e Searthquakes of 15 January 1944 (M 7.0) and 11 June 1952 (M 6.8)

solution for the 1944 earthquake are consistent with the parameters observed along the La Laja fault in the frontal part of the

and 33S results from the convergence between the extends near horizontal several hundred kilometers tothe east before descending into the mantle. Theflattening of the subducted Nazca plate initiated at

Earth and Planetary Science Letterseastern Precordillera that generated a 68-km-long coseismic surface rupture. The 1952 earthquake focal mechanism solution andits shallow source depth suggest it is related to faults in the eastern Precordillera, but a particular fault association is difficult. The1944 earthquake was clearly the most destructive event because its proximity to the most populated area in San Juan, large size andshallow focal depth. 2006 Elsevier B.V. All rights reserved.

Keywords: historical earthquake source; Andean backarc crust; Precordillera

1. Introduction and regional tectonics

The seismically active Andean backarc between 28S

oceanic Nazca plate and the continental South Americanplate at a rate of 6.3 cm yr1 [1] (Fig. 1a). In this region,the Nazca slab subducts to 100 km depth and thena Department of Geosciences, University of Arizona, USAb Department of Geophysics and Astronomy, Faculty of Exact, Physical and Mathematical Sciences, National University of San Juan, Argentina

Received 1 September 2005; received in revised form 18 December 2005; accepted 4 January 2006Available online 20 February 2006

Editor: S. King

Abstract

The backarc region of the Andes in the vicinity of San Juan, Argentina, is one of the most seismically active fold and thrust beltregions in the world. Four large damaging crustal earthquakes (1894, 1944, 1952 and 1977) occurred during the last 111 yr between30S and 32S. We have determined the source parameters for two of these important earthquakes, the 1944 and 1952 events, usinghistoric seismic records. The earthquake on 15 January 1944 had an epicentral location between the eastern thin-skinnedPrecordillera fold and thrust belt and the thick-skinned Sierras Pampeanas basement-cored uplifts. The 11 June 1952 earthquakeoccurred in the eastern Precordillera about 35 km southwest of the 1944 epicenter location. The P-wave first motions, long-periodteleseismic P waveform modeling, and SV/SH amplitude ratio indicate a thrust focal mechanism for the 1944 event (strike N45E,dip 35 to the southeast, and rake 110) with M0=3.0110

19 N m and Mw=7.0. The 1952 earthquake focal mechanism solutionindicates a more oblique mechanism (strike N40E, dip 75 to the southeast, and rake 30) with M0=2.2010

19 N m and Mw=6.8.Both the 1944 and 1952 earthquakes have focal depths b12 km and simple source time functions with one pulse of moment releasewith durations of 10 s and 8 s, respectively. Both the shallow focal depth and the east-dipping fault plane in the focal mechanismw w

Patricia Alvarado a,b,, Susan Beck aSource characterization of thCorresponding author. Department of Geosciences, University ofArizona, USA. Tel.: +1 520 621 3348; fax: +1 520 621 2672.

E-mail address: alvarado@geo.arizona.edu (P. Alvarado).

0012-821X/$ - see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.epsl.2006.01.015an Juan (Argentina) crustal

243 (2006) 615631www.elsevier.com/locate/epsl810 Ma [2,3]. This process is linked to the shut off ofthe volcanic arc, the eastern migration of the thin-

etary616 P. Alvarado, S. Beck / Earth and Planskinned Precordillera and the uplift of the SierrasPampeanas basement-cored blocks in the broad regionof the foreland [2,3] (Fig. 1). Although there is a largenumber of earthquakes with depths of 100 km which

Fig. 1. (a) Location map showing the PDE-NEIC seismicity during the perisubducted Nazca plate [5]. Also shown are the GPS plate convergence veloprovinces, large historical earthquakes from INPRES [29] (stars) (Table 2), anthe 1944 (Mw 7.0) (dashed lines) and 1952 (Mw 6.8) (dot-dashed lines) earthqSan Juan for both crustal earthquakes [30,33,43]. Note the maximum MM ievent. The arrow indicates the 68-km-long coseismic surface rupture along tare the foreshock (F) and mainshock (M) of the most recent 1977 (Mw 7.5)Science Letters 243 (2006) 615631define the flat slab geometry in the Andean backarc atabout 31S and correlate with the projection of thesubducting Juan Fernndez Ridge [47] (Fig. 1a), themost destructive earthquakes are the moderate-to-large

od 19942004. Contours indicate approximate depth to the top of thecity [1] and location of the Juan Fernndez Ridge (JFR). (b) Tectonicd contour map of Modified Mercalli (MM) seismic intensity points foruakes. Black dots are locations of MM intensity values in the vicinity ofntensity IX encircles the cities of San Juan and Albardn for the 1944he La Laja thrust fault caused by the 1944 earthquake [30]. Also shownCaucete earthquake [24,25,29].

etaryevents generated in the South American upper plate withdepths b35 km. Large crustal earthquakes in 1894,1944, 1952, and 1977 associated with the Precordilleraand the Sierras Pampeanas have caused extensivedamage in the vicinity of San Juan, Argentina (Fig. 1b).

The Precordillera is a thin-skinned fold and thrustbelt that makes up the foothills of the high elevationAndes (Fig. 1). Based on the deposits, timing and styleof deformation, it is divided in the western, central andeastern Precordillera [8,9] (Fig. 2). The western andcentral Precordillera are a series of mainly Paleozoicranges and linear valleys bounded by roughly NS eastverging reverse faults [8,10,11] (Fig. 2). In contrast, theeastern Precordillera is made up of Paleozoic, Triassicand Cenozoic rocks with both thin-skinned and thick-skinned structures and west verging thrust faults [8,12](Fig. 2). Farther east, the Sierras Pampeanas tectonicprovince comprises crystalline PrecambrianPaleozoicbasement blocks uplifted, tilted and folded by thrustfaults with a similar style to the Laramide structures inthe western U.S.A. [13] (Fig. 2). The Precordillera andthe Sierras Pampeanas are separated by broad internallydrained retroarc basins that include the Bermejo andTulum Valleys, which receive much of their sedimentsfrom the Andes but whose structures are controlled bythe Sierras Pampeanas active tectonics [12,14]. TheTulum Valley contains San Juan, a city with a popu-lation of about 350,000, and other smaller communi-ties (Figs. 1b and 2). This transition between thePrecordillera and the Sierras Pampeanas structurescorresponds to the region of many of the largedevastating crustal earthquakes in the last 111 yr andhence, it is a region with high seismic hazard.

