Earth and Planetary Science Letters 243 (2006) 615 – 631
www.elsevier.com/locate/epsl
Source characterization of the San Juan (Argentina) crustal
earthquakes of 15 January 1944 (Mw 7.0) and 11 June 1952 (Mw 6.8)
Patricia Alvarado a,b,⁎, Susan Beck a
b
a
Department of Geosciences, University of Arizona, USA
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 2006
Available 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 belt
regions in the world. Four large damaging crustal earthquakes (1894, 1944, 1952 and 1977) occurred during the last 111 yr between
30°S and 32°S. We have determined the source parameters for two of these important earthquakes, the 1944 and 1952 events, using
historic seismic records. The earthquake on 15 January 1944 had an epicentral location between the eastern thin-skinned
Precordillera fold and thrust belt and the thick-skinned Sierras Pampeanas basement-cored uplifts. The 11 June 1952 earthquake
occurred in the eastern Precordillera about 35 km southwest of the 1944 epicenter location. The P-wave first motions, long-period
teleseismic P waveform modeling, and SV/SH amplitude ratio indicate a thrust focal mechanism for the 1944 event (strike N45°E,
dip 35° to the southeast, and rake 110°) with M0 = 3.01 × 1019 N m and Mw = 7.0. The 1952 earthquake focal mechanism solution
indicates a more oblique mechanism (strike N40°E, dip 75° to the southeast, and rake 30°) with M0 = 2.20 × 1019 N m and Mw = 6.8.
Both the 1944 and 1952 earthquakes have focal depths b 12 km and simple source time functions with one pulse of moment release
with durations of 10 s and 8 s, respectively. Both the shallow focal depth and the east-dipping fault plane in the focal mechanism
solution for the 1944 earthquake are consistent with the parameters observed along the La Laja fault in the frontal part of the
eastern Precordillera that generated a 6–8-km-long coseismic surface rupture. The 1952 earthquake focal mechanism solution and
its shallow source depth suggest it is related to faults in the eastern Precordillera, but a particular fault association is difficult. The
1944 earthquake was clearly the most destructive event because its proximity to the most populated area in San Juan, large size and
shallow 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 28°S
and 33°S results from the convergence between the
⁎ Corresponding author. Department of Geosciences, University of
Arizona, 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.2006.01.015
oceanic Nazca plate and the continental South American
plate at a rate of 6.3 cm yr− 1 [1] (Fig. 1a). In this region,
the Nazca slab subducts to ∼ 100 km depth and then
extends near horizontal several hundred kilometers to
the east before descending into the mantle. The
flattening of the subducted Nazca plate initiated at
8–10 Ma [2,3]. This process is linked to the shut off of
the volcanic arc, the eastern migration of the thin-
616
P. Alvarado, S. Beck / Earth and Planetary Science Letters 243 (2006) 615–631
Fig. 1. (a) Location map showing the PDE-NEIC seismicity during the period 1994–2004. Contours indicate approximate depth to the top of the
subducted Nazca plate [5]. Also shown are the GPS plate convergence velocity [1] and location of the Juan Fernández Ridge (JFR). (b) Tectonic
provinces, large historical earthquakes from INPRES [29] (stars) (Table 2), and contour map of Modified Mercalli (MM) seismic intensity points for
the 1944 (Mw 7.0) (dashed lines) and 1952 (Mw 6.8) (dot-dashed lines) earthquakes. Black dots are locations of MM intensity values in the vicinity of
San Juan for both crustal earthquakes [30,33,43]. Note the maximum MM intensity IX encircles the cities of San Juan and Albardón for the 1944
event. The arrow indicates the 6–8-km-long coseismic surface rupture along the La Laja thrust fault caused by the 1944 earthquake [30]. Also shown
are the foreshock (F) and mainshock (M) of the most recent 1977 (Mw 7.5) Caucete earthquake [24,25,29].
skinned Precordillera and the uplift of the Sierras
Pampeanas basement-cored blocks in the broad region
of the foreland [2,3] (Fig. 1). Although there is a large
number of earthquakes with depths of ∼ 100 km which
define the flat slab geometry in the Andean backarc at
about 31°S and correlate with the projection of the
subducting Juan Fernández Ridge [4–7] (Fig. 1a), the
most destructive earthquakes are the moderate-to-large
P. Alvarado, S. Beck / Earth and Planetary Science Letters 243 (2006) 615–631
events generated in the South American upper plate with
depths b 35 km. Large crustal earthquakes in 1894,
1944, 1952, and 1977 associated with the Precordillera
and the Sierras Pampeanas have caused extensive
damage in the vicinity of San Juan, Argentina (Fig. 1b).
