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Reassessment of the 1892 Laguna Salada Earthquake: Fault

Kinematics and Rupture Patterns

by T. K. Rockwell, J. M. Fletcher, O. J. Teran, A. P. Hernandez, K. J. Mueller,J. B. Salisbury, S. O. Akciz, and P. Štěpančíková

Abstract We present the results of intensive field investigations of the scarpassociated with the 23 February 1892 earthquake in northern Baja California. Newlyrecognized additional offsets suggest the rupture was about 58 km in length, twice aslong as previous estimates. Slip produced in the 1892 event varied from purely dextralslip near the international border to roughly 1:1 oblique-normal slip farther south alongthe 2–4-km-deep portion of the Laguna Salada basin. The portion of the 1892 rupturewith oblique-normal slip comprises a number of short, poorly organized, and discon-tinuous fault scarps with heights that vary in concert with their strike. Slip was linkedfarther south to a short, purely normal fault that forms a large releasing bend at thesouthern termination of the fault zone. Given the distribution of slip along the earth-quake and a likely range of locking depths, we conclude the 1892 earthquake was be-tweenMw 7.1–7.3 in magnitude, consistent with previous estimates from macroseismicobservations. The length of the Laguna Salada fault that ruptured in 1892 also accom-modated minor normal sense displacement along much of its length in the recent 2010Mw 7.2 El Mayor–Cucapah earthquake, which guided the remapping effort.

Online Material: Table of displacement measurements with uncertainty, location,waypoint number, soil unit designation, and the strike, dip, and type of feature measured.

Introduction

The surface rupture for the 23 February 1892 earthquakewas first recognized and mapped by Mueller and Rockwell(1991, 1995) along the western flank of the Sierra Cucapahin Baja California (Figs. 1 and 2). In their study, they mapped22 km of rupture from Cañon Rojo northwest to the PasoInferior accommodation zone (PIAZ), which was later recog-nized as a structurally complex zone and named followingthe 2010 Mw 7.2 El Mayor–Cucapah earthquake (Fletcher,Rockwell, Teran, et al., 2010; Oskin et al., 2012; Fletcheret al., 2014; Teran et al., 2015). The maximum displacementsdocumented for the 1892 rupture exceeded 5 m of oblique slip(Mueller and Rockwell, 1995), which is larger than expectedfor a 22-km-long rupture, given the shallow depth to the baseof the seismogenic crust in this region and typical values fromdisplacement-length regressions. Hough and Elliot (2004) es-timated the magnitude of the 1892 earthquake as Mw 7.2,which is also larger than would be expected for a 22-km-longrupture but consistent with the observed large displacementsbased on scaling relationships (Wells and Coppersmith, 1994).Minor secondary rupture produced during the 2010 earth-quake occurred along most of the Laguna Salada fault fromCañon Rojo to as far north as the international border (Fletcheret al., 2014). During the field mapping of the 2010 surface

rupture, we recognized previously unmapped scarps that wenow attribute to the 1892 rupture, some of which rerupturedwith significant slip in the PIAZ in 2010 (Fig. 2). Furthermore,north of the accommodation zone, we identified minor slipsuperposed on major youthful rupture as far north as theinternational border (Figs. 3–5). Comparison of the degreeof scarp degradation and cross-cutting relationships is similarto that along the Laguna Salada fault to the south and indicatesthat these northern scarps are likely of the same age as thosedocumented by Mueller and Rockwell (1995). This extendsthe known rupture length of the 1892 earthquake considerablyfarther to the north for a total of at least 55 km. It is clear inhindsight that the distributed nature of slip, rapid subsidence,and recent sedimentation in the accommodation zone erasedevidence and led to the false impression that the 1892 slip diedout at the surface in this region.

