PUBLICATIONS - Stanford UniversityPeople experienced intensities of MMI VI and MMI V over areas of approximately 170km2 and 2200km2, respectively. Since the earthquake occurred at

The 17 May 2012M4.8 earthquake near Timpson,East Texas: An event possibly triggeredby fluid injectionCliff Frohlich1, William Ellsworth2, Wesley A. Brown3, Michael Brunt4, Jim Luetgert2,Tim MacDonald2, and Steve Walter2

1Institute for Geophysics, University of Texas at Austin, Austin, Texas, USA, 2Earthquake Science Center, U.S. GeologicalSurvey, Menlo Park, California, USA, 3Department of Geology, Stephen F. Austin State University, Nacogdoches, Texas, USA,4Eagle Pass High School, Eagle Pass, Texas, USA

Abstract This study summarizes our investigation of the 17 May 2012 MW-RMT4.8 earthquake nearTimpson, Texas, the largest earthquake recorded historically in eastern Texas. To identify preshocks andaftershocks of the 17 May event we examined the arrivals recorded at Nacogdoches (NATX) 30 km from the17 May epicenter, at nearby USArray Transportable Array stations, and at eight temporary stations deployedbetween 26 May 2012 and mid-2013. At NATX we identified seven preshocks, the earliest occurring in April2008. Reliably located aftershocks recorded by the temporary stations lie along a 6 km long NW-SE lineartrend corresponding to a previously mapped basement fault that extends across the highest-intensity (MMIVII) region of the 17 May main shock. Earthquakes in this sequence are relatively shallow—with focal depthsranging from 1.6 to 4.6 km. Evidence supporting these depths include: hypocentral locations of exceptionallywell-recorded aftershocks, S-P intervals at the nearest stations, and comparisons of synthetics and observedseismograms. Within 3 km of the linear trend of aftershock activity there are two Class II injection disposalwells injecting at 1.9 km depth beginning in August 2006 and February 2007, with injection rates averaging42,750m3/mo and 15,600m3/mo, respectively. Several observations support the hypothesis that fluidinjection triggered the Timpson sequence: well-located epicenters are situated near a mapped basementfault and near high-volume injection wells, focal depths are at or below the depths of injection, and theearliest preshock (April 2008) occurred after the onset of injection in 2006.

1. Introduction

On 17 May at 0812 UTC a MW-RMT4.8 earthquake occurred near Timpson, Texas. The quake awoke numerousresidents of Nacogdoches, Texas, 50 km to the southwest of Timpson and caused significant damage tochimneys, fireplaces, and brick veneer siding 5 km southwest of Timpson. The 17 May earthquake is thelargest earthquake in the historical record in East Texas (Figure 1). This paper discusses this earthquake andthe sequence of preshocks and aftershocks, including an MW-RMT4.0 foreshock on 10 May 2012 at 1515 UTCand aftershocks occurring on 25 January 2013 at 701 UTC (mbLg4.1) and 2 September 2013 at 1652 (mbLg4.1)and 1851 (MW-RMT4.3).

Regional tectonics in eastern Texas is dominated predominately by salt bodies; however, the Mt. Enterprisefault zone, a system of approximately east–west trending Cretaceous-Paleogene faults, is situated north andwest of the epicentral area (Figure 1) [see Ewing, 1990]. In the epicentral area of the 2012 earthquake Jackson[1982] and Geomap Company [2012] also indicate a northwest-southeast trending fault that is roughly par-allel to and slightly east of the Rusk-Shelby county line.

Seismicity was rare in this region prior to the events analyzed in this study. The nearest events discussed byFrohlich and Davis [2002] were a M4.0 8 January 1891 Rusk, Texas, event, 80 km to the west of the epicentralregion and the M3.0 9 June 1981 Center, Texas, earthquake, 25 km to the southeast. However, some inves-tigations have suggested the 1891 Rusk event might be spurious—a thunderstorm or a tornado—and the1981 Center earthquake was only locatable because it was recorded by a temporary local network deployedbetween June 1981 and August 1982 [Pennington and Carlson, 1984]. This network also recorded a micro-earthquake occurring on 11 December 1981 and located 25 km west of the 2012 epicenter.

FROHLICH ET AL. ©2014. American Geophysical Union. All Rights Reserved. 1

PUBLICATIONSJournal of Geophysical Research: Solid Earth

RESEARCH ARTICLE10.1002/2013JB010755

Key Points:• The 17 May 2012 earthquake is thelargest recorded historically in east Texas

• Its best-located aftershocks lay along aplane and had depths of 1.6-4.6 km

• Fluid injection at nearby disposal wellsprobably triggered these earthquakes

Supporting Information:• Readme• Figure S1• Table S1• Table S2• Table S3

Correspondence to:C. Frohlich,[email protected]

Citation:Frohlich, C., W. Ellsworth, W. A. Brown,M. Brunt, J. Luetgert, T. MacDonald, andS. Walter (2014), The 17 May 2012M4.8earthquake near Timpson, East Texas:An event possibly triggered by fluid in-jection, J. Geophys. Res. Solid Earth, 119,doi:10.1002/2013JB010755.

Received 4 OCT 2013Accepted 2 JAN 2014Accepted article online 6 JAN 2014

http://publications.agu.org/journals/http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)2169-9356http://dx.doi.org/10.1002/2013JB010755http://dx.doi.org/10.1002/2013JB010755

There are two high-volume injection disposal wells within 3 km of the highest-intensity region of the Timpsonearthquakes, and it is possible that injection triggered them. Since 2008, fluid injection appears to havetriggered several earthquake sequences in Texas and elsewhere in the Midwestern U.S. [Frohlich et al., 2011;Frohlich, 2012; Horton, 2012; Keranen et al., 2013; Ellsworth, 2013; Justinic et al., 2013].

We first present results from a felt report survey for the 17 May 2012 earthquake. We then summarize ouranalysis of seismograms to identify and locate earthquakes in the sequence; nearby stations include U.S.National Network station NATX in Nacogdoches located 25 km from the epicenter, several nearby EarthScopeTransportable Array stations, and eight temporary seismographs deployed following the 17 May earthquake.Finally, we describe the injection disposal wells situated near the epicentral area and discuss their possiblerelationship with seismicity.

2. Felt Reports From the May 2012 Earthquakes

The 10 May foreshock and 17 May main shock were strongly felt in the region around Timpson, TX, with thearea of high intensities including the zip code for the town of Garrison to the southwest. Instrumental epi-centers determined by the USGS, however, place the foreshock 10 km northwest of Timpson and the mainshock 4 km to the northeast (Figure 2). The ~10 km difference in their epicenters is consistent with the formaluncertainty in their locations, suggesting that they might be nearly colocated (as we might expect). To betterconstrain the epicentral area of these events we gathered felt report information (Figures 2 and 3) from threedifferent sources. Following the 17 May earthquake, we contacted the Nacogdoches Daily Sentinel and theTimpson and Tenaha News, who published felt report questionnaires that respondents mailed either to us orto newspaper offices. Two of the authors (M.B. and W.A.B.) spent 3 days in the epicentral region interviewingresidents, concentrating their efforts in the higher-intensity areas.

