). K. Pinto Wash Fault Zone (Yuha Basin and Mount Signal Quadrangles – Plate 1e & 1f) This zone of perhaps a half dozen previously unmapped fault strands was inspected on June 18-19, 2010 and February 24, 2011. Most of the faults showed minor extensional displacements with some right-stepping fracture patterns. The group of faults is named for Pinto Wash, which parallels many of the faults. At least one strand (Ke) may align roughly with a fault mapped to the south by Isaac (1987). Ka Post-earthquake observations This feature had not been identified early in the field investigations and largely parallels a heavily traveled road and active wash margin. No effort was made to make observations, and features related to faulting were not noted during several drives along the wash. Photo analysis This lineament is not evident in aerial imagery other than marking a somewhat linear wash margin.

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UAVSAR interpretation Displacement on Ka was indicated by the April-July interferogram, more strongly to the southwest and perhaps at depth extending to the northeast. Kb Post-earthquake observations No field observations were made. Photo analysis This lineament may be associated with some localized southeast-facing escarpments. UAVSAR interpretation Somewhat diffuse, but significant, displacement was indicated in the initial UAVSAR data. Kc Post-earthquake observations The northern end of this zone was extensively explored on June 18-19, 2010 and a relatively discontinuous zone of fracturing was mapped. One to two millimeters of extension was observed with a slight left-lateral sense. Surface rupture was difficult to follow to the south and the southern section was unexplored. A small northwest-facing scarp within the stepover between Kb and Kc had fine fractures along the base of the scarp that may have been shaking related, although minor tectonic displacement could not be ruled out. Photo analysis There were no fault related features interpreted in older aerial photography. UAVSAR interpretation Somewhat diffuse displacement, extending south of the border, was clearly indicated in the initial UAVSAR data. Kd Post-earthquake observations This relatively continuous to right-stepping, north-trending zone of rupture was first observed during helicopter reconnaissance by Ken Hudnut (USGS, personal communication) on August 21, 2010. Surface rupture was field checked and confirmed on February 24, 2011. This strand had up to 4 cm of vertical offset and 2 cm of left-lateral offset. Prior evidence of this sense of displacement was not evident in the landscape. Photo analysis There were no fault related features interpreted in older aerial photography. UAVSAR interpretation This fault was not detected in the UAVSAR data, perhaps due to the orientation and sense of motion of the fault with respect to the look direction of the instrument.

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Ke Post-earthquake observations Fault Ke is actually two en echelon fault strands, each of which had relatively continuous surface rupture as far as they were walked on June 18, 2010. Surface displacement could be seen continuing both to the southwest and the northeast of the limited traverses. Left-lateral displacements of 3-4 cm were measured along both strands. Along the northeast strand there was an impression that the surface rupture was parallel to bedding in the shallowly buried bedrock (Palm Spring Formation), suggesting that regional flexure and bedding-plane slip may be contributing to this northeast-trending set of ruptures. Prior surface expression of these faults was minimal. Photo analysis These fault strands were not specifically seen in aerial photo interpretation, but a few short linear scarps, parallel to the fault, suggested structural control. Considering the parallelism of the bedding it is likely the geomorphic features were bedding controlled rather than fault related. UAVSAR interpretation This en echelon fault set was very strongly indicated in the initial UAVSAR data (Figure 5a), with a possible continuation south of the border. Kf Post-earthquake observations Right-stepping fractures were mapped discontinuously along this trend. Photo analysis There were no fault related features along this trend interpreted in older aerial photography. A cross-trending, south-facing escarpment was not investigated. UAVSAR interpretation This fault is indicated by a moderately well defined lineament in the initial interferogram (Figure 5a). Kg Post-earthquake observations The southwestern end of this fault zone was field checked on June 18, 2010 and a weak zone of isolated fractures was noted. Some of the fractures appeared to be compressional. Photo Analysis Portions of this zone of faults were suggested by eroded scarps and lithologic contrasts visible in aerial imagery. UAVSAR interpretation This zone of fault is indicated by a moderately well defined lineaments in the initial interferogram (Figure 5a). This alignment extends along trend south of the border.