The Precordillera is a thin-skinned fold and thrust
belt that makes up the foothills of the high elevation
Andes (Fig. 1). Based on the deposits, timing and style
of deformation, it is divided in the western, central and
eastern Precordillera [8,9] (Fig. 2). The western and
central Precordillera are a series of mainly Paleozoic
ranges and linear valleys bounded by roughly N–S east
verging reverse faults [8,10,11] (Fig. 2). In contrast, the
eastern Precordillera is made up of Paleozoic, Triassic
and Cenozoic rocks with both thin-skinned and thickskinned structures and west verging thrust faults [8,12]
(Fig. 2). Farther east, the Sierras Pampeanas tectonic
province comprises crystalline Precambrian–Paleozoic
basement blocks uplifted, tilted and folded by thrust
faults with a similar style to the Laramide structures in
the western U.S.A. [13] (Fig. 2). The Precordillera and
the Sierras Pampeanas are separated by broad internally
drained retroarc basins that include the Bermejo and
Tulum Valleys, which receive much of their sediments
from the Andes but whose structures are controlled by
the Sierras Pampeanas active tectonics [12,14]. The
Tulum Valley contains San Juan, a city with a population of about ∼ 350,000, and other smaller communities (Figs. 1b and 2). This transition between the
Precordillera and the Sierras Pampeanas structures
corresponds to the region of many of the large
devastating crustal earthquakes in the last 111 yr and
hence, it is a region with high seismic hazard.
There is uncertainty in both the continuation at depth
of exposed Precordillera and Sierras Pampeanas structures and in how the structures are related. Regional
seismic studies around San Juan have shown crustal
seismicity to depths of ∼ 35 km related to the
Precordillera and the Sierras Pampeanas [15–17]. In
the Precordillera, this seismicity is mainly restricted to
the central and eastern subprovinces with almost no
seismic activity in the western Precordillera [16,18]. A
similar pattern of seismic activity is observed from
global catalogs (Fig. 1). Smalley et al. [16] have
determined small to moderate sized earthquakes of
depths between 5 and 35 km, which correlate with a
broad diffuse N–NE striking plane dipping to the
northwest in the region between the Sierra Villicum and
the Sierra Chica de Zonda, which is interpreted as a
basement structure. As these authors pointed out, this
structure (labeled as structure b in Fig. 2b) does not
match the exposed east-dipping faults (structures 1 and
617
2 in Fig. 2a) in the eastern Precordillera [16] (Fig. 2).
At the surface, the eastern Precordillera has a major
145-km-long east-dipping reverse fault system composed of several segments from north to south:
Villicum, Las Tapias, Chica de Zonda and Pedernal
[8,9]. Along each segment, the reverse faults uplift the
range of the same name in the hanging wall (Fig. 2).
Farther east, the Sierra Pie de Palo (uplift in the Sierras
Pampeanas) (Figs. 1 and 2) is seismically active with
crustal events located at depths between approximately
10 and 35 km [15,17]. Studies by Regnier et al. [15]
revealed seismicity at 20–25 km beneath the Sierra Pie
de Palo which may represent a décollement that
connects the eastern Precordillera with the Sierras
Pampeanas structures at depth [19–21] (Fig. 2b).
The only major crustal seismic event in the vicinity
of San Juan studied using local, regional and teleseismic
records 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 was
composed of two shocks separated by 64 km and 20 s
with focal depths of ∼ 17 km and ∼ 25–30 km,
respectively [24,25] (Fig. 1b). The geometry of the
fault activated during the mainshock is still debated
[25–28] (Fig. 2).