Figure 1. Felt area of 17 May 2012 MW-RMT4.8 earthquake. Roman numerals indicate regions experiencing modified Mercalli intensities MMI III,MMI IV, and MMI V; unlabeled regions within MMI V ellipse experienced MMI VI and MMI VII. Grey circles are historically reported earthquakesfrom Frohlich and Davis [2002]; labels indicate year of occurrence. Also plotted and labeled “PC” is 11 December 1981 microearthquake locatedby Pennington and Carlson [1984]. Red lines are regional faults as mapped by Ewing [1990]; the fault system north and west of the highest-in-tensity region is the Mt. Enterprise fault zone. Triangles are regional seismic stations nearest the 17 May epicenter; all but 239A, which closedDecember 2011, were used to locate it. Gray shaded regions are oil and gas fields as described by Galloway et al. [1983] and Kosters et al. [1989].Dashed lines are county boundaries. Rectangle within Texas icon at upper right indicates area mapped in this figure.

Journal of Geophysical Research: Solid Earth 10.1002/2013JB010755


We augmented these data with “Did you feel it?” (DYFI) information provided by the National EarthquakeInformation Center (NEIC). The DYFI program [Atkinson and Wald, 2007;Wald et al., 2011] is an internet-basedprogram where individuals can provide unsolicited responses to questions about their experiences andlocation during an earthquake. The responses are analyzed to assign a modified Mercalli intensity (MMI)value; the NEIC routinely presents summary onlinemaps of the MMI distributions. For this study the DYFI datawere especially useful for constraining boundaries for the MMI III and MMI IV regions (Figure 1).

Figure 2. Locations of felt reports for the 17 May 2012 earthquake. Key at bottom of figure shows symbols for felt intensities MMI III to MMIVII; larger symbols are reports collected by the authors; smaller symbols are DYFI reports provided by the NEIC. Yellow squares labeled “W”indicate the location of Class II injection wells; larger (smaller) symbols are wells with maximum monthly injection volumes exceeding (lessthan) 100,000 BWPM (16,000m3/mo). Red circles are NEIC epicenters determined for the 10 May and 17 May 2013 earthquakes. Solid linesare regional roads; broken lines are county boundaries.

Figure 3. Locations (circles) of felt reports and injection wells (squares) near the 17 May 2012 epicenter. County lines, highways, and key forMMI level are as in Figure 2. Ellipse labeled “VII” shows approximate extent of MMI VII area. Squares labeled W are locations of Class IIinjection disposal wells. The town of Timpson is at the junction of state highways 59 and 84.



We interviewed residents at eight locations who experienced the most severe intensities (MMI VII); theseoccurred within a 5.7 km× 2.3 km area (~10 km2) about 6 km southwest of Timpson, TX, in the direction ofGarrison (Figure 3). These individuals reported significant damage to chimneys, fireplaces, or masonry;objects such as pictures, mirrors, or deer heads fell off walls; objects fell off shelves or out of cabinets andoften broke. One respondent reported his fireplace came down inside his residence, and his south exteriorbrick wall “blew off” the house.

People experienced intensities of MMI VI and MMI V over areas of approximately 170 km2 and 2200 km2,respectively. Since the earthquake occurred at 3:12 A.M. local time, in the MMI V region typical respondentsnoted that they were awakened by the earthquake and realized that furniture was moving or objects wererattling. The MMI IV and MMI III felt areas covered approximately 7000 km2 and 20,000 km2, respectively.

We also collected felt report information for the 10 MayMW3.9 earthquake. This was much less severe than the17May earthquake and took place in themorning, 10:15 local time. Several residents of Timpson and the regionapproximately 5–10km to the southwest experienced intensities of MMI V; the region of highest intensity forthe 10 May event coincided roughly with the highest-intensity region for the 17 May main shock.

3. Analysis of Seismograms: Origin Times and Epicenters3.1. Seismic Station Coverage

Seismic station coverage in this part of East Texas has varied considerably over the past few years (Table S1 inthe supporting information). The nearest permanent seismic station is the U.S. National Network stationNATX in Nacogdoches, operational since 2004 and situated approximately 25 km southwest of the highest-intensity region (Figure 1). Between 2009 and 2011 EarthScope USArray Transportable Array (TA), stationswere deployed in eastern Texas; until December 2011 TA station 239A operated 15 km north of the epicentralregion. TA stations 140A, 240A, and 341A were installed some 65–140 km to the east between February 2011

Figure 4. Epicenters (circles) determined in this study (see Table S2). White circles are epicenters occurring before 26 May 2012 when thefirst temporary stations (triangles) were deployed. Green circles are epicenters occurring between 26 May 2012 and 5 February 2013; redcircles are best-recorded hypocenters occurring after 22 February 2013. Red line indicates basement fault from Jackson [1982]. Yellowsquares labeled W are locations of injection disposal wells as in Figures 2 and 3. Ellipses indicate MMI VI and MMI VII areas as in Figure 3.Beach ball is global centroid moment tensor reported by Columbia group (see www.globalcmt.org); note that the best recorded hypo-centers (red circles) lie along a NW-SE trend approximately parallel to one nodal plane. Labels A, A′, B, and B′ indicate locations of crosssections (Figure 6).



http://www.globalcmt.org

and February 2012. Because of the absence of stations in the epicentral area, we fixed focal depths at 2.5 kmto locate earthquakes occurring prior to 26 May 2012 (white circles in Figure 4) and their epicentral uncer-tainties are several km. In particular, for the 2011 toMay 2012 earthquakes in Table S2, our relocations found amedian value for the largest axis of the uncertainty ellipse to be ±2.9 km.

Following the 17 May 2012 earthquake weinstalled three temporary NetQuakes accelerome-ters (ETX01, ETX02, and ETX03; see Figure 4 andTable S1) between 26 May and 15 June. Becausethese instruments required access to power and aninternet connection, we only were able to find sitesfor them along HWY 59 between Nacogdochesand Timpson. Thus, for aftershocks occurring be-tween 26 May 2012 and February 2013, the sta-tions closest to the highest-intensity region (NATX,ETX01, ETX02, and ETX03) were situated approxi-mately along a northeast trending straight line.Two stations somewhat to the east (140A and240A) lay approximately along an extension ofthis line. Thus, arrivals at the remaining, relativelydistant station 341A (distance~1.2°) were criticalfor constraining aftershock locations along thenorthwest-southeast direction. There werereadable P and S phases at 341A for only threeearthquakes detected during the 26 May 2012 toFebruary 2013 period. For earthquakes duringthis period we determined locations (green cir-cles in Figure 4) with focal depths fixed at 2.5 km,and their epicentral positions along the NW-SEdirection are poorly determined.