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L. Centinela Fault Zone (Mount Signal Quadrangle – Plate 1f) These faults have not been previously mapped north of the border, and only very incompletely mapped to the south (Barnard, 1968). Some of the scarps were partially mapped in unpublished notes of Kahle (La & Lb), who also recorded details of a faceted scarp indicative of multiple offsets (locality L1). The zone name is taken from the Centinela Fault of Barnard (1968), a prominent fault which bounds Cerro Centinela on the west. La Post –earthquake observations Two pre-existing west-facing scarps were field checked on May 12, 2010. A very dissected scarp remnant at the northwest end of the zone (green line) had no evidence of fracturing, but there was a somewhat continuous zone of left-stepping fractures at the base of the more prominent scarp just to the east. Lateral offset was difficult to measure but was less than 1 centimeter, right-lateral. Surface rupture along an extension of this fault was also mapped in Baja California (Sinan Akciz, personal communication, shown on Plate 1f). Photo analysis The more continuous west-facing scarp was well-defined and readily identified in aerial imagery (USDA; Google Earth). The shorter scarp, although eroded, was evident by its truncation of the older fan surface remnant. UAVSAR interpretation Displacement was not detected in the UAVSAR data. Lb Post-earthquake observations The southern portion of this lineament was not field checked on the U.S. side of the border, but surface rupture was mapped south of the border (Sinan Akciz, personal communication). The northern portion was only checked almost a year after the earthquake (February 24, 2011) and no evidence of rupture or fault morphology was observed. Photo analysis Fairly clear scarps were interpreted in the southern part of this zone. The northern part of this lineament was not visible in the imagery that was interpreted. UAVSAR interpretation Diffuse displacement was suggested along most of this lineament in the initial data (Figure 5a), with a stronger expression near the border. Lc Post-earthquake observations This fault was field checked on June 18 & 19, 2010. Windblown sand had largely covered evidence of displacement but in one isolated area (L2) a zone of right-stepping fractures could be followed for a short distance. Left-lateral offsets of 5-10 mm were measured. A short, possibly related

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rupture was also mapped south of the border (Sinan Akciz, personal communication). There was no indication of prior geomorphic expression along this fault. Photo analysis Only a small part of this fault was suggested by a roughly linear zone of vegetation visible in Google Earth images. The remainder of this fault was not visible. UAVSAR interpretation The southern part of this fault was very strongly indicated in the initial UAVSAR data (Figure 5a), with a more diffuse expression to the north. Ld Post-earthquake observations This fault was field checked on June 19, 2010. Windblown sand had largely covered evidence of displacement, but in one isolated area (L3) a zone of right-stepping fractures could be followed. Offsets were difficult to determine but a right-stepping set of fractures and down to the east displacement suggested left-lateral extension, probably 1 cm or less. There was no indication of prior geomorphic expression. Photo analysis There was no evidence of this feature in the imagery that was reviewed. UAVSAR interpretation This fault was very strongly expressed in the initial UAVSAR data (Figure 5a). Le Post-earthquake observations No field observations were made. Photo analysis There was no evidence of this feature in the imagery that was reviewed. UAVSAR interpretation A moderately well-defined zone of displacement was indicated in the initial UAVSAR data. Lf Post-earthquake observations No field observations were made. Photo analysis The only suggestion of this feature in aerial images was a subtle tonal or vegetation contrast near the border.

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UAVSAR interpretation This lineament had a weak and diffuse expression in the initial UAVSAR data, somewhat west of the photo lineament.

DISCUSSION AND RECOMMENDATIONS Identification of active faults for zoning purposes requires both a reasonably accurate location of the fault such that “its trace is clearly detectable by a trained geologist as a physical feature at or just below the ground surface” and a finding of “evidence of Holocene displacement along one or more of its segments or branches” (Bryant and Hart, 2007). For this evaluation area the fault location is based, first, on GPS-located surface rupture or fault morphology identified in the field. Additional fault locations are derived from previous mapping and interpretation of aerial photography and other remote imagery projected within a GIS platform (MapInfo or Google Earth). Features interpreted in accurately registered imagery were used, in some instances, to relocate previously mapped faults. Much of the surface fault rupture associated with the El Mayor-Cucapah earthquake was observed in the field, but where InSAR lineaments were not field-verified we attempted to verify the fault presence and location based on interpretation of visible geologic structure and fault-related geomorphology in the various aerial imagery available. Faults interpreted and supported by several lines of evidence are recommended for zoning. Activity of unverified InSAR lineaments is judged primarily on expression in the InSAR data. Recency of faulting is judged first on evidence of historic displacement (direct observation and InSAR data) and, in the absence of surface rupture, on a combination of surface expression and inferred structural affinity with known active faults. It was a remarkable observation in this area that most of the faults had some geomorphic expression, most of which gave the appearance of being older, inactive features. Without recent rupture the activity of these faults would not have been recognized.