There is uncertainty in both the continuation at depthof exposed Precordillera and Sierras Pampeanas struc-tures and in how the structures are related. Regionalseismic studies around San Juan have shown crustalseismicity to depths of 35 km related to thePrecordillera and the Sierras Pampeanas [1517]. Inthe Precordillera, this seismicity is mainly restricted tothe central and eastern subprovinces with almost noseismic activity in the western Precordillera [16,18]. Asimilar pattern of seismic activity is observed fromglobal catalogs (Fig. 1). Smalley et al. [16] havedetermined small to moderate sized earthquakes ofdepths between 5 and 35 km, which correlate with abroad diffuse NNE striking plane dipping to thenorthwest in the region between the Sierra Villicum andthe Sierra Chica de Zonda, which is interpreted as abasement structure. As these authors pointed out, thisstructure (labeled as structure b in Fig. 2b) does not

P. Alvarado, S. Beck / Earth and Planmatch the exposed east-dipping faults (structures 1 and2 in Fig. 2a) in the eastern Precordillera [16] (Fig. 2).At the surface, the eastern Precordillera has a major145-km-long east-dipping reverse fault system com-posed of several segments from north to south:Villicum, Las Tapias, Chica de Zonda and Pedernal[8,9]. Along each segment, the reverse faults uplift therange of the same name in the hanging wall (Fig. 2).Farther east, the Sierra Pie de Palo (uplift in the SierrasPampeanas) (Figs. 1 and 2) is seismically active withcrustal events located at depths between approximately10 and 35 km [15,17]. Studies by Regnier et al. [15]revealed seismicity at 2025 km beneath the Sierra Piede Palo which may represent a dcollement thatconnects the eastern Precordillera with the SierrasPampeanas structures at depth [1921] (Fig. 2b).

The only major crustal seismic event in the vicinityof San Juan studied using local, regional and teleseismicrecords is the most recent 1977 earthquake (Ms=7.4)that killed 65 people [22]. This thrust earthquake [23]was located in the Sierra Pie de Palo [22] and wascomposed of two shocks separated by 64 km and 20 swith focal depths of 17 km and 2530 km,respectively [24,25] (Fig. 1b). The geometry of thefault activated during the mainshock is still debated[2528] (Fig. 2).

The earthquake on 15 January 1944 (Ms=7.3) wasby far the most damaging. Its epicenter is locatedbetween the eastern Precordillera and the westernSierras Pampeanas [29]. The event devastated SanJuan and caused numerous deaths [30,31] (Fig. 1b). Asurface rupture of 6 to 8 km long was observedimmediately after this large earthquake in the Pre-cordillera along the La Laja fault [30,32,33] (Figs. 1band 2). The relatively short exposed rupture and theexisting middle to lower crust seismicity detected bylocal seismic studies [16] (Fig. 2) have given rise todebate about interpreting the La Laja reverse fault as themain fault activated during the 1944 earthquake or as asecondary structure [16,20,21,24,26,30,3337].

Earthquakes in 1894 (Ms=7.6) and 1952 (Ms=7.0)are generally associated with the Precordillera [35,38](Fig. 1b). Although less damaging events, their study isimportant to characterize the large crustal seismicityaround San Juan related to the Precordillera. The studyof the 1894 event using seismic records is difficultbecause of its occurrence during the early stages of theinstrumental period. In contrast, there are a large numberof seismic records for the 1944 and 1952 earthquakesthat can be analyzed to extract useful quantitativeinformation.

We have collected, digitized and analyzed mainly

617Science Letters 243 (2006) 615631long-period teleseismic records for the 1944 and 1952

etary618 P. Alvarado, S. Beck / Earth and PlanSan Juan earthquakes. We determined P-wave firstmotion focal mechanisms and carried out a depth-phaseanalysis for both events. We have also done a grid search

Fig. 2. (a) Map of the epicentral area of the 1944 and 1952 San Juan earthquaksolutions for the 1944 (Mw 7.0) and the 1952 (Mw 6.8) earthquakes (thisindicating compressional quadrants. Brackets at 31.4S show the locationearthquakes are from INPRES [29] (see Table 2). Also shown are focal mechaLa Laja fault, (2) Sierra VillicumLas Tapias, (3) North Pie de Palo lineameMolle megafracture, (7) Sierra Chica de Zonda, (8) Suture, (9) Niquizanga falineament, (a) 1944 fault scarp (La Laja), (b) 1944 fault scarp(?) (Ullum), (WSierras Pampeanas. Geological references as in (b). (b) 3D-view and schemathe 1944 earthquake focal mechanism solution and its focal depth are consistsoutheast. An error bar shows uncertainties around the focal depth for the maj(c) [25,28], (d) [15], (e) [19], (f) [15,20,21], (g) [24], (h) [10], (i) [17].Science Letters 243 (2006) 615631to determine the best focal mechanism and source depthand obtained the source time function and seismicmoment by modeling teleseismic P-waves. We have

es showing topography and major faults in the region. Focal mechanismstudy) are shown by lower hemisphere projections with dark colorsof the cross-section shown in (b). Epicenters for the 1944 and 1952nism solutions for the 1977 earthquake from [25]. Key to symbols: (1)nt, (4) Sierra Pie de Palo, (5) GuayampaLima megafracture, (6) Delult system, (10) Valle Frtil fault, (11) Sierra Pedernal, (12) LANDSAT-CP) Western-Central Precordillera, (EP) Eastern Precordillera, (SP)

tic cross-section at 31.4S along A-A in (a). The vertical projection ofent with a surface projection along the La Laja fault, which dips to theor moment release. Key to symbols and references: (a) [20,21], (b) [16],

compared these results with neotectonic studies andseismic intensity information. The study of the 1944 and1952 earthquake sources completes the characterizationof the large crustal earthquakes in the vicinity of SanJuan during the last century. This is useful for theunderstanding of the crustal earthquake backarc defor-mation related to different major geological andstructural provinces over the flat-slab Andean subduc-tion segment. It is also important for the assessment ofthe seismic hazard in this region.