The region was shaken strongly a third time on 25January 2013 at 0701 UTC by a mbLg4.1 after-shock. This event was well recorded by the threeNetQuakes accelerometers. Peak acceleration andpeak velocity at the closest instrument ETX01 were62% of gravity and 22 cm/s, respectively (Figure 5).

Seconds Seconds

Acc

eler

atio

n in

g

Vel

ocity

in c

m/s

Figure 5. Accelerations (left) and velocities recorded at ETX01 for the 25 January 2013 0701 UTCmbLg4.1 aftershock. The 1.08 s S-P time at this closest station to the event requires a shallowfocal depth for the earthquake.

Figure 6. Cross sections A-A′ and B-B′ (see Figure 4) of best recordedhypocenters (red circles) determined in this study. Symbols are as inFigure 4; blue lines extending beneath injection wells (yellow squares)indicate depth extent of injection wells. Note that hypocenters in sec-tion A-A′ form a linear group extending from ~1.5 to 4.5 km depth,suggesting they may occur along a southwest dipping planar surfacetrending ~35° west of north.



In response to this earthquake we installed fiveadditional stations (ETX04–ETX08, see Figure 4)equipped with both broadband seismometersand accelerometers to surround the epicentralarea. These stations were operational starting on22 February 2013 and remained in the field untilearly August 2013. For this paper, we have ana-lyzed a subset of the earthquakes recorded by thecombined network of temporary stations andNATX to determine hypocenters with free depths(red circles in Figures 4 and 6). We consider theepicentral locations and focal depths to be reli-able for these events because they are locatedwithin the network—median values are ±0.3 kmfor the largest axis of the epicentral uncertaintyellipsoid and ±0.6 km for focal depth. These errorestimates refer to the precision of the relativelocation only, as we lack a calibration shot toconstrain the absolute location uncertainty.Absolute errors are likely less than 1 km, consider-ing the network geometry, timing precision, anduncertainty in crustal velocity model.

On 2 September 2013 two more earthquakesoccurred, at 1652 UTC with mbLG4.1 and at 1851UTC with MW-RMT4.3. Because ETX04–ETX08 hadbeen removed, we do not have accurate loca-tions for these events. However, they are clearlyin the same epicentral zone as the earlier events,with S-P times of ~1.0 s at ETX01. These earth-quakes also produced excellent records at NATXthat we have modeled to evaluate focal depth(see section 3.4).

3.2. Foreshocks and Aftershocks; (S-P) Times

NATX is the only regional station that has beencontinuously operational for more than 2 years.Using a seismogram from the 10 May 2012 eventas a cross-correlation template, we searchedNATX records from January 2005 to May 2012 andidentified seven preshock events with magni-tudes 0.5–2.2 (Table S2). All the events found had(S-P) times of 4.0–4.5 s at NATX (Figure 7, top),similar to that of the 10 May 2012 foreshock andthe 17 May 2012 main shock, which had (S-P)times of 4.30 and 4.20 s, respectively. The earliestevent (with S-P of 4.35 s) occurred in April 2008and apparently went unnoticed by local residents.

For all Timpson earthquakes recorded by NATXwe determined magnitudes (see Table S2). Weperformed a linear least squares fit between thelogarithm of the peak-to-peak amplitudes andNEIC-reported magnitudes for 10 of the Timpsonearthquakes reported by the NEIC. A frequency-

Figure 7. Cumulative distribution of S-P intervals observed at threestations recording the Timpson sequence. Open circles indicate less-reliable data where either the S or P reading was of lower quality.Grey area between vertical lines indicates the middle two thirds ofthe distribution. (top) S-P intervals at station NATX; times labeled Mand F are S-P intervals for 17 May 2012 main shock and 10 May 2012foreshock. (middle) S-P intervals at ETX01. The observation that S-Pintervals are ~1.1 s indicates focal depths must be ~4 km or shallower(see text). (bottom) S-P intervals at ETX02. The observation that S-Pintervals form two groups suggests that the earthquakes originatefrom at least two distinct clusters.



magnitude plot of these data (Figure 8) indi-cates that NATX detects all Timpson earth-quakes with magnitudes of ~2.0 and greater.The b value for the sequence is 0.570 ± 0.17(95% confidence interval) as determined usingAki’s [1965] method for events having magni-tude of 2.0 and larger.

In the year following the 17 May 2012 mainshock we recorded 55 aftershocks; many werewell recorded on the temporary stations ETX01and ETX02. At ETX01, the station recording thesmallest S-P intervals, all but one interval fellbetween 1.00 and 1.23 s (Figure 7, middle). AtETX02 the intervals formed two distinct groups(Figure 7, bottom), one clustered at 1.45 s andthe other at 1.65 s.

3.3. Velocity Model and Locations

Historically, earthquakes have been rare in EastTexas, and regional station coverage has been poor; thus, there is no well-established velocity model for location.For the 17 May 2012 earthquake the St. Louis group [see Herrmann et al., 2011] obtained a better fit to regionalseismograms using their model WUS (western U.S.) than with model CUS (central U.S). In comparison with theCUS model, the WUS model has significantly lower velocities in the uppermost 8 km. Velocity logs for the depthinterval ~1.0–3.5 km, including one well situated about 3 km north of the town of Timpson (API: 42–41931360),indicated that P velocity increased approximately linearly from about 3 km/s to 5 km/s in this depth range.

We developed a starting location model based on this information and then modified it to obtain a finalmodel by applying the VELEST computer program to the best-recorded Timpson earthquakes (Group 3 inTable S2). VELEST solves the simultaneous hypocenter and 1-D velocity model tomography problem for localearthquakes as initial reference models for 3-D seismic tomography [Kissling et al., 1994]. Our final model(Table S3) has relatively low velocities with VP/VS~ 2 to depths of 2.5 km and then VP/VS~ 1.72 at greaterdepths. Because of the limited station coverage and relatively restricted range of focal depths, the model isdetermined only to about 4 km depth.

The deepest horizon plotted on Jackson [1982] summary cross sections of East Texas regional geology is thetop of the Louann Salt at 4.5 km; the Louann is only a short distance above Paleozoic Ouachita basement.This is in general agreement with Lutter and Novack’s [1990] analysis of the Program for Array Seismic Studies ofthe Continental Lithosphere (PASSCAL) Ouchita experiment that found Mezozoic sediments extending to adepth of 5 km and also well logs shown by Hammes et al. [2011] from two wells about 20–30 km south of the2012 epicenter.