In this article, we present new mapping along theLaguna Salada fault conducted after the 2010 El Mayor–Cucapah earthquake, using Google Earth Pro imagery as abase map. We made hundreds of new field measurementson displaced channel walls, alluvial bars, channel thalwegs,and other linear features to compile a more complete recordof rupture kinematics for the 1892 earthquake. We also

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Bulletin of the Seismological Society of America, Vol. 105, No. 6, pp. –, December 2015, doi: 10.1785/0120140274

present the displacement field for the minor rerupture of theLaguna Salada fault that resulted from the 2010 earthquake.We calculate the moment release for the 1892 earthquake,based on the new mapping, and compare it with the new es-timate of magnitude determined from historically reporteddamage (Hough and Elliot, 2004).

The Laguna Salada Fault

The Laguna Salada fault and related faults to the east thatruptured in the 1892 and 2010 earthquakes comprise a zone ofoblique transtension that is the southern continuation of theElsinore fault zone into the Gulf Extensional Province (Mu-eller and Rockwell, 1995; Fletcher, Rockwell, Hudnut, et al.,2010; Fletcher et al., 2014). The Laguna Salada fault is themain basin-bounding structure for the northern half of the La-guna Salada rift basin, which is filled by up to 4 km of Plio-Quaternary sediment (D. Kelm, unpublished thesis, 1971).Slip on the Laguna Salada fault accommodates relative upliftof the Sierra Cucapah, a basement-cored range that bounds thewestern margin of Mexicali Valley (Fig. 1). Total displacementis not well constrained, but the Laguna Salada fault must ac-commodate significant vertical offset because the sedimentaryfill in its hanging wall reaches 4 km in thickness (García-Ab-deslem et al., 2001; Fletcher and Spelz, 2009). Near thesouthern limit of the Laguna Salada fault where it crossesthe block of uplifted crystalline basement, it juxtaposes differ-

ent rock packages (metasedimentary rocks to the south withmeta-igneous rocks to the north) over a strike length of∼11 km, which thus represents a minimum amount of dextralstrike-slip displacement. The metasedimentary rocks to thesouth of the Laguna Salada fault exist in the footwall of theCañada David detachment, which has accommodated 5–7 kmof vertical exhumation (Axen et al., 2000). Moreover, the14 km of total dip slip accommodated by the Cañada Daviddetachment (Fletcher and Spelz, 2009) should be similar to thetotal slip accommodated by the Laguna Salada fault if a long-term linkage of the two faults has been maintained. However,it is possible that other ruptures with distinct strike-slipkinematics may have also occurred, which would have signifi-cantly increased the net amount of strike slip relative to west-down dip slip (Mueller and Rockwell, 1991).

Our new estimates of total displacement on the LagunaSalada fault in excess of 11 km is much greater than the 2.5–5 km of lateral offset observed on the southern Elsinore fault(Lampe et al., 1988; Magistrale and Rockwell, 1996). Thisimplies a northward propagation of the fault and/or significantslip transfer to structures in the Yuha Desert south of the Elsi-nore fault. A fault strand that bounds the margin of the SierraCucapah in the southern portion of the region mapped byMueller and Rockwell (1991, 1995) is a likely candidate for anearlier history of more purely dextral slip based on the geomor-phology of the range front and its near-vertical geometry.

The slip rate on the Laguna Salada fault is estimated atabout 3 mm=yr, based on displaced Holocene alluvial fandeposits that have ages inferred from development of a soilchronosequence (Mueller and Rockwell, 1991, 1995). This isconsistent with geodetic estimates of 3 mm=yr (Savage et al.,1994) of right-lateral shear that are based primarily on trilat-eration data.

The 1892 Earthquake

The 23 February 1892 earthquake is notable because it isthe largest historical earthquake to have produced strongground shaking in southernmost California and northernBaja California (Toppozada et al., 1981) prior to the 2010earthquake. The earthquake was felt and reported fromSan Quintin in Baja California to Tulare in central California(a distance of approximately 700 km) and produced modifiedMercalli intensity (MMI) VI–VII damage in San Diego, withhigher intensities recorded in the Imperial and MexicaliValley region (C. Strand, unpublished thesis, 1980; Toppo-zada et al., 1981; Hough and Elliot, 2004). Chimneys weretoppled in San Diego (MMI VI–VII) with complete destruc-tion of adobe structures at Carrizo station (MMI VIII–IX).From the reported damage, the magnitude of this earthquakewas estimated at Mw 7.8 by C. Strand (unpublished thesis,1980) and Mw 7.2 by Hough and Elliot (2004).