Faults recommended for zoning under the APEFZ Act are plotted on Plates 2a-2f. Individual judgments relevant to zoning are discussed below. NORTHWEST-TRENDING FAULTS A. Laguna Salada Fault Zone (Coyote Wells & Yuha Basin quadrangles – Plates 2d & 2e) These faults, which define a complex northwest-trending zone, are part of an important active fault zone, transferring slip across the border region between the Elsinore Fault Zone and the Laguna Salada Fault Zone in Baja California. The recent earthquake has demonstrated the role of both strands of the Laguna Salada Fault Zone in this process. Furthermore, the M5.7 aftershock of 14 June 2010 and the related rupture has illuminated a complex zone of faults, both at the surface and at depth, that help transmit slip from the Laguna Salada Fault Zone to the Elsinore Fault Zone. The Ocotillo Fault Zone (area G) appears to be an important element in this structural system. In addition, it has been seen that the western branch of the Laguna Salada Fault Zone merges at its northern end with the northeast-trending left-lateral faults of the Yuha Well Fault Zone through a zone of normal extensional faults.

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Aa. Laguna Salada Fault, east branch – The location of this branch of the Laguna Salada Fault Zone is based primarily on prominent topographic expression and recently expressed surface rupture. Activity is based primarily on the April rupture, where up to 4 cm of dextral strike-slip displacement was documented. The northern extent of the recommended zone is based on aerial photo interpretation, InSAR data and previous mapping (Clark, 1982; Isaac, 1987). Activity of the northern extent is inferred from InSAR data as well as its continuity with the clearly active strand to the south. The more northerly extension mapped by Dibblee is not included as it was not verified in imagery or activity (as indicated by InSAR) and also would appear to be truncated by the Yuha Well Fault. Ab. Laguna Salada Fault, west branch – The multiple strands of this fault were located based on field mapping of recent rupture and the excellent UAVSAR data. Recent activity was verified for almost the entire zone, except for the concealed traces beneath Pinto Wash. The entire fault set is recommended for zoning. Ac. intermediate faults – The faults in this zone are subtle with generally poor geomorphic expression. The locations are based on mapping by Clark (1982) and supported by aerial photo interpretation. Only one of these faults had minor fracturing from the April earthquake and none showed expression of displacement in the InSAR imagery. Nevertheless, there is a remarkable similarity of this group of faults and its association with the Laguna Salada, east branch (Aa) and the Yuha Well Fault (Hf) to the active extensional zone a few kilometers to the west (Hd), and its association with the Laguna Salada, west branch (Ab) and the Vista de Anza Fault (Hc). This close association with demonstrated active faults is considered sufficient to consider them active as well.

Ad. west of west branch – Section Ad is well-defined and considered active based on surface expression and recent rupture on at least the northern part of the fault. Displacement on its southern portion is suggested by the InSAR data. Zoning is recommended

B. Laguna Salada Fault Zone, northwest extension (Painted Gorge & Coyote Wells quadrangles – Plates 2b & 2d) The principal trace (Ba) of this zone is defined by a discontinuous set of aerial photo lineaments which correspond well with a diffuse InSAR lineament. One short well-defined scarp, as well as a less sharp generalized topographic escarpment also mark the feature and support a fault origin for the general trend. Activity is indicated by the InSAR lineament, along with a few scattered observations of minor fracturing. In spite of the weak definition, there are sufficient features to indicate that this fault is likely to rupture the surface in larger or closer earthquakes than the April 2010 event and it is therefore recommended for zoning. Three parallel faults to the east (Bb-Bd) exhibited minor rupture in the recent earthquake and should also be zoned . Fault section Be is identified from mapped surface fracturing within a localized cluster. The stronger northwest trend of faulting within this cluster is suggestive of an association with faults Ad to the southeast, although there is a considerable gap between them. Although Be does not closely correspond