For the 22 best-recorded earthquakes in Group 3, locations determined with this model had small residuals(average RMS error 0.02 s), with epicenters extending along a NW-SE line approximately 6 km long through thecenter of highest-intensity region (Figures 4 and 6, red circles). Focal depths are well constrained and rangedbetween 1.6 and 4.6 km. For our preferred velocity model (Table S3) the observed S-P intervals ranging from 1.0to 1.2 s at ETX01 (Figures 5 and 7) are inconsistent with any focal depths exceeding 3.3–4.6 km.

We fixed focal depths at 2.5 km for the 30 epicenters in Group 2 (green circles in Figure 4). Thesewere events withfewer observations at temporary local stations than the Group 3 earthquakes. Although the station distributionfor Group 2 earthquakes provided relatively poor constraint along the NW-SE direction, our preferred locationswere generally situated along the same NW-SE linear trend as the well-constrained Group 3 earthquakes.

Epicenters determined for earthquakes occurring before the installation of local stations (Group 1 inTable S2; white circles in Figure 4) were about 3 km east of the better-recorded events. These locations arenot considered reliable; they are controlled by stations 140A and 341A situated 110–130 km east of thehighest-intensity area; it is thus plausible that Group 1 locations are systematically mislocated by ~ 3 km asthe velocity model is uncalibrated along their raypaths. The (S-P) intervals observed at station NATX for

Figure 8. Magnitude distribution of Timpson earthquakes recorded at sta-tion NATX. Circles indicate events assigned magnitudes by NEIC; othermagnitudes are as determined in this study.



Group 1 earthquakes, including the 10 May foreshock and 17 May aftershock, were similar to (S-P) intervalsobserved after installation of the local network (see Figure 7, top). This observation suggests that theGroup 1 earthquakes occurred in nearly the same location as the later Group 2 and Group 3 events withbetter-constrained locations.

3.4. Modeling Seismograms Recorded at NATX

A notable feature of the transverse horizontal broadband seismograms recorded at NATX for the 17May 2012main shock are several high-amplitude phases arriving in the 20 s interval following the S arrival (Figures 9and 10) on the transverse component of motion. Similar phases are visible on seismograms of the 10 May2012 MW-RMT3.9 and 25 January 2013 mbLg4.1 earthquakes.

We interpret these arrivals as crustal reverberations—sometimes called “whispering gallery phases” [Aki andRichards, 1980] —generated when a shallow source lies near the boundary between a low-velocity gradientoverlying a higher-velocity layer, and significant amounts of energy become trapped in the upper layer. Wemodeled the seismograms recorded at NATX for the 10 May 2012 foreshock, the 17 May main shock, the 25January 2013 aftershock, and both 2 September 2013 aftershocks using the velocity model obtained withVELEST (Table S3) and the Global centroid moment tensor focal mechanism (Figure 4). Syntheticseismograms were computed using program FK [Zhu and Rivera, 2002] at the appropriate epicentral distance

Figure 9. Transverse component displacement seismograms (black) for a double couple point source (strike =330, dip= 85, rake=!15) at a range of 24.5 km and azimuth of 240° in the struc-ture in Table S3 for depths between 2 and 5.5 km. The near-field ramp begins at the Pwave arrival at approximately 5 s and is interrupted by the Swave arrival at approximately 10 s travel time. Itis followed by multiple, impulsive arrivals, corresponding to whispering gallery arrivals from waves trapped in the sediments above basement. Note the sensitivity of the relative amplitudes ofthe whispering gallery phases to focal depth. Later arrivals have the largest amplitude at shallow depth, while the first arrival has the largest amplitude for deeper focal depths. Observedtransverse displacement at NATX (red) for the 17 May 2012 main shock best matches a focal depth of 4.5 km. Both observed and synthetic displacements were low-pass filtered at 2Hz.



for a range of focal depths. Both the synthetic and observed seismograms were low-pass filtered at 2 Hz andintegrated to displacement for comparison. We obtained the best match between the NATX displacementseismograms and the relative amplitudes of the whispering gallery phases for synthetic sources (Figure 9) atdepths of 3.5, 4.5, 2.75, 4.0, and 4.5 km for these five events, respectively (Figure 10).

4. Injection Wells

In Texas, the Texas Railroad Commission (RRC), which no longer regulates railroads, is responsible forregulating activity related to petroleum operations in the state. The RRC issues permits for drilling wellsand monitors production and underground disposal activities. By law operators annually provide the RRCwith certain information concerning fluid injection, both when it is used to stimulate production andalso when it is used to dispose of fluid wastes. The RRC’s database is publically available online and in-cludes information for individual wells and leases which is mostly complete for the past two decades;for each well the data include monthly volumes of water and gas injected and volumes of oil, water, andgas extracted.

Although East Texas has been the focus of recent unconventional gas development, in the immediate vicinityof the epicenters located in this study, there were no wells actively producing petroleum between 2000 and2011 (see Figure S1). However, there were several such wells situated several kilometers to the southeast inRusk and Nacogdoches Counties. Many of the gas wells began operation in 2005 or later and produced fromformations within the Trinity at depths of 2.2–2.9 km.

There are, however, two relatively high-volume and two lower-volume injection disposal wells situatedwithin several kilometers of the highest-intensity region of the 17 May 2012 earthquake (Figures 4 and 11).All four wells injected at depths of about 1.8–1.9 km; here Paleozoic basement is at ~5 km depth [Jackson,1982]; Lutter and Novack, 1990; Hammes et al., 2011]. The highest injected volumes, averaging 269,000 barrels ofwater per month (BWPM) (42,750m3/mo) since September 2006, were at a well at 4 km southwest of thecenter of the MMI VII region (API 42–40133833; labeled “south” in Figure 4); RRC data indicates that injectionwas into the Rodessa of the Trinity formation. At a well about 2 km north of the MMI VII centroid (API 42–41931083; labeled “north” in Figure 4) monthly volumes averaged 98,000 BWPM (15,600m3/mo); injectionhere was also into the Rodessa. The total injected volumes for the north and south wells were 1.05 million m3

and 2.90 million m3, respectively.

Seconds

May 10, 2012

May 17, 2012

January 25, 2013

September 2a, 2013

September 2b, 2013

2

0

-2

100

-10

4

0

-4

2

0

-2

5

0

-5

3.5 km

4.5 km

2.75 km

4.0 km

4.5 km

Figure 10. Transverse component displacement seismograms recorded at NATX (red) for the five principal earthquakes. Synthetic seismograms (black) are shown for the best fitting pointsource focal depth for the double-couple orientation used in Figure 9. Both observed and synthetic displacements were low-pass filtered at 2 Hz.



While these are relatively high-volume wells, they are not unusual for Texas injection wells. There are morethan 10,000 injection wells in Texas that have been active since 2000; of these more than 1300 have reportedinjection volumes exceeding 100,000 BWPM (16,000m3/mo) for 12 or more months since 2000.