2. Data and methods

We have collected paper seismograms and instru-ment responses from seismic observatories around theworld for the 1944 and 1952 earthquakes (Fig. 3a andTable 1). We have scanned and digitized all available

analog seismograms using the interactive softwareSeisDig [39] (Fig. 3b, c). We used as many records aspossible to estimate the first-motion focal mechanismfor each event. In some cases, we utilized horizontalcomponents and P-diffracted waves in order to increasethe usable records. We also included available local andregional P-wave data to place more constraints on thefocal mechanism for both events as shown in Fig. 4a forthe 1944 earthquake.

To help determine the focal mechanism and sourcedepth for both earthquakes, we identified possible depthphases and used their relative amplitudes and timing.We tested a set of possible fault planes and source depthspreviously determined by predicting the observedrelative P-wave and depth-phase amplitudes (P, pP, sP)and timing at each teleseismic station [40]. However, forearthquakes with magnitudes of near 7 it can be difficult

ect torthqua

619P. Alvarado, S. Beck / Earth and Planetary Science Letters 243 (2006) 615631Fig. 3. (a) Global seismic station distribution used in this study with respperiod vertical component paper seismogram for the 1944 (Mw 7.0) ea

distance=80.3, azimuth Az=320; see Table 1). (c) Digitized seismogramfirst arrival for this vertical component record.the 1944 and 1952 earthquake epicenters (stars). (b) Example of a long-ke recorded at station PAS in Pasadena, California, U.S.A. (epicentral

of the 1944 mainshock recorded at station PAS. Note the clear P-wave

Table 1Information for seismic stations used in this study of the 1944 and 1952 San Juan, Argentina earthquakes including the instrument responses

Event Station Comp. Lat.()

Long.()

H(m)

()

Az()

Baz()

Instrument T0(s)

Tg(s)

Damp Vm Speed(mm/min)

Take-off()

19441952 TUC z 32.25 110.83 770 75.1 323.8 143.4 Benioff 1 77 0.8 3000 30 1819441952 SJP s 18.38 66.11 80 49.5 2.9 182.5 Wenner M-1 9.8 17 1280 16 24

e Wenner M-1 9.8 14.5 1180 161944 PAS z 34.14 118.17 295 80.3 320 13.8 Benioff 1 90 0.8 3000 30 1719441952 DBNa z 52.1 5.17 2 105 37.8 238.1 Galitzin 12 12 740 30 1219441952 BRK z 37.87 112.26 49 85.3 320.1 136.2 Galitzin 12 12 0.06 600 30 1619441952 WES z 42.38 71.32 60 73.5 357.7 177.4 Benioff 1 60 0.8 3000 30 1919441952 CHR z 43.5 172.62 8 86.8 219.2 131.5 Galitzin 13 12.9 0.02 465 30 161944 SITa e 57.05 135.32 19 104.5 328.7 125.6 Wenner 13 8 0.8 1000 15 131944 COLa n 64.86 147.84 159 113.7 332.7 113.4 Benioff 1.5 0.45 0.01 8000 15 13

e Benioff 1.5 0.45 0.01 8000 151944 HUAa z 12.04 75.34 3350 20.3 340.1 162.7 Benioff 361952 HUAa n Wenner 1952 STL a n 33.44 70.64 595 2.5 222.7 43.7 Bosch-Omori 631944 BAAa n 34.6 58.48 25 9 114.7 289.3 Wiechert 63

e Wiechert 631944 MENa n 32.9 68.85 826 1.5 194.3 14.5 Bosch-Omori 631952 SLMa z 38.63 90.24 161 72.7 342.4 143.6 Macelwane-Sprengnether 2.6 1.3 15,000 30 191952 FLO z 38.8 90.37 160 72.9 342.3 239.1 Galitzin-Wilips 12 11.7 0.08 694 30 191952 STRa e 48.58 7.76 161 104.8 41.9 239.1 Galitzin 12 11.6 0.2 710 30 14

Comp. is the seismic component (z, vertical component; n, northsouth, e, eastwest), Lat. the station location latitude, Long. the station location longitude, H the station altitude, the epicentraldistance, Az the azimuth to the station, Baz the back-azimuth from the station, T0 the period of the seismometer, Tg the period of the galvanometer, Damp the damping [67], Vm the instrumentamplification, Speed the rotating drum velocity and Take-off the angle for a seismic ray leaving the source.a Used only P-wave first motion.

620P.

Alvarado,

S.Beck

/Earth

andPlanetary

ScienceLetters

243(2006)

615631

Fig. 4. Results for the 15 January 1944 San Juan earthquake. (a) Ourpreferred P-wave first-motion focal mechanism solution (strikeN45E, dip 35 to the southeast, rake 110) in lower hemisphereprojection. The solid circles are compressional first motions and theopen circles are dilatational first motions. The P-waveforms used areplotted showing the station component names for the positive (up)arrivals (Fig. 3a and Table 1). (b) Observed and synthetic long-period P-waves and source time function results from the multi-station inversion for the best focal mechanism shown in (a) using afocal depth of 11 km. (c) Multistation P-wave inversion results forthe best focal mechanism (a), using a focal depth of 20 km. Notehow the P-waves are better predicted when using a shallower focaldepth (b).

P. Alvarado, S. Beck / Earth and Planetaryto separate out the depth phases because of the durationof the source time function or the instrument response.Hence, we also did a grid-search inversion using the fullteleseismic P-waveforms to constrain the focal mecha-nism and depth.