The epicenter was estimated to have been along theLaguna Salada fault (C. Strand, unpublished thesis, 1980)and near Jacumba, based on observations of ground fissuringand liquefaction in that area (Toppozada et al., 1981). Top-

Figure 1. The Laguna Salada fault in northern Baja California.Note the location of Figure 2 and the portion of the Laguna Saladafault inferred by Mueller and Rockwell (1995) to have ruptured.Also, note the locations of places mentioned in the text, includingSan Diego (SD), Yuma (Y), Tulare (T), San Quintin (SQ) in theinset and Jacumba and Carrizo Station on the main figure. The colorversion of this figure is available only in the electronic edition.

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pozada did not believe the epicenter occurred on the LagunaSalada fault because of the lack of aftershocks reportednearby from Yuma, compared to the many reports from SanDiego, although he noted that attenuation through the thickfill of Mexicali Valley could partially explain the difference.Soon after these estimates on the location of the 1892 earth-quake were made, K. Mueller (unpublished thesis, 1984)mapped a very young 22-km-long rupture on the Laguna Sal-ada fault from Cañon Rojo to the northwest, which he attrib-uted to the 1892 earthquake (Mueller and Rockwell, 1991,1995). The 1892 earthquake is a reasonable choice for thesource of the scarps along the Laguna Salada fault, not onlybecause of the damage pattern interpreted from historical in-tensity estimates, but because free faces in alluvial depositsand planar bedrock scarps are well preserved along a signifi-cant length of the fault, which indicate a very recent earth-quake in the historical period (Fig. 6).

Mueller and Rockwell (1995) mapped on 1:50,000-scaleblack-and-white aerial photographs in which details of therupture were difficult to resolve, and no rupture was clear inthe aerial images to the north of the PIAZ. However, with theadvent of modern imagery, in particular the imagery em-bedded in Google Earth and a Light Detection and Ranging(lidar) survey conducted in 2010 following the El Mayor–Cucapah earthquake (Oskin et al., 2012), it is now possibleto map broad regions at high resolution and search for veryyoung scarps and offsets, as we did here for the entire lengthof the Laguna Salada fault (Figs. 3–5).

Field Methods

The 1892 rupture is clearly visible in Google Earthimagery (Fig. 3), at a resolution that pointed the way for de-tailed mapping of fresh-looking scarps, offset channels, andother indicators of young rupture prior to actual field map-ping and measurements of offset features. Location pins inGoogle Earth Pro were placed along the fault and enlargedimagery was printed at a scale that would allow easy map-ping in the field. Using the images as a base map, we tookfield measurements of every offset feature that could belocated by walking the rupture from the PIAZ northward tothe international border, as well as selected sections farthersouth. Locating the fault was typically quite simple, even inareas of active sedimentation, as most of the northern Laguna

Salada fault sustained minor (2–3 cm, and locally as much as40 cm) slip in the 2010 earthquake (Fletcher et al., 2014).This allowed for a high degree of confidence that deflectionsand offsets were collocated on the actual fault trace.

All measurements were made with a steel tape by 3–4geologists, such that the piercing points and uncertainty weredebated for every measurement. The locations of the meas-urement points were surveyed with Global Positioning Sys-tems to a resolution of a few meters; and, when combinedwith the imagery, allowed for a very precise and detailedmap of the 1892 rupture. Ⓔ The 1892 surface displacementdata are presented in Table S1, available in the electronicsupplement to this article.