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with any other known or inferred faults* its orientation and systematic pattern suggest fault control and a narrowly-defined zone is recommended. *A weakly defined northeast trend is considered only coincidentally aligned with the probably mislocated “Yuha Wells Fault” of Rockwell et al (1990) [see discussion of fault group H, earlier in this report]. C. Elsinore Fault Zone (Carrizo Mtn & Painted Gorge quadrangles – Plates 2a & 2b) The Elsinore Fault Zone is a recognized active fault in southern California (Jennings & Bryant, 2010). The recent earthquake and particularly its aftershocks have highlighted a more southeasterly extent of the fault zone than has been previously recognized. Sections Ca-Cc are recommended for zoning, extending the existing APEFZ to the southeast. The traces identified here are located based on prior mapping (Christensen, 1957; Clark, 1982; Kahle, 1988) as well as recent surface rupture as identified from InSAR and ground inspection. Some splays (Ca) and connective lineaments are derived from aerial imagery and limited field confirmation. Recency of activity is based on the June 2010 ground rupture and continuity to a recognized active fault zone to the northwest. Northwest-trending lineaments (section Cd) were mapped by Kahle (1988) and Christensen (1957). Only one of these had some weak expression in the post-aftershock InSAR, but no surface rupture was seen along that possible trace. There is not sufficient evidence to indicate that these faults are well-defined or active and they are not recommended for zoning. The existing APEFZ to the northwest, within the Carrizo Mtn. quadrangle, is based on older unpublished mapping of Clark’s, but Clark’s 1982 map is not significantly different for zoning purposes and no changes are recommended except as shown at the southeast end of the fault. D. Other NW-trending faults (Coyote Wells quadrangle – Plate 2d) Da - Davies Valley Fault This fault is defined by a well-expressed valley-margin scarp. The scarps are at least as clear as many of those along recently active faults to the east (Kahle, unpublished notes) and activity is judged based on the scarps and a diffuse but strong InSAR lineament in the valley. An APEFZ is recommended for this fault. Db - fault along range front Elements of this zone were identified by Kahle (1988 and unpublished notes) and are also prominent in aerial imagery (NAIP & USDA). It is marked by both east and west-facing scarps. It is, however, rather discontinuous with no expression on some intermediate age fans, and there were neither observable fractures nor InSAR lineaments along this trend. Zoning is not recommended. E. Other NW-trending faults (Plaster City & Yuha Basin quadrangles – Plates 2c & 2e) Ea - unnamed fault This unnamed fault has discontinuous geomorphic expression and a moderately defined InSAR lineament to provide a reasonable fault location. Activity is indicated by the InSAR signature and supported by drainage anomalies. Zoning is recommended.

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Eb - Dixieland Fault This fault is, as yet, still poorly defined beyond the area adjacent to the road. The cause of historically noted deformation is unclear and zoning is not recommended at this time. Other lineaments to the northwest, from Kahle (1988), are largely on military land and have not been evaluated. NORTHEAST AND NORTH-TRENDING FAULTS F. Painted Gorge Wash Fault (Painted Gorge quadrangle – Plate 2b) The Painted Gorge Wash Fault may play a similar role to that of the Extra Fault and the Elmore Ranch Fault in the transfer of regional strain between major northwest-trending right-lateral faults. The southernmost section of this fault zone (Fa) was identified from InSAR imagery following the 14 June 2010 aftershock and corroborated by surface rupture mapping and aerial photo interpretation. The central section (Fb) is well-defined by scarps and other geomorphic features. Recency of activity is interpreted from youthful scarps and other fault morphology (Kahle, 1988; Morgan & Morgan, 2006; mapping for this evaluation) and additionally inferred from the apparent continuity with active faulting to the south. The northern section (Fc) is well-located based on prior mapping (Kahle, 1988) and aerial photo interpretation. Recency of activity is inferred from the geomorphic expression across younger alluvial fan surfaces and continuity with active faulting to the south. All of the indicated faults are recommended for zoning. G. Ocotillo Fault Zone and related faults, south of the Elsinore Fault (Painted Gorge and Coyote Wells quadrangles – Plates 2b & 2d) The Ocotillo Fault Zone appears to be involved in facilitating slip transfer between the Laguna Salada Fault Zone and the Elsinore Fault Zone. The prominent normal component of displacement in this zone is in accord with its northerly orientation and its role within a short right step and releasing bend between these two major structures. The Ocotillo Fault (Gb) is well-located based on geomorphic expression and prior mapping and its activity was demonstrated by surface rupture following the 14 June 2010 aftershock. A similarly oriented fault to the south (Ge) is located and considered active based on surface rupture following the April 4 earthquake and aftershock sequence. One well-defined parallel scarp to the west is as prominent as scarps associated with other active faults and can be projected across younger fans to a similar scarp 400-500 m to the north. A more northeasterly trending fault strand (Ga) at the northern end of this zone was located based on InSAR and field mapping of surface rupture. Other related fault strands (Gc, Gd & Gf) are located based on aerial photo interpretation, InSAR and limited field verification. Activity is inferred based on the InSAR data and association with related active structures (Ga, Gb & Ge).