We carried out a long-period teleseismic P-waveinversion using the multistation omnilinear techniqueof Ruff [41] (Fig. 4b). We initially assumed a focalmechanism and a source depth previously determinedwith P-wave first motions and depth-phase analysis. Asa result, we obtained the source time function andseismic moment. The technique minimizes scatter in theamplitudes, which results in a better match betweenobserved and synthetic seismograms. We used a 2-sinterval for the source time function and 40 s of record,and a crustal velocity structure of P-wave velocity6.6 km s1 and density 2.7 g cm3. We then performeda grid search for the focal mechanism and source depthby stepping through a full suit of possible strike, dipand rake values to determine the best source parametersfor each earthquake.

3. Results and discussion

3.1. The 15 January 1944 earthquake

The earthquake on 15 January 1944 that occurred ona late Saturday evening (23:49:27 GMT) was the mostdestructive event during the last century in the vicinityof San Juan, Argentina. The mainshock and anaftershock that occurred 36 h later produced the collapseof 13,000 houses and historical sites, and devastated80% of downtown San Juan [30]. Many reports wereimmediately published describing the effects of thisearthquake, eyewitness accounts, and the local geology[3033]. This earthquake caused 5000 deaths in apopulation of 80,00090,000 people. For this reason,the 1944 earthquake is considered the largest naturaldisaster in the Argentinean history [42]. Residents ofdowntown San Juan and Albardn (Figs. 1b and 2a)reported one very sharp shock of short duration [30,33].We interpolated the Modified Mercalli (MM) seismicintensities reported for 39 localities in Argentina for the1944 earthquake [30,33,43]. The intensities show asmall area of maximum MM intensity IX in the vicinityof San Juan (Fig. 1b). For completeness, we list all thereported seismic locations published for the 1944earthquake in Table 2. Most likely none of theearthquake depths are very well constrained. TheINPRES seismic location [29] lies in the region ofmaximum intensity and used local and regional seismic

621Science Letters 243 (2006) 615631data; hence, this is our preferred epicenter. From this

etaryTable 2Summary of published locations and depths for the 1944 and 1952 SanJuan, Argentina earthquakes

1944 Earthquake

Reference Lat.()

Long.()

Depth(km)

MMintensity

LPA Bulletin [45] 31.51 68.55 12 IXKadisnky-Cade [24] 31.60

0.468.500.6

Shallow IX

INPRES (on-linecatalog) [29]

31.40 68.40 30 IX

Castellanos [30] N of San JuanCity

15 IX

CALTECH PasadenaBulletin (1945) a

31.50 68.00 50 VIII

622 P. Alvarado, S. Beck / Earth and Planinformation alone it is difficult to estimate sourceparameters for this large earthquake.

All of the reports are consistent describing a 6- to8-km-long fault scarp forming with the east side up onthe La Laja reverse fault after the 1944 earthquake(Fig. 2). The La Laja fault has been interpreted as asplay or branch of the larger eastern Precordillerathrust fault system of NNE trend and west vergence,composed of several segments between 31S and32.30S [16,20] (Fig. 2a). The surface rupture whichappeared after the occurrence of the 1944 eventshowed a dip-slip displacement of 0.30 m immediatelyafter the earthquake, which increased to 0.60 m in thefollowing days [30,33]. Harrington [33] also observeda horizontal right-lateral strike-slip displacement of

Jesuit SeismologicalAsoc. Bulletin(1944) a

31.50 67.60 4050 VIII

NEIC 31.60 68.50 50 IXISS a 31.50 68.60 50 IXGutenberg andRitcher (1945) a

31.25 68.75 50 VIII

1952 Earthquake

Reference Lat.()

Long.()

Depth(km)

INPRES (on-linecatalog) [29]

31.60 68.60 30

NEIC 31.50 68.60 30ISS a 31.50 68.60 BCIS (Bureau CentralInternational deSismologie) a

31.10 67.90

Gutenberg andRitcher (1945) a

31.50 67.50 35

MM intensities for the 1944 earthquake [43] represent the MaximumModified Mercalli intensity that would correspond to each hypocentrallocation. We used the epicentral location from INPRES [29] for the1944 and the 1952 events in our study (Figs. 1b and 2).a References from reports in the ISC archive data.0.25 m on the same segment. Based on scaling rela-tionships, the expected rupture and average displace-ment for an earthquake of magnitude 7.0 might be of40 to 50 km and 1.4 m, respectively [44].

The 1944 earthquake was well recorded at tele-seismic distances and the P-wave first motions areconsistent with a thrust focal mechanism but neitherplane is well constrained. All of the teleseismic P-waverecords we obtained show a compressional first arrival.We also used horizontal seismic records which areplotted in Fig. 4a to be consistent with the conventionthat a downward motion on the seismogram correspondsto a P-wave dilatation. Local and regional componentsfrom stations in Mendoza (MEN) and Buenos Aires(BAA) in Argentina and Huancayo (HUA) in Per(Table 1) show dilatational first motions. More infor-mation reported in the LPA bulletin from the threecomponent seismic station in La Plata (LPA), Argentina,located at 34.91S and 57.93W, also indicates adilatational P-wave arrival [45]. These data from closerstations to the 1944 epicenter (Fig. 3a and Table 1) areconsistent with a thrust focal solution (Fig. 4a). Becausewe have so few records compared to modern events andof the importance of determining the focal mechanismand depth, we have used several approaches to constrainthe source parameters.

Previous depth-phase analysis results using thevertical component of the stations with epicentraldistances between 50 and 87 indicate a thrust focalmechanism of fault planes oriented in a northsouthdirection and a shallow focal depth b12 km, approxi-mately [40].