Results of the New Mapping

The 1892 earthquake activated slip on two main faultsthat intersect near the southern end of the rupture (Muellerand Rockwell, 1995). The Cañon Rojo fault strikes north-northeast and ends against the northwest-striking LagunaSalada fault. The Cañon Rojo fault is likely an active strandof the Cañada David detachment, which forms a similar T-shaped intersection with the Laguna Salada fault located10 km to the southeast (Fletcher and Spelz, 2009). The La-guna Salada fault is composed of two strands that are sub-parallel in strike, which varies from 310° to 320° along mostof its length but gradually bends to the west (290°) near theinternational border. In general, the Laguna Salada faultstrands are also subparallel in dip, which generally variesfrom 60° to 90° to the southwest (Mueller and Rockwell,1991, 1995), but there are sections where the basal strandsdip less steeply and intersect steeper fault strands formingshallow flower structures at depth. In addition, some of thesteep segments of the Laguna Salada fault were observed totilt to the opposite direction and dip 80° to the northeast.Regardless of the changes in polarity of dip direction, theLaguna Salada fault accommodates southwest-side-downdip slip. Everywhere along its length, the northeast strandjuxtaposes Mesozoic crystalline basement with synrift basinfill. The southwest strand is located within the sedimentarysequence and commonly uplifts Holocene fan surfaces in itsfootwall. The across-strike spacing between the strands istypically 400–800 m. The 1892 rupture typically passesrepeatedly from one strand to another, but the activated

Figure 2. Detail of area shown in Figure 1 where the 1892 rupture (thick bold line) was reported by Mueller and Rockwell (1995). Otherfaults (thin lines) were also mapped by Mueller and Rockwell but are not considered as part of the 1892 rupture. Faults in bold black (mediumthickness lines) ruptured in 1892 but were mapped after the 2010 earthquake. Locations of Figure 3b–d are shown. The color version of thisfigure is available only in the electronic edition.

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sections generally have little overlap. The trace of the south-west strand becomes buried along sections where the mostrecent consecutive ruptures (1892 plus the penultimate event)have been localized on the northeast strand.

Figure 7 shows the strands of the Laguna Salada fault,along which there is evidence of young rupture that is likely

attributable to the 1892 earthquake. For evidence of involve-ment in the 1892 earthquake, we used observations of over-steepened scarps and free faces and the offset of very lateHolocene rills and channel deposits (i.e., those with essen-tially no soil development that corresponds to the Q2 depos-its of Mueller and Rockwell, 1995).

Figure 4. Photo of 1892 rupture in the field, between the PIAZ and Mexican Highway 2. The color version of this figure is available onlyin the electronic edition.

Figure 3. Oblique aerial images from Google Earth Pro of the most recent rupture trace of the Laguna Salada fault (LSF; imagery takenprior to the 2010 earthquake). (a) The fault location shown in Figure 7, north of the Paso Inferior accommodation zone (PIAZ), exhibits thesame degree of expression and scarp degradation as the scarps south of the PIAZ, indicating that they are the same age. (b)–(d) Locationsmapped by Mueller and Rockwell (1995), shown in Figures 2 and 7. The color version of this figure is available only in the electronic edition.

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Lateral offsets along the southern several kilometerswere difficult to resolve for two reasons. First, the verticaldisplacement was so large (several meters) that it could bedifficult to resolve evidence for lateral slip. Second, therewere no piercing points that we could confidently correlateacross the high vertical scarps in the alluvial fan deposits;and, south of fan 3 of Mueller and Rockwell (1995), allscarps are in alluvium until the Cañon Rojo area.