Lineaments in area Gg are prominent but are strongly suspect as erosional features because of their alignment with the dominant surface flow direction. The mostly compelling argument for fault origin might be the indication of surface displacement in the UAVSAR data. However, differential settlement of younger fan and channel deposits with respect to older fan remnants might also explain the InSAR expression. Lineament Gh is very poorly defined in the InSAR data and its weak geomorphic expression may be attributable to erosion parallel to bedding. All of the indicated strands, with the exception of Gg and Gh, are recommended for zoning. Other lineaments to the west of Gd did not have indications of recent activity, based on InSAR data and limited field examination.

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H. Yuha Well Fault Zone (Plaster City, Coyote Wells, and Yuha Basin Quadrangles – Plates 2c, 2d & 2e) This zone of faults was largely identified from InSAR image interpretation, with indication therein of moderate to strong offsets. Some associated faults were indicated by prior mapping. This set of faults appears to be conjugate to the Laguna Salada Fault Zone, with which it is intricately enmeshed. Wherever the stronger InSAR lineaments were field checked, surface rupture (primarily left-lateral displacement) was corroborated. Most interpreted faults also have support from aerial photo interpretation and those with multiple lines of supporting evidence are recommended for zoning. There are two principal faults and numerous smaller faults that are recognized here. All faults discussed below are recommended for zoning, except as noted.

Ha1 is reasonably well defined by geomorphology and its activity is indicated by the July UAVSAR data. Another set of short parallel faults (Ha2) is separated from Ha1 by the cross-cutting northwestern extension of the Laguna Salada Fault Zone (Ba). Ha2 is not recommended for zoning, except at its southernmost end. Although based on prior mapping by both Clark (1982) and Kahle (1988), the faults were not shown with high confidence and, if they are present, do not show geomorphic or historic indications of Holocene displacement. The most youthful appearing of these faults was seen at locality H1, where it may be influenced by deformation along the northwestern extension of the Laguna Salada Fault Zone; only this southern part of Ha2 is recommended for zoning. The Vista de Anza Fault (Hc) had nearly continuous rupture where explored, confirming both its location and activity. A probably related west-northwest trending graben appears related to slip and perhaps block rotation associated with this fault. The northern extent of the Vista de Anza Fault is extrapolated from InSAR data. At its southern end it appears to mark the western margin of a zone of extensional and normal-oblique faults (Hd), most of which also had confirmed rupture. A slightly shorter set of en echelon fault strands (Hb) to the northwest of the Vista de Anza Fault is defined by scarps and InSAR and its activity is further indicated by minor surface rupture at the southern end. This fault (Hb) was part of the originally mapped “Yuha Wells” fault. The Yuha Well Fault (Hf & Hg) had confirmed surface rupture where it was inspected and is otherwise reasonably well defined by InSAR data. A set of northeast-trending splay faults in the middle of the zone is not included in these recommendations. Hh is parallel to the Yuha Well Fault and is defined by InSAR, with local corroboration of recent surface rupture. Hi is suggested for zoning because of its strength of geomorphic expression and proximity to other active faults. Section He is located based on prior mapping (Clark, 1982; Kahle, 1988) and aerial photo interpretation (NAIP & USDA). Fault expression includes a degraded west-facing scarp. Recent activity is indicated by a moderate expression in the InSAR data. I. Yuha Basin Faults (Plaster City & Yuha Basin quadrangles – Plates 2c & 2e) The existence of the Shell Beds Fault (Ib) was confirmed in the field at two localities, and features were noted suggestive of recent displacement. It also displayed moderate to strong expression in the InSAR data and was judged on this basis to represent an active fault. Several additional sub-parallel faults (Ia, Ic & Id) are interpreted to have had recent rupture based on the InSAR data. A scarp associated with set Id (on the east) is included due to its geomorphic expression and proximity to other faults that appear to have moved in the recent earthquake. This scarp is better expressed in the aerial imagery than many other faults with documented historic displacement.