We have performed a grid-search for a range of strike,dip and rake values by inverting the data at a series offixed focal depths for the source time function using P-waves recorded on the vertical component at stationsPAS, TUC, WES, BRK and CHR. All of these recordsare observed at teleseismic distances and have clear firstarrivals (Fig. 4a). We carefully examined the predictedphases for each waveform modeling, the normalizederror between observed and synthetic amplitudes, andthe source time function results in the grid search. Westarted exploring fault planes with dip between 0 and90, rake between 30 and 150, using a grid spacing of10 and a fixed strike for different focal depths. The bestresults occur for a dip of 3040 to the east and a rake of100130. We then fixed the rake and varied the dipbetween 0 and 90, and the strike between 340 and100, in steps of 10. Acceptable solutions have a strikebetween 20 and 50 and a dip of 3040. Forsimplicity, we present amplitude-misfit error results of

Science Letters 243 (2006) 615631modeling for the best depth (11 km) using a fixed strike

of N45E in Fig. 5a and rake of 110 in Fig. 5b,respectively. Our preferred solution (source time func-tion mostly positive and best comparison betweenobserved and synthetic waveforms with normalizederror 0.53) has a strike of N45E, a dip of 35 to thesoutheast, and a rake of 110 as shown in Fig. 4b.

Another constrain used to test the focal mechanismconsisted of the S-wave amplitude ratio and polarityobserved at station SJP (Puerto Rico Observatory). Theback-azimuth from SJP to the earthquake is 182.5 (Fig.3a and Table 1), hence, the S waves are almost naturallyrotated to SV and SH waves. Both SV and SH phaseshave impulsive arrivals with opposite polarities at SJP(Fig. 6). At SJP, the ratio of the initial pulse of the SV toSH is 0.52 with SV polarity up (north) and SH polaritydown (west). Most of the focal mechanisms thatproduced the correct SV/SH ratio and P-wave firstmotion polarities have a thrust focal mechanism with astrike of 35085, a dip of 2065 to the east, and a rakeof 70140, including our preferred focal mechanismsolution (Fig. 4a).

Fig. 5. Example of the grid-search results for the focal mechanismparameters of the 1944 earthquake using the multistation P-waveinversion. (a) Map of the normalized errors between synthetic andobserved seismograms for each inversion varying the dip and rake ofthe fault plane in steps of 10, for a fixed strike of N45E and a sourcedepth of 11 km. (b) Same as (a) but varying the dip and the strike of thefault plane in steps of 10, for a fixed rake of 110. (c) Amplitude-misfit errors between observed and synthetic seismograms as afunction of fixed focal depths from the P-wave inversion assuming ourpreferred focal mechanism of strike 45, dip 35 and rake 110. Errorbar in the focal depth is based on acceptable synthetic fits to theobserved teleseismic P-waveform data.

623P. Alvarado, S. Beck / Earth and Planetary Science Letters 243 (2006) 615631Fig. 6. Plot of the SH and SV components recorded at station SJP for

the 1944 earthquake. The S waveforms are naturally rotated for thisstation (Fig. 3a).

especially with magnitudes near 7.0. There are manyexamples of intraplate earthquakes related to blindthrusts not reaching the surface despite magnitudesbetween 6.5 and 7.0 [47]. Another empirical regressioncurve indicates a maximum displacement of 2.5 m for amagnitude Mw 7.0 [44], which is similar to our estimatebased on the seismic moment and fault areas assumedabove.

We have observed that the teleseismic P-waveforminversion is not sensitive to the different epicentrallocations reported for the 1944 earthquake (Table 2).However, the hypocentral location determined usinglocal and regional seismic data by INPRES [29]

Fig. 7. Example of the source time functions for the 15 January 1944San Juan earthquake deconvolved from the simultaneous inversion ofvertical components at stations TUC, BRK, WES, PAS and CHR(Table 1) at depths from 1 to 40 km. Note the increase in ringing of thelatter part of the source time function at greater depths.

etaryWe evaluated the best focal mechanism at a seriesof fixed focal depths using P-waveform inversions(Fig. 5c). The normalized error versus focal depthcurve shows a minimum at a depth of 11 km, althoughacceptable synthetic P-waveforms occur in the depthrange between 9 km and 13 km (Figs. 2b and 5c). Theresultant source time function indicates one simplepulse with duration of 10 s (Fig. 4b). For comparison,we present the results for a focal depth of 20 km inFig. 4c which show a higher amplitude misfit, a morenegative source time function (corresponding to asmaller seismic moment and a smaller moment mag-nitude) and synthetic P-waveforms more inconsistentwith the observed waveform data.

We also investigated the behavior of the source timefunction with increasing depth. We find that at depthsgreater than 13 km, the source time function begins toexhibit the periodic ringing indicative of a depthoverestimation [46] (Fig. 7). In fact, at the best depth(11 km), the seismic moment is concentrated toward thebeginning of the deconvolved source time function(Fig. 4b). It seems likely that the local residents didfeel one single event of short duration. We concludedthat the majority of the seismic moment releaseoccurred at shallow depths with the best point sourceat 11 km, although we cannot rule out small amountsof moment release further down-dip.

The source time function for the 1944 earthquake hasone simple pulse with duration of 10 s, suggesting thatmost of the moment release occurred near the epicenter.Assuming a rupture velocity of 2.5 km s1, and a 10-sduration, we estimate that most of the seismic momentwas released within approximately 25 km of theepicenter. Depending on if the earthquake had a uni-lateral or bilateral rupture, the fault length should bebetween 25 km and 50 km, respectively. We determineda seismic moment M0=3.0110

19 N m which gives amoment magnitude Mw=7.0 (using the equationMw=0.67 log M06.0). We can calculate the averagedisplacement (D) using M0=DA. For this purpose, weused two possible fault areas (12 km25 km for aunilateral rupture and 12 km50 km for a bilateralrupture) and a rigidity of 31010 N m2, and obtainedan average slip of 3.3 m and 1.7 m, respectively.