Injection volumes were considerably less at two other regional wells. At well API 42–40131974 (labeled“south-2” in Figure 4) injection took place only from April 2009 to January 2010, and injection volumes neverexceeded 50,000 BWPM (8000m3/mo). At a fourth well (API 42–41931287; labeled “north-2” in Figure 4)injection began in March 2009 and volumes averaged about 50,000 BWPM.

The RRC also reports monthly values for maximum and average injection pressures at disposal wells(Figure 11). At both the higher-volume wells, in 2010 and afterward, injection volumes decrease even asinjection pressures hold steady, as at the south well, or increase, as at the north well. Variations in the injectedvolume presumably reflect demand and nothing more. Increasing injection pressure with decreasing volumeinjected could mean that the well bore has become constricted from a buildup of scale or plugging of ascreen, or it could reflect increased resistance from the formation to accept additional fluid.

Modeling of subsurface hydrology and stress conditions in response to the pressure/volume histories atindividual wells would help achieve a more complete understanding of the relationship between fluid injectionand seismicity. However, this will requiremore information about injection pressures and well properties than isavailable from RRC data. The RRC-reported pressures are average and maximum surface pressures; these datado not reflect whether injection was episodic or continuous throughout the month. Moreover, the surfacepressures for a pumping system do not account for frictional losses depending on casing diameters, etc., andthus are not related simply to the pressures at depth where well perforations permit fluid to disperse intosubsurface strata.

Figure 11. Monthly injection volumes for the wells indicated labeled north-2, north, south, and south-2 in Figure 4. Scale bar at left of histo-grams is 100,000 BWPM (16,000m3/mo). Green and white circles and right scale are average and maximum monthly injection pressures(1000 PSIG is 6895 kPa). Red circles indicate times of earthquakes identified in this study (see Table S2). Labels at right indicate depth intervalof injection. At time of publication, information for February 2013 and later was not yet available concerning monthly injection volume andpressures for the south well.



5. Discussion

The most significant conclusions of the present study are that:

1. Epicenters of aftershocks of the 17 May 2012 earthquake occurred along a ~6 km long N35°W striking lin-ear zone that coincides with a mapped basement fault [see Jackson, 1982; Geomap Company, 2012].

2. The epicentral area of aftershocks extends across the highest-intensity region of the 17 May 2012 MW-RMT4.8main shock, where local residents experienced intensities ofMMI VII. Moreover, the aftershocks trend parallelto one nodal plane of the predominantly strike-slip focal mechanisms reported for this event by both theColumbia (Figure 4) and the St. Louis groups [Ekström et al., 2012; Herrmann et al., 2011].

3. The better-determined focal depths of sequence events are relatively shallow, with depths ranging from1.6 to 4.6 km as determined by a network of temporarily deployed stations. For the five largest earth-quakes in the sequence, modeling of the seismograms recorded at station NATX at a distance of24.5 km indicates focal depths between 2.75 and 4.5 km.

4. Two relatively high-volume injection disposal wells are situated within ~3 km of the linear trend of epicen-ters and near the highest-intensity region of the 17 May main shock. Injection at these wells began in2006–2007 prior to the first recorded sequence earthquake in April 2008.

5.1. Origin: Human-Triggered or Natural?

The above observations demonstrate that the Timpson earthquakes share many features in common withrecent earthquakes elsewhere in the Midwestern U.S. that have been inferred to be triggered or induced. Likethe 1962–1968 earthquakes in Denver [Healy, et al., 1968; Herrmann and Park, 1981], the 2008–2009 earth-quakes in Dallas-Fort Worth [Frohlich et al., 2011], the 2009 earthquakes near Cleburne, TX, [Justinic et al.,2013] and the 2011 earthquake in Youngstown, OH [Kim, 2013], the Timpson earthquakes are the first knownearthquakes in this location and began only after injection began. As in Denver, Dallas-Fort Worth, Cleburne,2011 in Arkansas [Horton, 2012], and 2011 in Youngstown OH [Kim, 2013], the Timpson earthquakes had focaldepths at or exceeding the depths of injection and occurred along a linear trend situated within only a fewkilometers of the site of injection. Elsewhere in Texas, Frohlich [2012] found that seismic activity at distancesof ~3 km is sometimes associated with wells having maximum monthly rates exceeding 150,000 BWPM(24,000m3/mo); in Timpson, injection rates averaged 269,000 BWPM (43,000m3/mo) since 2007 at the southwell (Figures 4 and 11). All the above observations are consistent with the hypothesis that the Timpsonearthquakes were triggered by nearby injection disposal wells; the likelihood that Timpson earthquakes areinduced is comparable to that for the Denver, Dallas-Fort Worth, Cleburne, and Arkansas sequences.

Like the Dallas-Fort Worth earthquake sequence, the linear trend of aftershocks in Timpson approximatelycoincides with a mapped fault. In both cases we have been unable to locate the primary evidence confirmingthe faults’ existence and location. The fault location indicated in Figure 4 is from Jackson [1982], as plottedon his map of faults at the top of Paleozoic basement; he does not place the fault on maps of faulting atshallower depths. Geomap Company [2012] shows the fault 2–3 km east of Jackson’s location. The Timpsonregion has been the focus of recent exploration efforts, and we are seeking more recent data that might fixthe location and dip of this fault, and thus resolve the possible discrepancy.

Although the preponderance of evidence favors the conclusion that the Timpson earthquakes were inducedor triggered by fluid injection, our investigation cannot rule out the possibility that they are of natural origin.The historical record [Pennington and Carlson, 1984; Frohlich and Davis, 2002] indicates that in 1891 and 1981,well before injection began,M4.0 andM3.2 earthquakes occurred at Rusk, Texas, 80 km west of Timpson, andat Center, Texas, 25 km to the southeast (Figure 1). These could not have been triggered by human activity;rather, they have been attributed to the Mt. Enterprise fault zone (Figure 1). One of the focal planes for a focalmechanism reported for the 17 May 2012 earthquake (see Figure 4) is parallel to faults in the Mt. Enterprisefault zone as mapped by Ewing [1990]; the other focal plane is parallel to the mapped fault reported byJackson [1982] and Geomap Company [2012].

5.2. Magnitude and Scalar Moment

The 17 May 2012 Timpson earthquake, with magnitudeMW-RMT4.8 and scalar momentMo of 2.21× 1023 dyn-

cm, is the largest earthquake to have occurred in eastern Texas. The largest previous earthquakes in easternand Central Texas are the 19 March 1957M4.7 Gladewater earthquake, attributed to extraction in the EastTexas oil field by several investigators, including Frohlich and Davis [2002] and the 20 October 2011MW-RMT4.8



Fashing earthquake (Mo 1.8× 1023 dyn-cm), with an epicenter that coincides with several previous earth-

quakes that Pennington et al. [1986], Davis et al. [1995], and Frohlich and Brunt [2013] attributed to extractionin the Fashing gas field. The nearest historical earthquakes larger than the 17 May 2012 Timpson event aremore than 400 km distant; e.g., in Oklahoma earthquakes with magnitudes of 5.5, 5.7, and 5.6 occurred in1882, 1952, and 2011.