The vertical displacement in 1892 was also difficult tomeasure, but it is large along the southern part of the rupture.The main problem is that vertical displacements of several me-ters were superposed on pre-existing Holocene scarps suchthat some scarps now measure nearly 20 m. In some cases,as in Figure 6, free faces that may be due only to the 1892earthquake are preserved that are 2–3 m high, and Muellerand Rockwell (1995) determined about 5 m of dip slip onthe Cañon Rojo and Laguna Salada faults at their intersectionnear Cañon Rojo. This stretch of the fault borders the deepestpart of the basin fill (D. Kelm, unpublished thesis, 1971), con-sistent with the largest dip-slip motion occurring here in 1892and previous earthquakes. This interpretation is consistentwith the observed free faces and suggests that on the orderof 4–5 m of vertical displacement occurred along the southernpart of the 1892 rupture, with vertical slip increasing to thecorner of the pull-apart basin at Cañon Rojo (Mueller andRockwell, 1995).

At Cañon Rojo, Mueller and Rockwell (1995) docu-mented 90° rakes for striae on the Cañon Rojo fault itself,with oblique mullions on the southernmost part of the La-guna Salada fault involved in the 1892 rupture. These kin-ematic indicators of slip have similar orientations, indicatinga consistent slip vector, which also coincides with the inter-section of the two faults. The 1892 scarp on the Cañon Rojofault is about 5–5.5 m in height, consistent with 2.8–3.5 m ofright-lateral slip on the Laguna Salada fault (Fig. 8).

At the surface, the ratio of lateral to vertical coseismicslip on the Laguna Salada fault is subparallel to and thuslikely controlled by its intersection with the steeply dipping(∼60°) Cañon Rojo fault (Fig. 8). Therefore, the northwardincrease in the ratio of lateral to vertical slip as shown inFigure 8 may indicate a listric shallowing of the Cañon Rojofault with depth. This is consistent with the fault geometryproposed by Fletcher and Spelz (2009), albeit with a moresubtle listric bend in the lower seismogenic zone, and sug-gests that much of the 1892 rupture could have been accom-modated by slip on deeper portions of the Cañada Daviddetachment.

North of the PIAZ, vertical slip on the Laguna Saladafault slowly dies out to the northwest until it is nearly zeroin the vicinity of Mexican Highway 2. This pattern is differ-ent than the 2010 rupture, where vertical displacement on thePaso Superior fault is expressed northward to nearly theinternational border (Fletcher et al., 2014). For the 1892 rup-ture of the Laguna Salada fault, vertical slip is primarilylocalized only adjacent to the modern lacustrine depressionof Laguna Salada itself, demonstrating that the basin is kin-ematically tied to the fault and is formed by repeated obliqueslip. Offsets measured along the northern Laguna Saladafault immediately south of the U.S. border were essentiallystrike slip, with a variable sense of vertical separation. Thetransition in kinematics coincides with a prominent left stepin the Laguna Salada fault and a westward bending of thefault such that strike-slip-dominated strands north of High-way 2 have a more westerly strike. This change in kinematicswith fault orientation is consistent with regional transtensionof the plate margin, which was also observed in the 2010rupture (Fletcher et al., 2014).

Figure 8 shows the magnitude of horizontal and verticaldisplacements measured along the 1892 rupture trace. Notethe absence of much data for strike slip in the south due to the

Figure 5. Lateral offset of a small rill along the northern LagunaSalada fault attributed to the 1892 earthquake. The 2010 crackingand minor rupture was coincident with the 1892 trace. The colorversion of this figure is available only in the electronic edition.

Figure 6. Scarp along the Laguna Salada fault crossing Holocenealluvial fan deposits with a clearly defined free face in unconsolidatedgravel (Mueller and Rockwell, 1995). The expression of the free facein gravel indicates that this is a very young rupture, with the 1892earthquake the only reasonable historical candidate. The color versionof this figure is available only in the electronic edition.