According to Wells and Coppersmith [44], anempirical scaling relationship between the momentmagnitude and the surface rupture length (SRL) isgiven by Mw=5.08+1.16 log (SRL). This relationshippredicts a surface rupture length of 45 km for the 1944event rather than the 68 km observed. However, thereis a large amount of scatter in this relationship for

624 P. Alvarado, S. Beck / Earth and Plancontinental shallow thrust or reverse fault earthquakesScience Letters 243 (2006) 615631(Table 2) is consistent with the east dipping fault-plane

etarygeometry suggested by the observed surface rupture(Fig. 2), and by our focal mechanism solution (Fig. 4a).Our results are consistent with the 1944 earthquakerupturing upward mostly or in part on the La Laja fault.In fact, the fault parameters measured on the 1944 faultscarp by Perucca and Paredes [36] yield a strike ofN45E, a dip between 25 and 45 to the southeast anda rake of 90, as measured in Neogene strata and aQuaternary terrace. These parameters are comparableto those observed by INPRES [48] and Costa et al.[26]. Our solution is also consistent with the sense ofcoseismic displacement vectors (vertical thrust dip-slipand horizontal right-lateral strike-slip) observed on theLa Laja fault after the 1944 earthquake [30,33]. Hence,we suggest the southeast-dipping nodal plane (strikeN45E, dip 35, rake 110) is the fault plane thatruptured and the northwest-dipping plane (strike 201,dip 57, rake 76) is the auxiliary plane in the focalmechanism solution.

Besides the clear 68-km-long coseismic rupture[30,32,33], Castellanos [30] describes another less well-developed break further south in the vicinity of Ullumrelated to the 1944 earthquake. It was observed in softsediments of 1.5-m vertical displacement with orien-tation along the strike of the La Laja fault (Fig. 2a).Harrington [33] also pointed out that the La Laja faultmight have ruptured for more than 20 km during the1944 event but the presence of sediments andagricultural sites makes its trace confusing. In addition,Smalley et al. [16] have observed a LANDSATlineament of N45E-trending near La Laja fault thatcontinues to the south ending at the Villicum-Zondasegment in the eastern Precordillera fault (Fig. 2a).Taken together this suggests that the La Laja fault mightbe 40 to 50 km long and capable of a magnitude 7.0earthquake (Fig. 2).

Based on the regional crustal seismicity around SanJuan, the 50-km focal depth reported by the ISS for the1944 earthquake (Table 2), and the 1977 earthquakesource studies, several authors have suggested a deepsource (2030 km) for the 1944 earthquake[16,20,21,24,34,37]. Smalley et al. [16] investigatedthe local seismicity in this area and found an activeplanar structure dipping 35 to the northwest between 19km and 35 km depth and more diffuse seismicitybetween 5 km and 15 km beneath the easternPrecordillera (Fig. 2). Thus, these authors speculatedthat the 1944 event probably had a deep crustal sourcelocated to the west of the eastern Precordillera, and mayhave ruptured in multiple faults, like the 1977earthquake in the Sierras Pampeanas [24,25] (Fig. 2a),

P. Alvarado, S. Beck / Earth and Planwithout generating a main surface rupture and inter-preting the La Laja fault as a secondary feature[16,34]. For focal depths greater than 15 km and faultplanes dipping 35 to the northwest, which are 22less inclined than our estimated dip (57) for theauxiliary fault plane, we find significantly worse fitsto the observed P-waveforms (Figs. 4b,c, 5 and 7). Infact, we note that the dip is the best constrainedparameter in our focal mechanism grid-search with anuncertainty of 5 (Fig. 5a, b). The simple sourcetime function (Fig. 4b) also indicates that the 1944earthquake occurred most likely as a single event.Hence, we suggest the 1944 earthquake is unlikely tohave ruptured on the deep west-dipping fault and as acomplex multiple event.

Siame et al. [20,21] consider the La Laja fault as partof the N40E-trending fault system that branches fromthe main eastern Precordillera fault composed of severalsegments. The same authors used structural geology,geomorphology and exposure age dating analysis in thisarea to suggest the 1944 earthquakemay have ruptured atdepth on the VillicumLas Tapias segment. Accordingto their study, this segment is a 65-km-long reverse fault,oriented in a N20E direction with a dip of 60 (15) tothe east at the surface but becoming much shallower at adepth of 2025 km [20,21] (Fig. 2). The strike N20E isconsistent with the waveforms but not the high angle(60) dip. We note that if we assume this focalmechanism with a high angle east dipping fault planefor modeling, larger errors result between our observedand synthetic P-waveforms (Fig. 5). These authors alsosuggested a seismic source for the 1944 earthquakelocated at 2025 km depth but with the focus to the eastof the eastern Precordillera in a flat-lying part of theVillicumLas Tapias fault that did not produce anysurface rupture along this fault and distributed thedeformation in secondary fractures [20,21]. Our resultsfor an inversion of the P-waves with the moment releaseat depths N13 km (Figs. 4c, 5c and 7) do not produce thebest results. Our evidences indicate that it is more likelythat it ruptured on a shallower thrust fault with ageometry similar to that observed for the La Laja fault.However, it is possible that the 1944 event also producedcoseismic folding and faulting deformation and relatedground features. The 1971 (Mw 6.7) San Fernando,California earthquake is another example of a shallow(12 km) thrust event that ruptured spreading out alongseveral minor structures as it approached the surface[50,51]. Given that we do not know the subsurfacegeometry of active faults beneath the Tulum valley(Fig. 2) from supporting industry oil-well or seismicreflection data we cannot rule out rupture on the Vil-

625Science Letters 243 (2006) 615631licumLas Tapias segment. But if the seismic moment

dip of 3090 and a rake of 570. We used these focalmechanisms in the depth-phase analysis. The relativedepth phase amplitudes were well predicted when usinga roughly northeast fault plane of a steep dip of at least50 with an important left-lateral strike-slip component(Fig. 8a). We estimated a focal depth of 1012 km fromthe predicted depth-phase arrivals.

Teleseismic records at stations TUC, FLO, WES,BRK and CHR (Fig. 3a and Table 1) were used in thesimultaneous inversion for the source time functionfollowing the same methodology used for the 1944event. In this grid search, we explored a wide range offault-plane focal mechanisms and focal depths. Fig. 9

etary Science Letters 243 (2006) 615631release occurred at a shallow depth then wewould expectsome surface rupture on the VillicumLas Tapias faultwhich is clearly not observed [20,21,26,52,53].