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Only some of the faults in set Ie are recommended for zoning, based on geomorphic expression and InSAR data. However, the scarp-like feature on the western side of Ie is most likely related to shoreline erosion of ancient Lake Cahuilla and evidence of co-located faulting is not convincing. Similarly, many scarp-like features along the eastern side of the proposed zone also appear related to shoreline processes. The Yuha Fault (If, Ig & Ih) appears to be a prominent conjugate fault to the Laguna Salada Fault Zone and, along with the Yuha Well Fault Zone and the other Yuha Basin Fault Zones, helps distribute strain northward to the San Jacinto Fault. The fault is well defined and active on the basis of recent faulting as well as geomorphic expression. The recommendation for zoning is extended beyond the recent surface rupture based on aerial photo interpretation and InSAR data. J. “Route 247 fault” (Yuha Basin & Mount Signal Quadrangles – Plates 2e & 2f) This fault is well-located based on aerial photo interpretation. Activity is based on limited ground rupture along parts of its southern extent. A short southwest-trending branch at the north end had modest expression in the UAVSAR data and is on trend with the Yuha Fault. The faults as defined by the data should be zoned. K. Pinto Wash Fault Zone (Yuha Basin and Mount Signal Quadrangles – Plates 2e and 2f) This set of mostly northeast-trending faults is located primarily from InSAR data and locally confirmed in both position and activity by ground observation of recent surface rupture. Two inferred strands (Ka & Kb) were not confirmed as faults, but are recommended for zoning based on their InSAR expression. Kg is somewhat discontinuous, but otherwise forms a rather coherent set of en echelon faults, some confirmed on the ground, some suggested by the UAVSAR data and some suggested by topographic features. Aerial image interpretation took precedence for some fault locations over less sharp InSAR data and the zone as a whole should be zoned. If the recent rupture in this area is the result of bedding plane slip this is a mechanism that can be repeated in future earthquakes and it is considered appropriate to zone these faults. L . Centinela Fault Zone (Mount Signal Quadrangle – Plate 2f) These faults are an apparent extension of a broad zone of active faults branching northward from the Laguna Salada Fault Zone and projecting around and through El Centinela (Mt. Signal). Prominent faulting was mapped along trend south of fault La and more generally south of the other faults (Akciz, pers.comm., 2010). Fault La was defined by an eroded paleoscarp and recent rupture and should be zoned. Fault Lb had only weak expression in the InSAR data, except at its southern end, and was only confirmed in the field south of the border; only the stronger southern lineaments should be zoned. Faults Lc, Ld and Le are indicated by very pronounced InSAR lineaments and confirmed rupture on two of the faults and are recommended for zoning. The northern portion of Lb and lineament Lf were only defined by very diffuse InSAR lineaments and do not have sufficient evidence to zone as surface rupture hazards at this time.

Jerome A. Treiman CEG 1035

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IMAGERY USED Google Earth Google Earth proved to be a very useful source of imagery on which pre-existing faults could be interpreted, and it was a very useful platform on which to do interpretation. Much of the imagery used for the study area was from 2005, 2008 or 2009. It is credited as Digital Globe imagery but some of it appears identical to NAIP imagery that we also used. Resolution of the Google Earth image was generally better than the similar county-wide image that we used with the MapInfo platform. NAIP (National Agriculture Imagery Program) NAIP imagery from 2005 was used for interpretation of fault-related lineaments and topography. We used a county-wide image that was somewhat degraded in resolution but was sufficient to locate features identified in other imagery. The images were viewed within the GIS platform (MapInfo) and interpretation done directly on screen. NASA (National Aeronautics and Space Administration) Synthetic Aperture Radar (SAR) data from NASA’s UAVSAR (Unoccupied Aerial Vehicle SAR) instrument. UAVSAR data acquired under Donnellan et al. EarthScope UAVSAR project. Interferograms processed by Scott Hensley and the UAVSAR team. The below image pairs were portrayed with both 12-cm and 5-cm wrap on the color fringes. The web links below only access the 12-cm wrap. SanAnd_26501_09083-010_10028-000_0174d_s01_L090HH_01 (October 21, 2009 to April 13, 2010) SanAnd_26501_10028-000_10057-100_0079d_s01_L090HH_01 (April 13, 2010 to July 1, 2010) SanAnd_26501_10057-100_10071-001_0089d_s01_L090HH_01 (July 1, 2010 to September 28, 2010) Interferograms available at: http://uavsar.jpl.nasa.gov/kml/RPI/SanAnd_26501_09083-010_10028-000_0174d_s01_L090_01.htm http://uavsar.jpl.nasa.gov/kml/RPI/SanAnd_26501_10028-000_10057-100_0079d_s01_L090_01.htm http://uavsar.jpl.nasa.gov/kml/RPI/SanAnd_26501_10057-100_10071-001_0089d_s01_L090_01.htm U.S. Bureau of Land Management BLM_C_M000 scale 1:10,000 color 9x9 contact prints