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large vertical scarps in unconsolidated alluvium or alongsections of fault that were eroded by the historical highstands of large ephemeral lakes in Laguna Salada. Slip diesnorthward toward the border, where we measured lateraldisplacements of 1–1.5 m (Fig. 5), and we used limited ob-servations of deflected rills and gullies north of the bordermade by S. Isaac (unpublished thesis, 1987) in the YuhaDesert to estimate a half-meter of slip in that area. In all,we estimate that the 1892 earthquake ruptured for a distanceof about 58 km and produced oblique dextral-normal slipwith systematically varying ratios of dip to strike slip, de-pending on local fault strike. A strong correlation betweenslip direction and fault orientation was also observed in the2010 surface rupture, which provides another exampledemonstrating that multiple fault sets are activated in singleearthquakes to accommodate the 3D strain of oblique ex-tension in the northern Gulf Extensional Province (Fletcheret al., 2014).

Displacement Associated with the 2010 Earthquakealong the Laguna Salada Fault

The same strands of the Laguna Salada fault that accom-modated slip in the 1892 rupture were ubiquitously observedto have been reactivated in 2010. However, unlike the 1892rupture trace, no part of the Cañon Rojo fault was reacti-vated. Moreover, 2010 faulting and fracturing extends to thesouthern extreme of the Laguna Salada fault (Fletcher et al.,2014), which is ∼20 km beyond the southern limit of the1892 rupture. Although the 2010 rupture utilized the samestrands of the Laguna Salada fault system as the 1892 rup-ture, the magnitude and sense of slip was distinctly differentin the two events.

In the PIAZ, the magnitude of the 2010 reactivationreaches 127 cm, which is up to an order of magnitude greaterthan the reactivated slip on other sections of the LagunaSalada fault to the north and south (Fig. 9; Fletcher et al.,

Figure 8. Slip distribution estimated for the 1892 earthquake along the Laguna Salada fault. The zero point marks the northern end of thefault in the Yuha Desert. The color version of this figure is available only in the electronic edition.

Figure 7. Map of strands that likely ruptured in the 1892 earthquake (bolded thick faults). Observations south of the border are from thisstudy, with additional observations from the Yuha Desert north of the border from S. Isaac (unpublished thesis, 1987). Other strands may alsohave been involved in 1892 but no longer have sufficient expression to be recognized as a young rupture. Locations of Figure 3 panels areindicated (see Ⓔ GoogleEarth KML file, available in the electronic supplement to this article). The color version of this figure is availableonly in the electronic edition.

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2014). In this area, 2010 displacement is typically located onshort isolated scarps that show very little evidence for offset inthe penultimate event at the surface. However, due to rapidsubsidence and sedimentation in this area, both the LagunaSalada fault and scarps of the 1892 surface rupture have beenburied, and evidence for the 1892 slip is generally only ob-served in trench walls that were excavated to study the faultinghistory (Hernandez Flores, 2015). Thus, it is likely that 1892slip was similar in magnitude to that observed along well-ex-posed portions of the Laguna Salada fault to the north andsouth (Fig. 8). Outside of the PIAZ, 2010 reactivation is gen-erally characterized by fracture arrays that exhibit minor totaloffset <10 cm. Slip magnitudes reach 40 cm south of thePIAZ (Fig. 9); and, as slip magnitude increases, it becomesfocused onto a single continuous fault that is collocated withthe middle of older 1892 scarps.

Although slip associated with both earthquakes had anoverall oblique dextral-normal shear sense, the component ofthe 2010 slip that was partitioned onto the Laguna Saladafault was overwhelmingly dominated by the normal senseof west-down dip slip. In general, all of the lateral componentand most of the normal dip-slip component of the 2010 rup-ture was accommodated on faults to the east of the LagunaSalada fault, typically with an order of magnitude greateramount of coseismic slip (Fletcher et al., 2014).