The magnitudes of both the 17 May 2012 earthquake and 10 May foreshock were adjusted upward severaldays after they occurred. That is, the 17 May magnitude was initially reported as mbGS4.3 from short-periodbody waves, and the 10May foreshockmagnitude asmbGS3.7. Then, following analysis of broadband regionalsignals to determine the scalar moments and focal mechanisms for both events, the magnitudes wereupgraded to MW-RMT4.8 and MW-RMT3.9. This suggests that the body-wave amplitudes at frequencies of ~1 sused to determinembGS give magnitude values somewhat smaller than predicted by the Atkinson and Boore[2006; 2011] Central U.S. strong motion model. It also suggests that the magnitudes cataloged for somehistorical earthquakes in East Texas and along the Gulf Coastal Plain—determined from body waves—mightbe smaller than moment magnitudes that might have been determined using modern methods hadbroadband observations been available.

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Economic Geol, Austin, Tex.Geomap Company (2012), Executive reference map 302: East Texas, scale 1:386,000, Dallas, Tex.Hammes, U., H. S. Hamlin, and T. E. Ewing (2011), Geologic analysis of Upper Jurrasic Haynesville shale in East Texas and west Louisiana, AAPG

Bull., 95, 1643–1666, doi:10.1306/02141110128.Healy, J. H., W. W. Rubey, D. T. Griggs, and C. B. Raleigh (1968), The Denver earthquakes, Science, 161, 1301–1310.Herrmann, R. B., and S.-K. Park (1981), The Denver earthquakes of 1967–1968, Bull. Seismol. Soc. Am., 71, 731–745.Herrmann, R. B., H. Benz, and C. J. Ammon (2011), Monitoring the earthquake source process in North America, Bull. Seismol. Soc. Am., 101,

2609–2625, doi:10.1785/0120110095.Horton, S. (2012), Disposal of hydrofracking waste fluid by injection into subsurface aquifers triggers earthquake swarm in central Arkansas

with potential for damaging earthquake, Seismol. Res. Lett., 83, 250–260, doi:10.1785/gssrl.83.2.250.Jackson, M. P. A. (1982), Fault Tectonics of the East Texas Basin, Geol. Circular, vol. 82–4, p. 31, Univ. Texas Bureau Economic Geol.,

Austin, Tex.Justinic, A. H., B. Stump, C. Hayward, and C. Frohlich (2013), Analysis of the Cleburne, Texas, earthquake sequence from June 2009 to June

2010, Bull. Seismol. Soc. Am., 103, 3083–3093, doi:10.1785/0120120336.Keranen, K. M., H. M. Savage, G. A. Abers, and E. S. Cochran (2013), Potentially induced earthquakes in Oklahoma, USA: Links between

wastewater injection and the MW 5.7 earthquake sequence, Geology, 41, 609–702, doi:10.1130/G34045.1.Kim, W.-Y. (2013), Induced seismicity associated with fluid injection into a deep well in Youngstown, Ohio, J. Geophys. Res. Solid Earth,

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AcknowledgmentsWe are indebted to the property ownerswho graciously allowed us to install andoperate seismographs on private prop-erty. We thank David Wald and VinceQuitoriano, who provided did-you-feel-it(DYFI) information for constraining theMMI III and MMI IV intensity areas,allowing us to focus our field studies inthe higher-intensity areas. We thank JuliaGale and Martin Jackson for providingvaluable information about regionalgeology and Wayne Pennington, ArtMcGarr, Andrea Llenos, and an anony-mous reviewer for helpful suggestionsthat improved the manuscript. This re-search was partially supported by theUSGS, Department of the Interior, underUSGS award G12AP20001 andG13AP00023; and by ResearchPartnership to Secure Energy for America(RPSEA) subcontract 11122-27 throughthe “Ultra-Deepwater andUnconventional Natural Gas and OtherPetroleum Resources” program autho-rized by the U.S. Energy Policy Act of2005. RPSEA (www.rpsea.org) is a non-profit corporation whose mission is toprovide a stewardship role in ensuringthe focused research, development, anddeployment of safe and environmentallyresponsible technology that can effec-tively deliver hydrocarbons from domes-tic resources to the citizens of the UnitedStates. RPSEA, operating as a consortiumof premier U.S. energy research universi-ties, industry, and independent researchorganizations, manages the programunder a contract with the U.S.Department of Energy’s National EnergyTechnology Laboratory. The views andconclusions contained in this documentare those of the authors and should notbe interpreted as representing the officialpolicies, either expressed or implied, ofthe U.S. Government.

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http://dx.doi.org/10.4401/ag&hyphen;5354

Auxiliary)material)for))

The 17 May 2012 M4.8 earthquake near Timpson, east Texas: An event

possibly triggered by fluid injection

Cliff Frohlich1, William Ellsworth2, Wesley A. Brown3, Michael Brunt4, Jim Luetgert2,

Tim MacDonald2 and Steve Walter2

Journal of Geophysical Research Solid Earth

)Introduction*

We)provide)a)figure)“fs01.eps”)that)shows)producing)oil)and)gas)wells)near)the)

Timpson)sequence)as)well)as)injection)disposal)wells.)We)also)provide)three)tables:)

“ts01.txt”)provides)the)coordinates)of)the)temporary)and)permanent)seismic)

stations)deployed)in)the)Timpson)area)and)used)to)locate)earthquakes;)“ts02.txt”)

presents)the)preferred)epicentral)and)hypocentral)locations)for)earthquakes)in)the)

Timpson)sequence;)and)“ts03.txt”)is)the)velocity)model)we)determined)from)the)data)

and)used)to)locate)hypocenters)and)epicenters)in)this)study.)

)

1.)fs01.eps)For the region mapped in Figure 4, injection disposal wells (yellow squares

labeled “W”) and producing wells (blue squares labeled ‘P’) actively extracting oil or

water during the 2000-2011 interval. Larger blue squares are wells producing monthly

))))))))))))))))))))))))))))))))))))))))))))))))))))))))1 Institute for Geophysics, University of Texas at Austin, Austin TX 78758-4445 2)Earthquake)Science)Center,)U.S.)Geological)Survey,)Menlo)Park)CA)94025R3591)3)Department)of)Geology,)Stephen)F.)Austin)State)University,)Nacogdoches)TX)75962)4)Eagle)Pass)High)School,)2020)2nd)Street,)Eagle)Pass)TX)78852))) Corresponding)author:)Cliff)Frohlich,)Institute)for)Geophysics,)10100)Burnet)Rd.)(R2200),)Austin,)TX)78758R4445)([email protected]))

volumes of water that exceeded 10,000 barrels (1600 m3) for one or more months

between 2000-2011. Note that producing wells are absent in the vicinity of the seismic

activity (circles). Other symbols are as in Figure 4.