The earthquake on 15 January 1944 (Ms = 7.3) was
by far the most damaging. Its epicenter is located
between the eastern Precordillera and the western
Sierras Pampeanas [29]. The event devastated San
Juan and caused numerous deaths [30,31] (Fig. 1b). A
surface rupture of 6 to 8 km long was observed
immediately after this large earthquake in the Precordillera along the La Laja fault [30,32,33] (Figs. 1b
and 2). The relatively short exposed rupture and the
existing middle to lower crust seismicity detected by
local seismic studies [16] (Fig. 2) have given rise to
debate about interpreting the La Laja reverse fault as the
main fault activated during the 1944 earthquake or as a
secondary structure [16,20,21,24,26,30,33–37].
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 is
important to characterize the large crustal seismicity
around San Juan related to the Precordillera. The study
of the 1894 event using seismic records is difficult
because of its occurrence during the early stages of the
instrumental period. In contrast, there are a large number
of seismic records for the 1944 and 1952 earthquakes
that can be analyzed to extract useful quantitative
information.
We have collected, digitized and analyzed mainly
long-period teleseismic records for the 1944 and 1952
618
P. Alvarado, S. Beck / Earth and Planetary Science Letters 243 (2006) 615–631
Fig. 2. (a) Map of the epicentral area of the 1944 and 1952 San Juan earthquakes showing topography and major faults in the region. Focal mechanism
solutions for the 1944 (Mw 7.0) and the 1952 (Mw 6.8) earthquakes (this study) are shown by lower hemisphere projections with dark colors
indicating compressional quadrants. Brackets at 31.4°S show the location of the cross-section shown in (b). Epicenters for the 1944 and 1952
earthquakes are from INPRES [29] (see Table 2). Also shown are focal mechanism solutions for the 1977 earthquake from [25]. Key to symbols: (1)
La Laja fault, (2) Sierra Villicum–Las Tapias, (3) North Pie de Palo lineament, (4) Sierra Pie de Palo, (5) Guayampa–Lima megafracture, (6) Del
Molle megafracture, (7) Sierra Chica de Zonda, (8) Suture, (9) Niquizanga fault system, (10) Valle Fértil fault, (11) Sierra Pedernal, (12) LANDSAT
lineament, (a) 1944 fault scarp (La Laja), (b) 1944 fault scarp(?) (Ullum), (W-CP) Western-Central Precordillera, (EP) Eastern Precordillera, (SP)
Sierras Pampeanas. Geological references as in (b). (b) 3D-view and schematic cross-section at 31.4°S along A-A′ in (a). The vertical projection of
the 1944 earthquake focal mechanism solution and its focal depth are consistent with a surface projection along the La Laja fault, which dips to the
southeast. An error bar shows uncertainties around the focal depth for the major moment release. Key to symbols and references: (a) [20,21], (b) [16],
(c) [25,28], (d) [15], (e) [19], (f) [15,20,21], (g) [24], (h) [10], (i) [17].
San Juan earthquakes. We determined P-wave first
motion focal mechanisms and carried out a depth-phase
analysis for both events. We have also done a grid search
to determine the best focal mechanism and source depth
and obtained the source time function and seismic
moment by modeling teleseismic P-waves. We have
P. Alvarado, S. Beck / Earth and Planetary Science Letters 243 (2006) 615–631
compared these results with neotectonic studies and
seismic intensity information. The study of the 1944 and
1952 earthquake sources completes the characterization
of the large crustal earthquakes in the vicinity of San
Juan during the last century. This is useful for the
understanding of the crustal earthquake backarc deformation related to different major geological and
structural provinces over the flat-slab Andean subduction segment. It is also important for the assessment of
the seismic hazard in this region.
2. Data and methods
We have collected paper seismograms and instrument responses from seismic observatories around the
world for the 1944 and 1952 earthquakes (Fig. 3a and
Table 1). We have scanned and digitized all available
619
analog seismograms using the interactive software
SeisDig [39] (Fig. 3b, c). We used as many records as
possible to estimate the first-motion focal mechanism
for each event. In some cases, we utilized horizontal
components and P-diffracted waves in order to increase
the usable records. We also included available local and
regional P-wave data to place more constraints on the
focal mechanism for both events as shown in Fig. 4a for
the 1944 earthquake.