Our preferred solution is consistent with the geologicmodel proposed by Ramos et al. [19] between thePrecordillera and the Sierra Pie de Palo (Fig. 2). Theseauthors inferred one major east-dipping thrust basementfault extending up to 1015 km depth beneath theTulum valley with several thrust branches as it becomesshallower. We note that the crustal seismicity observedin the same region by Smalley et al. between 5 km and15 km in a diffuse distribution [16] may be related tothe same east-dipping thrust structures. Our interpreta-tion, together with the already recognized deeper activebasement structure of a 35-northwest dip by Smalley etal. [16] (Fig. 2), can have direct implications in theseismic risk around San Juan from both concealed andexposed faults. A comparable example was evidenced inthe San Fernando Valley, California after the 1971 and1994 earthquakes [51,54].

3.2. The 11 June 1952 earthquake

After the 1944 earthquake San Juan was rebuiltquickly using a modern building code for earthquake-resistant design [55]. Another damaging crustal earth-quake occurred in 1952 [29]. However, the smallermagnitude of this event, its location further away fromdensely populated centers, and improved buildingsprevented widespread destruction.

The 1952 earthquake occurred on June 11 at00:31:37 GMT. Its epicenter was located 15 km tothe SSW of downtown San Juan, in the vicinity of theSierra Chica de Zonda (Figs. 1b and 2 and Table 2).Almost 1000 houses collapsed or suffered severedamage in Carpintera, Pocito, and Zonda localities,among others, reaching a maximum MM intensity VIIIin this area [43] (Fig. 1b). Beyond this area, damage wasminor to moderate and mostly limited to old non-earthquake-proof design constructions as shown by ourinterpolation for the MM intensities reported inArgentina [43]. The mainshock caused 1 death inCarpintera (Fig. 2) and 14 injured people [56,57]. Thereports indicate that the local residents felt 11 after-shocks during that night and point out the strongest oneat 03:00:33 GMT [57,58].

We used first motion P-waves (stations STL, HUA,SJP, SLM, FLO, WES, TUC, BRK, CHR), P-diffractedwaves (stations STR, DBN), and the SV/SH ratio (0.49)and positive polarities (SV north and SH east) observedat station SJP as input to estimate the focal mechanism.

626 P. Alvarado, S. Beck / Earth and PlanIn general, possible solutions have a strike of 0120, aFig. 8. (a) Lower hemisphere P-wave first-motion focal mechanismplot for the 11 June 1952 San Juan earthquake. First motionconvention as in Fig. 4a. The P-waveforms used are also plottedshowing the station component names for the positive (up) arrivals. (b)Observed and synthetic long-period P-waves, focal mechanism, and

source time function results from the multistation inversion for the1952 San Juan earthquake. Station information is listed in Table 1.

earthquake contributes to the next on an adjacentsegment [61,62]. It is likely that both earthquakes arerelated to the interaction of the Precordillera and Sierras

Fig. 9. Example of the grid-search results for the focal mechanismparameters of the 1952 earthquake using the multistation P-waveinversion. (a) Map of the normalized errors between synthetic andobserved seismograms for each inversion varying the dip and the rakeof the fault plane, in steps of 10, and fixing the strike at 40 and source

627etary Science Letters 243 (2006) 615631shows the results of the normalized errors betweenobserved and synthetic data obtained for each inversion.In the first step, we varied the dip from 10 to 90 andthe rake from 0 to 120 in steps of 10, respectively,maintaining the strike at N40E (Fig. 9a). Secondly, wefixed the rake at 30 and varied the dip as before and thestrike from 340 through 90, using a grid spacing of10. The minimum amplitude-misfit errors, best com-parison between predicted and observed P-waveforms,and source time function mainly positive, are found for afocal mechanism with a fault plane of strike of 2040,dip of 7080 to the southeast, and rake of 2040(Figs. 8b and 9a,b).

Fig. 9c shows the behavior of our preferred focalmechanism solution (strike N40E, dip 75, rake 30)when varying the source depth from 1 km to 25 km, insteps of 1 km. The best fit between observed andsynthetic data (amplitude-misfit error of 0.402) occurs ata point source depth of 12 km, but acceptable fits resultbetween 10 km and 13 km focal depth, which is inagreement with the depth-phase identification. Inaddition, solutions with depths larger than 15 kmproduce a ringing in the source time function indicativeof a depth overestimation [46].

Our best results (Fig. 8b) indicate a single pulse forthe source time function with a duration of 6 to 8 s andseismic moment M0=2.210

19 N m, which corre-sponds to a moment magnitude Mw=6.8.

Although there are uncertainties in the epicentrallocation of the 1952 earthquake, we can speculate on theassociation of this event with the local geology.Considering the epicentral information from INPRES,NEIC and ISS (Table 2) and the maximum MMintensities for this earthquake [43] (Fig. 1b), it ispossible that it is related to the structures in the SierraChica de Zonda in the eastern Precordillera as suggestedby [26,48,52] (Fig. 2). This portion of the easternPrecordillera has shown little crustal seismicity in a verydisperse pattern during local seismic deployments[16,59]. However, neotectonic studies of the interactionbetween the Precordillera and the Sierras Pampeanas byMartinez and Perez [60] have identified important left-lateral horizontal displacements along active faults ofNNE orientation in this sector of the Precordillera. TheNE striking plane in our focal mechanism solution isconsistent with this style of deformation.

Interestingly, both the 1944 and the 1952 earthquakesseem to be related to the active eastern Precordillerasystem and separated by 8 yr and a distance of about 35km. Recent studies about earthquake interaction, basedon the Coulomb stress transference and seismicity rate

P. Alvarado, S. Beck / Earth and Planchanges, have proposed that the occurrence of onedepth at 12 km. (b) Same as (a) but varying the dip and strike in stepsof 10, while fixing the rake at 30. (c) Amplitude-misfit errorsbetween observed and synthetic seismograms versus focal depths fromthe P-wave inversion assuming our preferred focal mechanism of

strike 40, dip 75 and rake 30. Error bar in the focal depth is based onacceptable synthetic fits to the observed teleseismic P-waveform data.

etaryPampeanas fault systems. Our constrains might help totest this possibility which requires further study.