2. ts01.txt Seismic stations providing data for this study.

2.1 Column “Station Code”, four-character identifier for seismic stations providing data

in this study.

2.2 Column “latitude”, latitude of seismic station

2.3 Column “longitude”, longitude of seismic station

2.4 Column “installed”, date of station installation

2.5 Column “removed”, date of station removal (when available)

2.6 Column “instrument”, instrument type or array identifier (when available)

3. ts02.txt Preferred locations for epicenters and hypocenters determined in this study.

Locations in Group 1 (white circles in Figure 4) epicenters were determined with focal

depths fixed at 2.5 km; epicenters are least reliable because of the absence of stations

within ~5 km. Stations recording events in Group 2 (green circles in Figure 4) were

situated along a southwest-northeast-trending line; epicenters were determined with fixed

focal depths. Temporary stations ETX01-ETX08 were in place to record events in Group

3 (red circles in Figure 4). Group 3 hypocenters were determined with free depths and are

considered relatively reliable.

3.1 Column “year”, origin time year of epicenter or hypocenter.

3.2 Column “date”, monty, day, hour, and minute of origin time

3.3 Column “lat”, latitude of epicenter or hypocenter (degrees)

3.4 Column “long”, longitude of epicenter or hypocenter (degrees)

3.5 Column “depth”, depth of epicenter (as fixed; see above) or hypocenter (km)

3.6 Column “res”, residual of epicenter or hypocenter as determined in this study (sec);

see text.

3.7 Column “gap”, largest azimuthal angle (degrees) between stations used to locate

epicenter or hypocenter.

3.8 Column “magnitude”, magnitude and source of magnitude for epicenter or

hypocenter (when available).

4. ts03.txt Velocity model used for location in this study. 4.1)Column)“layer”,)layer)number,)with)“1”)and)surface)and)deeper)layers)“2”,”3”,)

etc.)at)increasingly)greater)depths.)

4.2)Column)“PRvelocity)(km/sec)”,)compressional)velocity)(km/sec))

4.3)Column)“SRvelocity)(km/sec)”,)shear)velocity)(km/sec))

4.4)Column)“thickness)(km)”,)thickness)of)layer)()km))

4.5)Column)“depth)(km)”,)depth)range)of)top)and)bottom)of)layer)(km),)with)surface)

at)0)km.)

)

)

)

Table ts01.txt Seismic stations providing data for this study.

Station code latitude longitude installed removed instrument

NATX 31.7598°N 94.6610°W 2004/05/15 --- STS-2

140A 32.6408°N 93.5740°W 2011/02/12 2012/11/10 USArray141A 32.6046°N 92.9049°W 2011/02/14 2012/11/12 USArray239A 32.0179°N 94.4707°W 2010/02/21 2011/12/11 USArray240A 32.0387°N 93.7626°W 2011/02/11 2012/12/01 USArray241A 32.0227°N 92.9188°W 2011/02/10 2012/12/01 USArray340A 31.4167°N 93.8896°W 2010/02/19 2011/12/10 USArray341A 31.3334°N 93.1681°W 2011/03/08 2012/12/15 USArray

ETX1 31.8747°N 94.4278°W 2012/06/07 --- NetQuakesETX2 31.9091°N 94.4058°W 2012/05/24 --- NetQuakesETX3 31.8273°N 94.5000°W 2012/05/30 --- NetQuakesETX4 31.8668°N 94.3810°W 2013/02/22 2013/08/ETX5 31.8463°N 94.3863°W 2013/02/22 2013/08/ETX6 31.8675°N 94.4525°W 2013/02/22 2013/08/ETX7 31.8965°N 94.4701°W 2013/02/22 2013/08/ETX8 31.8369°N 94.4182°W 2013/02/22 2013/08/

2/6/2014 onlinelibrary.wiley.com.ezproxy.lib.utexas.edu/store/10.1002/2013JB010755/asset/supinfo/ts02.txt?v=1&s=8c90f54e61eabb9452020ed0319539fbc7884e8f

http://onlinelibrary.wiley.com.ezproxy.lib.utexas.edu/store/10.1002/2013JB010755/asset/supinfo/ts02.txt?v=1&s=8c90f54e61eabb9452020ed0319539fbc7884e8f 1/2

Table ts02.txt. Preferred locations for epicenters determined in this study. Locations in Group 1 (white circles in Figure 4) epicenters were determined with focal depths fixed at 2.5 km; epicenters are least reliable because of the absence of stations within ~5 km. Stations recording events in Group 2 (green circles in Figure 4) were situated along a southwest-northeast-trending line; epicenters were determined with fixed focal depths. Temporary stations ETX01-ETX08 were in place to record events in Group 3 (red circles in Figure 4). Group 3 hypocenters were determined with free depths and are considered relatively reliable.

Group 1: events recorded prior to deployment of stations ETX01-ETX03

year date origin time lat long depth res gap magnitude2008 Apr 08 22 39 [S-P 4.35 sec at NATX] 1.6 this study2010 Jun 11 11 56 10.14 31.860 -94.393 2.50 0.30 159¡ 1.2 this study2010 Dec 12 10 12 [S-P 2.975 sec at 239A] 0.5 this study2011 Jan 30 4 22 14.04 31.863 -94.383 2.50 0.35 162¡ 1 5 this study2011 Jul 04 2 36 35.03 31.886 -94.406 2.50 0.44 122¡ 2.2 mblg pde2011 Nov 26 21 22 12.56 31.871 -94.411 2.50 0.30 126¡ 2.2 this study2012 May 10 15 15 38.80 31.881 -94.401 2.50 0.31 161¡ 3.9 mwr pde2012 May 17 8 12 0.94 31.887 -94.406 2.50 0.34 163¡ 4.8 mwr pde2012 May 17 10 58 52.99 31.874 -94.416 2.50 0.54 162¡ 2.1 this study2012 May 20 19 09 [S-P 4.250 sec at NATX] 1.0 this study2012 May 20 18 28 33.73 31.882 -94.404 2.50 0.36 162¡ 2.7 mblg pde