To help determine the focal mechanism and source
depth for both earthquakes, we identified possible depth
phases and used their relative amplitudes and timing.
We tested a set of possible fault planes and source depths
previously determined by predicting the observed
relative P-wave and depth-phase amplitudes (P, pP, sP)
and timing at each teleseismic station [40]. However, for
earthquakes with magnitudes of near 7 it can be difficult
Fig. 3. (a) Global seismic station distribution used in this study with respect to the 1944 and 1952 earthquake epicenters (stars). (b) Example of a longperiod vertical component paper seismogram for the 1944 (Mw 7.0) earthquake recorded at station PAS in Pasadena, California, U.S.A. (epicentral
distance Δ = 80.3°, azimuth Az = 320°; see Table 1). (c) Digitized seismogram of the 1944 mainshock recorded at station PAS. Note the clear P-wave
first arrival for this vertical component record.
620
Table 1
Information for seismic stations used in this study of the 1944 and 1952 San Juan, Argentina earthquakes including the instrument responses
Station
Comp.
1944–1952
1944–1952
TUC
SJP
1944
1944–1952
1944–1952
1944–1952
1944–1952
1944
1944
PAS
DBN a
BRK
WES
CHR
SIT a
COL a
1944
1952
1952
1944
HUA a
HUA a
STL a
BAA a
1944
1952
1952
1952
MEN a
SLM a
FLO
STR a
z
s
e
z
z
z
z
z
e
n
e
z
n
n
n
e
n
z
z
e
Lat.
(°)
Long.
(°)
H
(m)
Δ
(°)
Az
(°)
Baz
(°)
Instrument
T0
(s)
Tg
(s)
Damp
Benioff
Wenner M-1
Wenner M-1
Benioff
Galitzin
Galitzin
Benioff
Galitzin
Wenner
Benioff
Benioff
Benioff
Wenner
Bosch-Omori
Wiechert
Wiechert
Bosch-Omori
Macelwane-Sprengnether
Galitzin-Wilips
Galitzin
1
9.8
9.8
1
12
12
1
13
13
1.5
1.5
–
–
–
–
–
–
2.6
12
12
77
17
14.5
90
12
12
60
12.9
8
0.45
0.45
–
–
–
–
–
–
1.3
11.7
11.6
0.8
–
–
0.8
–
0.06
0.8
0.02
0.8
0.01
0.01
–
–
–
–
–
–
–
0.08
0.2
32.25
18.38
− 110.83
− 66.11
770
80
75.1
49.5
323.8
2.9
143.4
182.5
34.14
52.1
37.87
42.38
− 43.5
57.05
64.86
− 118.17
5.17
− 112.26
− 71.32
172.62
− 135.32
− 147.84
295
2
49
60
8
19
159
80.3
105
85.3
73.5
86.8
104.5
113.7
320
37.8
320.1
357.7
219.2
328.7
332.7
13.8
238.1
136.2
177.4
131.5
125.6
113.4
− 12.04
− 75.34
3350
20.3
340.1
162.7
− 33.44
− 34.6
− 70.64
− 58.48
595
25
2.5
9
222.7
114.7
43.7
289.3
− 32.9
38.63
38.8
48.58
− 68.85
− 90.24
− 90.37
7.76
826
161
160
161
1.5
72.7
72.9
104.8
194.3
342.4
342.3
41.9
14.5
143.6
239.1
239.1
Vm
3000
1280
1180
3000
740
600
3000
465
1000
8000
8000
–
–
–
–
–
–
15,000
694
710
Speed
(mm/min)
Take-off
(°)
30
16
16
30
30
30
30
30
15
15
15
–
–
–
–
–
–
30
30
30
18
24
17
12
16
19
16
13
13
36
63
63
63
63
19
19
14
Comp. is the seismic component (z, vertical component; n, north–south, e, east–west), Lat. the station location latitude, Long. the station location longitude, H the station altitude, Δ the epicentral
distance, 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 instrument
amplification, Speed the rotating drum velocity and Take-off the angle for a seismic ray leaving the source.
a
Used only P-wave first motion.