Re-Evaluation of Moment Magnitude

Our new mapping extends the length of the knownrupture associated with the 1892 Laguna Salada earthquaketo between 55 and 60 km, with about 53 km of rupture on theLaguna Salada fault itself and an additional 5 km of normalrupture along the Cañon Rojo fault (Fig. 8; Mueller andRockwell, 1995). Vertical displacement reaches a maximumvalue near the intersection of the Laguna Salada and CañonRojo faults, whereas lateral displacement appears to be larg-est in the south-central part of the rupture. Figure 10 shows aslip model with which we estimate the moment release in the1892 earthquake to compare to the historical estimates of thesize of this earthquake. For simplicity, we estimate the aver-age horizontal and vertical displacement along a section offault to calculate total oblique slip for the estimation ofmoment release. For this model, we use a range of lockingdepths of 12� 2 km (10–14 km) based on the maximumdepth of local seismicity (Hauksson et al., 2010; Wei et al.,2011) and apply the standard equation for momentM0, whichis the product of rupture area, average displacement, and shearrigidity of the crust, assumed to be 3:1 × 1011 dyn·cm2

(Hanks and Kanamori, 1979).A 12 km locking depth on a 60°–90° dipping fault

yields a fault width of about 12–14 km; and, using the

Figure 9. (a) Map of the surface rupture associated with the 1892 and 2010 earthquakes. (b) Vertical displacement along the LagunaSalada fault associated with rupture during the 2010 El Mayor–Cucapah earthquake (modified from Fletcher et al., 2014; seeⒺGoogleEarthKML file). (CRF, Cañon Rojo fault; LSF, Laguna Salada fault; PIAZ, Paso Inferior accommodation zone.) The color version of this figure isavailable only in the electronic edition.

Figure 10. Slip model for the 1892 earthquake based on the remapping and new measurements of displacement that we attribute to thatearthquake. Box displacements are used for simplicity. The color version of this figure is available only in the electronic edition.

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slip model in Figure 9, we estimate the moment to be6:65–7:65 × 1026 dyn·cm, which corresponds to an Mw 7.2earthquake. Varying the locking depth by an additional cou-ple kilometers (�2 km) suggests that the likely magnitude forthe 1892 earthquake falls in the 7.1–7.3 range. This magnitudevalue is consistent with the reanalysis of historical felt reportsby Hough and Elliot (2004), who estimated the magnitude atMw 7.2, but it is considerably smaller than the Mw 7.8 esti-mated by C. Strand (unpublished thesis, 1980).

Comparison of the damage and felt effects between the1892 and 2010 earthquakes (Fig. 11) also suggests that theywere of similar magnitude. For this comparison, we apply thecorrection-curve approach of Hough (2014) to the estimated1892 intensities from Hough and Elliot (2004) so that thetraditional (media-based) intensities for the 1892 earthquakecan be compared to the “Did You Feel It?” (DYFI) intensitiesfor the 2010 event. Overall, the corrected intensities for the1892 event overlay the distribution of DYFI intensities forthe 2010 earthquake, with perhaps some tendency towardhigher values for the 1892 event at distances of 100–300 km.It is interesting to note that the 2010 rupture was nearly twiceas long as 1892 but that the 1892 rupture exhibits greater dis-placement, suggesting the 1892 earthquake might have had ahigher stress drop than the 2010 event. Although speculative,this suggestion is consistent with the slight tendency towardhigher intensities for the 1892 earthquake, because, as dis-cussed by Boore (1983), intensities depend quite strongly onstress drop.

Conclusions

Reassessment of the extent of rupture in the 1892Laguna Salada earthquake through new mapping followingthe 2010 El Mayor–Cucapah earthquake has led to recogni-

tion of up to 58 km of surface rupture, with maximum strikeslip of about 4 m (Mueller and Rockwell, 1995). The newwork is congruent with recent reassessments of the damageand felt reports for this earthquake (Hough and Elliot, 2004),which suggest that the magnitude was in the Mw 7.1–7.3range. The widespread reactivation of the 1892 rupture dur-ing the 2010 earthquake demonstrates the close linkage be-tween these faults and suggests that these and other faults inthe Sierra Cucapah should be considered as parts of a linkedsystem that work together to accommodate oblique extensionin the northern Gulf of California structural province.