Group 2: events recorded after deployment of stations ETX01-ETX03

year date origin time lat long depth res gap magnitude2012 May 26 5 42 24.31 31.932 -94.475 2.50 0.09 184¡ 2.1 this study2012 May 26 5 47 57.52 31.845 -94.385 2.50 0.44 134¡2012 May 26 5 58 26.11 31.842 -94.397 2.50 0.21 134¡ 2.5 mblg pde2012 May 27 17 35 26.11 31.943 -94.476 2.50 0.44 187¡ 1.8 this study2012 Jun 01 07 472012 Jun 03 11 10 50.55 31.865 -94.427 2.50 0.03 216¡ 0.6 this study2012 Jun 06 10 46 11.71 31.873 -94.452 2.50 0.02 174¡2012 Jun 07 0 34 47.80 31.874 -94.449 2.50 0.07 176¡ 1.0 this study2012 Jun 15 21 50 21.95 31.866 -94.435 2.50 0.22 201¡ 0.7 this study2012 Jun 16 8 58 11.91 31.916 -94.458 2.50 0.55 178¡ 2.1 mblg pde2012 Jun 20 19 13 55.47 31.868 -94.427 2.50 0.18 214¡2012 Jul 15 8 57 27.56 31.860 -94.429 2.50 0.01 220¡2012 Jul 19 19 7 59.08 31.836 -94.394 2.50 0.39 211¡ 2.1 this study2012 Sep 19 13 13 53.42 31.857 -94.410 2.50 0.14 241¡ 1.1 this study2012 Dec 07 19 38 38.97 31.874 -94.449 2.50 0.02 168¡ 2.8 mblg pde2012 Dec 22 17 55 12.89 31.868 -94.447 2.50 0.02 189¡ 2.6 mblg pde2012 Dec 30 14 47 24.28 31.868 -94.444 2.50 0.03 183¡ 1.8 this study2013 Jan 25 7 1 19.48 31.865 -94.441 2.50 0.02 186¡ 4.1 MbLg us2013 Jan 25 7 45 58.51 31.870 -94.446 2.50 0.03 163¡ 1.5 this study2013 Jan 25 9 40 2.91 31.856 -94.430 2.50 0.03 222¡ 0.9 this study2013 Jan 25 22 41 13.45 31.855 -94.430 2.50 0.05 223¡ 0.6 this study2013 Jan 26 1 9 13.77 31.864 -94.438 2.50 0.06 196¡ 0.7 this study2013 Jan 26 22 37 46.61 31.854 -94.425 2.50 0.08 228¡ 0.7 this study2013 Jan 29 12 31 33.39 31.851 -94.429 2.50 0.08 226¡ 2.8 MbLg us2013 Jan 31 18 37 37.93 31.850 -94.431 2.50 0.07 226¡ 1.0 this study2013 Jan 31 21 20 32.89 31.848 -94.428 2.50 0.07 228¡ 3.0 Md us2013 Feb 01 20 40 32.61 31.846 -94.417 2.50 0.18 239¡ 1.1 this study2013 Feb 02 20 59 51.72 31.849 -94.426 2.50 0.10 230¡ 1.0 this study2013 Feb 02 21 14 23.28 31.856 -94.426 2.50 0.07 226¡ 0.8 this study2013 Feb 03 6 41 0.60 31.873 -94.451 2.50 0.03 169¡ 2.1 MbLg us2013 Feb 05 0 36 19.00 31.854 -94.426 2.50 0.05 228¡ 0.5 this study2013 Feb 05 5 58 40.79 31.855 -94.426 2.50 0.05 227¡ 1.5 this study

2/6/2014 onlinelibrary.wiley.com.ezproxy.lib.utexas.edu/store/10.1002/2013JB010755/asset/supinfo/ts02.txt?v=1&s=8c90f54e61eabb9452020ed0319539fbc7884e8f

http://onlinelibrary.wiley.com.ezproxy.lib.utexas.edu/store/10.1002/2013JB010755/asset/supinfo/ts02.txt?v=1&s=8c90f54e61eabb9452020ed0319539fbc7884e8f 2/2

Group 3: events recorded after deployment of stations ETX04-ETX08

year date origin time lat long depth res gap magnitude2013 Feb 22 5 10 15.30 31.8871 -94.4473 2.42 0.043 182¡ 0.9 this study2013 Feb 26 10 35 7.19 31.8509 -94.4243 3.40 0.033 145¡2013 Feb 26 12 57 7.20 31.8512 -94.4242 3.31 0.033 143¡2013 Mar 01 6 52 50.68 31.8548 -94.4297 3.88 0.030 94¡2013 Mar 02 10 34 59.87 31.8639 -94.4255 1.68 0.022 74¡2013 Mar 02 20 48 37.11 31.8617 -94.4294 2.82 0.026 128¡ 2013 Mar 04 2 53 52.22 31.8486 -94.4211 3.03 0.033 85¡2013 Mar 04 15 8 25.03 31.8529 -94.4262 3.48 0.043 91¡2013 Mar 04 17 4 28.64 31.8370 -94.4146 3.98 0.046 184¡ 2013 Mar 05 4 56 41.41 31.8522 -94.4241 3.17 0.034 87¡2013 Mar 09 0 26 1.69 31.8539 -94.4231 2.86 0.033 80¡ 0.8 this study2013 Mar 17 9 41 49.58 31.8776 -94.4457 3.56 0.033 184¡ 2013 Mar 18 2 14 58.64 31.8778 -94.4456 3.40 0.028 102¡ 0.6 this study2013 Mar 18 5 28 30.78 31.8781 -94.4453 3.56 0.019 101¡ 2013 Mar 20 16 57 32.85 31.8452 -94.4235 3.97 0.032 102¡ 2013 May 05 5 38 1.54 31.8682 -94.4138 4.62 0.027 177¡ 2013 May 06 2 31 38.05 31.8507 -94.4254 3.83 0.027 151¡ 2013 May 07 4 53 37.54 31.8808 -94.4504 3.55 0.033 176¡ 0.9 this study2013 May 08 8 19 5.61 31.8571 -94.4296 3.63 0.009 134¡ 2013 May 11 22 47 18.33 31.8896 -94.4523 2.50 0.021 209¡2013 May 12 13 32 0.09 31.8644 -94.4452 4.32 0.016 104¡ 2013 May 31 10 1 17.89 31.8693 -94.4320 4.55 0.043 201¡ 1.9 this study

Table ts03.txt. Velocity model used for location in this study.

layer P-velocity (km/sec) S-velocity (km/sec) thickness (km) depth (km) 1 2.82 1.20 3.5 0-0.5 2 3.30 1.52 0.5 0.5-1.0 3 3.79 1.82 0.5 1.0-1.5 4 4.28 2.11 0.5 1.5-2.0 5 4.76 2.40 0.5 2.0-2.5 6 5.11 2.79 0.5 2.5-3.0 7 5.12 2.93 -- 3.0+

1

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PUBLICATIONS - Stanford UniversityPeople experienced intensities of MMI VI and MMI V over areas of approximately 170km2 and 2200km2, respectively. Since the earthquake occurred at