4. Conclusions

We have analyzed waveforms for the historicalearthquakes in 1944 and 1952 in San Juan, Argentina.We have used a combination of several seismictechniques to constrain the source parameters for bothcrustal events. We obtained a thrust focal mechanism(strike 45, dip 35, rake 110), a seismic momentM0=3.0110

19 N m, a moment magnitude Mw=7.0and a focal depth of 11 km for the 1944 earthquake. Theresults for the 1952 event indicate an oblique focalmechanism solution (strike 40, dip 75, rake 30), withM0=2.2010

19 N m, Mw=6.8 and focal depth of12 km. We found one single pulse for the source timefunction of each event with duration of 10 s and 8 s,respectively. Although it is difficult to make a formalerror analysis, we estimate the uncertainty in the strike,dip and rake, and focal depth by trial-and-errorsensitivity tests using teleseismic P-waveform model-ing. This gives uncertainties of about 15 in the strikeand rake, and 5 in the dip. The same analysis for thefocal depth gives a total range of uncertainty of 5 km,approximately.

Based on our results and the geological observations,we think that the 1944 earthquake is a thrust event thatoccurred on a northeast striking plane, dipping 35 tothe southeast. The 1952 earthquake, of a more obliquefocal mechanism, is consistent with the active NNEfaults of the eastern Precordillera that show left-lateralneotectonic displacements [60] in the Sierra Chica deZonda segment, but a particular association to the activefaults in the area is difficult (Fig. 2). We note that therelocation study of some of the larger aftershocksmentioned in the historical reports could help toconstrain the fault plane activated during each earth-quake. Unfortunately, we do not have seismic recordsfor them.

We find good agreement between the exposed LaLaja thrust fault and the seismogenic fault that producedthe 1944 Mw=7.0 earthquake. This is mainly based onthe following reasons:

The focal depth and geometry of the east-dippingthrust fault plane solutions are consistent with theparameters (strike, dip and rake) observed for the LaLaja fault.

The east-dipping focal mechanism fault planesolution is compatible with the sense of the slip

628 P. Alvarado, S. Beck / Earth and Planvectors (thrust and right-lateral) observed at thesurface on the La Laja fault scarp during the 1944earthquake.

Given the shallow focal depth (11 km) of this largeearthquake, it is likely to have occurred on a faultplane that ruptured to or near the surface.

Some reports of disruption at Ullum during the 1944earthquake might be an indication of additionalactivity along the same (La Laja) fault.

The La Laja fault has been predicted to continue tothe south for about 40 km until it meets the VillicumZonda fault segment in the eastern Precordillera[16,33,52], although the presence of unconsolidatedsediments and agriculture sites to the south of the LaLaja fault scarp make it difficult to map (Fig. 2a).

There were no other major faults that had anycoseismic rupture during the 1944 earthquake.

Our results for the 1944 earthquake show that thestyle of deformation is similar to that observed in theSierras Pampeanas [13,19] where the basement blocksare mainly uplifted by east-dipping faults (Fig. 2).Although our source time function results predict theactivation of one main fault (one simple source withone pulse of moment release), the 1944 earthquakemay have caused coseismic deformation in theassociated folding and terraces around the La Lajafault [37,49,53]. Considering the deep west-dippingactive structure recognized by Smalley et al. inprevious seismic studies [16] and the activation ofan east-dipping thrust fault that might have reachedthe surface during the 1944 earthquake (Fig. 2), thereis a high seismic hazard posed to San Juan from bothconcealed and exposed faults related to the frontal partof the eastern Precordillera.

The source characterization of the 1944 (Mw=7.0)and 1952 (Mw=6.8) earthquakes completes the study ofthe largest damaging crustal earthquakes during the lastcentury in the vicinity of San Juan. Both events exhibit asimple source time function, a shorter duration and ashallower focal depth than the most recent 1977(Mw=7.5) multiple shock earthquake in the SierrasPampeanas [25]. This backarc deformation is consistentwith the two dcollement levels proposed by [15,63,64]in the vicinity of San Juan. The crustal earthquakes in1944 (this study) and 1977 [25] both showed predom-inantly thrust focal mechanisms (Fig. 2), but theydiffered in their focal depths and in the complexity oftheir coseismic ruptures. Since the occurrence of largeearthquakes is sporadic, the study of historical seismo-grams can help in identifying active structures that areimportant in the mapping of the seismic hazard in this

Science Letters 243 (2006) 615631area.

629P. Alvarado, S. Beck / Earth and Planetary Science Letters 243 (2006) 615631Acknowledgments

We thank Jim Dewey, Keith Koper, Rick McKenzie,Tony Monfret, Luis Rivera, John Ebel, Victor Cruz andBrian Ferris, for helping us to obtain paper copies of thehistoric earthquakes. Maureen Aspreen from theInternational Seismological Centre (ISC) provided ususeful historic documents. We also thank Alberto Sezfor helping us to obtain copies of old Argentineannewspapers. We are grateful to Carlos Costa forinteresting discussions. We acknowledge Jim Dewey,and two anonymous reviewers for their critical reviewsand suggestions to improve this paper. The material isbased on work partially funded by NSF grant EAR-9811878 and awarded by LASPAU-University ofHarvard under Lewis A. Tyler Trustees' Fund Award(2003), the National PERISHIP Dissertation Award(2004), and CHEVRON-TEXACO Summer Scholar-ships 2004 and 2005 to P. Alvarado.

Maps were generated using Generic Mapping Toolssoftware [65]. We also used SAC2000 [66] to processseismic data and used seismological data from the ISC,and the USGS-NEIC Bulletins. Mauro Sez helped us insetting up the database for this project and with thefigures in this paper.

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631P. Alvarado, S. Beck / Earth and Planetary Science Letters 243 (2006) 615631

Source characterization of the San Juan (Argentina) crustal earthquakes of 15 January 1944 (Mw .....Introduction and regional tectonicsData and methodsResults and discussionThe 15 January 1944 earthquakeThe 11 June 1952 earthquake

ConclusionsAcknowledgmentsReferences