Data and Resources

All field data used in this article were collected by theauthors. Ⓔ Table S1 (available in the electronic supplementto this article), containing the 1892 surface displacementdata, was compiled at Centro de Investigación Científica yde Educació n Superior de Ensenada (CICESE) and is madeavailable along with detailed mapping of the rupture traces asseparate layers in aⒺGoogle Earth KML file (.kml file). The2010 surface displacement data published by Fletcher et al.(2014) are accessible at http://dx.doi.org/10.1130/GES00933.S4, and the mapping of the 2010 rupture by Teran et al. (2015)can be obtained from http://dx.doi.org/10.1130/GES01078.S1.The “Did You Feel It” data are from the U.S. Geological Sur-vey’s “Did You Feel It” website, and all other data are fromcited papers. Unpublished theses by D. Kelm (1971), C. Strand(1980), K. Mueller (1984), and S. Isaac (1987) are on file at theSan Diego State University Library.

Acknowledgments

We are particularly grateful to Susan Hough, who contributed to thispaper but was unable to be a coauthor. We deeply thank Rosario Garcia Gon-zalez, an elder of the Cucapah tribe, for granting access to the entire SierraCucapah for this study. We also thank Roman Manjarrez, David Lynch, andAnand Pandey for assistance in the field. Mike Oskin, Rich Koehler, RyanGold, Kate Scharer, and an anonymous reviewer provided excellent reviews,which substantially improved the presentation of these data.

This work was supported by CONACYT Grant 239818 to JohnFletcher. This research was also supported by a Southern California Earth-quake Center (SCEC) grant to Fletcher and Rockwell, and travel support forPetra Štěpančíková was provided by the CzechMinistry of Education, Youthand Sports Project Number LH12078 and the long-term conceptual develop-ment research organization RVO: 67985891. SCEC is funded by NationalScience Foundation (NSF) Cooperative Agreement EAR-1033462 and U.S.Geological Survey (USGS) Cooperative Agreement G12AC20038. TheSCEC Contribution Number for this article is 1961.

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Boore, D. M. (1983). Stochastic simulation of high-frequency groundmotions based on seismological models of radiated spectra, Bull.Seismol. Soc. Am. 73, 1865–1894.

Figure 11. Intensities for the 2010 Baja earthquake from theU.S. Geological Survey’s “Did You Feel It?” (DYFI) system (smalldots) are shown together with the intensities assigned by Hough andElliot (2004) for the 1892 earthquake (large circles). The lattervalues are adjusted using the correction-curve approach of Hough(2014) so that the traditional intensity values can be compared withDYFI intensities. The color version of this figure is available only inthe electronic edition.

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Fletcher, J. M., and R. M. Spelz (2009). Patterns of Quaternary deformationand rupture propagation associated with an active low-angle normalfault, Laguna Salada, Mexico: Evidence of a rolling hinge? Geosphere5, no. 4, 385–407.

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Geological SciencesSan Diego State UniversitySan Diego, California 92182trockwell@mail.sdsu.edu

(T.K.R.)

Departamento de GeologíaCentro de Investigación Científica y de Educació n Superior de Ensenada(CICESE)Carretera Ensenada Tijuana No. 3918Zona PlayitasC.P. 22860 Ensenada, Baja California, Mexico

(J.M.F., O.J.T., A.P.H.)

Geological SciencesUniversity of ColoradoBoulder, Colorado 80309

(K.J.M.)

School of Earth and Space ExplorationArizona State UniversityP.O. Box 876004Tempe, Arizona 85287-6004

(J.B.S.)

University of California, Los AngelesDepartment of Earth, Planetary, and Space SciencesLos Angeles, California 90095

(S.O.A.)

Academy of Sciences of the Czech RepublicInstitute of Rock Structure and MechanicsPrague, Czech Republic 18209

(P.Š.)

Manuscript received 3 September 2015;Published Online 20 October 2015

Reassessment of the 1892 Laguna Salada Earthquake: Fault Kinematics and Rupture Patterns 9

BSSA Early Edition