flight line 1 frames 227-241 11/27/1972 flight line 2 frames 255-269 11/27/1972 flight line 3 frames 193-206 11/27/1972 flight line 4 frames 167-181 11/27/1972 flight line 5 frames 147-166 11/26/1972 flight line 6 frames 127-146 11/26/1972 flight line 7 frames 107-126 11/26/1972 flight line 8 frames 92-106 11/26/1972 flight line 9 frames 82 - 91 11/26/1972

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U.S. Department of Agriculture, Soil Conservation Service ABN series scale 1:20,000 b/w 9x9 contact prints

flight line 1M frames 14 – 33 3/30/1953 flight line 1M frames 169-187 3/30/1953 flight line 9M frames 92 – 95 4/12/1953 flight line 9M frames 189-193 4/12/1953 flight line 9M frames 197-201 4/12/1953 flight line 10M frames 138-142 4/18/1953 flight line 10M frames 147-151 4/18/1953 flight line 10M frames 199-212 5/21/1953 flight line 11M frames 40 – 55 4/18/1953 flight line 11M frames 56 – 70 4/18/1953 flight line 11M frames 109-112 4/26/1953 flight line 11M frames 212-226 4/26/1953 flight line 12M frames 79 – 86 4/26/1953

REFERENCES Barnard, F.L., 1968, Structural geology of the Sierra de los Cucapas, northeastern Baja California,

Mexico, and Imperial County, California: unpublished PhD thesis, University of Colorado, 154p. Bryant, W.A., and Hart, E.W., 2007, Fault-rupture hazard zones in California: California Geological

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acknowledgements: UAVSAR data was acquired under Donnellan et al. EarthScope UAVSAR project, processed by Scott

Hensley and the UAVSAR team; data courtesy NASA/JPL-Caltech I thank Eric Fielding (JPL/Caltech) for sharing UAVSAR imagery and valuable discussions. I am grateful to George and J.R. Morgan for graciously sharing their field mapping and knowledge of the

Painted Gorge Wash Fault. I offer my sincere thanks to Michael Rymer, Katherine Kendrick, Janis Hernandez, Pam Irvine, George

Morgan, J.R. Morgan and Martin Siem for their collaboration during field mapping.

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Table of Contents Introduction 1 Rupture Synopsis 3 Seismicity 3 Previous Mapping 5 Aerial Photo Interpretation 7 Synthetic Aperture Radar Interferometry 7 Post-Earthquake Field Observations 15 Summary of Faults Evaluated 17 Northwest-trending faults A-E 17 Northeast and north-trending faults F-L 26 Discussion and Recommendations 45 Northwest-trending faults 45 Northeast and north-trending faults 48 Imagery Used 51 References 52 Acknowledgements 54

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List of Figures and Plates Figure 1 – index map to study area p.1 Figure 2 – index map showing faults and quadrangles with recent rupture p.2 Figure 3a,b – seismicity, three intervals shown on two frames p.4 Figure 4 - previous mapping overview p.6 Figure 5a-b -- April InSAR; April detail p.8 Figure 5c -- July InSAR p.9 Figure 6a-f -- UAVSAR lineaments pp.10-14 Figure 7 – areas visited p.15 Figure 8 -- fault sets p.16 Figure 9 -- mapping by Clark p.18 Figure 10a,b – Painted Gorge Wash Fault p.27 Figure 11 -- Shell Beds Fault p.36 Figure 12 a,b – Shell Beds Fault p.37 Plate 1a-f - aerial photo interpretation, InSAR, previous mapping, field observations, selected slip measurements Plate 2a-f – recommended zoning