P. Alvarado, S. Beck / Earth and Planetary Science Letters 243 (2006) 615–631
Event
P. Alvarado, S. Beck / Earth and Planetary Science Letters 243 (2006) 615–631
621
to separate out the depth phases because of the duration
of the source time function or the instrument response.
Hence, we also did a grid-search inversion using the full
teleseismic P-waveforms to constrain the focal mechanism and depth.
We carried out a long-period teleseismic P-wave
inversion using the multistation omnilinear technique
of Ruff [41] (Fig. 4b). We initially assumed a focal
mechanism and a source depth previously determined
with P-wave first motions and depth-phase analysis. As
a result, we obtained the source time function and
seismic moment. The technique minimizes scatter in the
amplitudes, which results in a better match between
observed and synthetic seismograms. We used a 2-s
interval for the source time function and 40 s of record,
and a crustal velocity structure of P-wave velocity
6.6 km s− 1 and density 2.7 g cm− 3. We then performed
a grid search for the focal mechanism and source depth
by stepping through a full suit of possible strike, dip
and rake values to determine the best source parameters
for each earthquake.
3. Results and discussion
3.1. The 15 January 1944 earthquake
Fig. 4. Results for the 15 January 1944 San Juan earthquake. (a) Our
preferred P-wave first-motion focal mechanism solution (strike
N45°E, dip 35° to the southeast, rake 110°) in lower hemisphere
projection. The solid circles are compressional first motions and the
open circles are dilatational first motions. The P-waveforms used are
plotted showing the station component names for the positive (up)
arrivals (Fig. 3a and Table 1). (b) Observed and synthetic longperiod P-waves and source time function results from the multistation inversion for the best focal mechanism shown in (a) using a
focal depth of 11 km. (c) Multistation P-wave inversion results for
the best focal mechanism (a), using a focal depth of 20 km. Note
how the P-waves are better predicted when using a shallower focal
depth (b).
The earthquake on 15 January 1944 that occurred on
a late Saturday evening (23:49:27 GMT) was the most
destructive event during the last century in the vicinity
of San Juan, Argentina. The mainshock and an
aftershock that occurred 36 h later produced the collapse
of ∼ 13,000 houses and historical sites, and devastated
80% of downtown San Juan [30]. Many reports were
immediately published describing the effects of this
earthquake, eyewitness accounts, and the local geology
[30–33]. This earthquake caused ∼ 5000 deaths in a
population of 80,000–90,000 people. For this reason,
the 1944 earthquake is considered the largest natural
disaster in the Argentinean history [42]. Residents of
downtown San Juan and Albardón (Figs. 1b and 2a)
reported one very sharp shock of short duration [30,33].
We interpolated the Modified Mercalli (MM) seismic
intensities reported for 39 localities in Argentina for the
1944 earthquake [30,33,43]. The intensities show a
small area of maximum MM intensity IX in the vicinity
of San Juan (Fig. 1b). For completeness, we list all the
reported seismic locations published for the 1944
earthquake in Table 2. Most likely none of the
earthquake depths are very well constrained. The
INPRES seismic location [29] lies in the region of
maximum intensity and used local and regional seismic
data; hence, this is our preferred epicenter. From this
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P. Alvarado, S. Beck / Earth and Planetary Science Letters 243 (2006) 615–631
Table 2
Summary of published locations and depths for the 1944 and 1952 San
Juan, Argentina earthquakes
1944 Earthquake
Reference
Lat.
(°)
Long.
(°)
LPA Bulletin [45]
Kadisnky-Cade [24]
− 31.51
− 31.60
± 0.4°
− 31.40
− 68.55 12
IX
− 68.50 Shallow IX
± 0.6°
− 68.40 30
IX
INPRES (on-line
catalog) [29]
Castellanos [30]
Depth
(km)
MM
intensity
N of San Juan
∼ 15
City
− 31.50
− 68.00 50
IX
− 31.50
− 67.60 40–50
VIII
− 31.60
− 31.50
− 31.25
− 68.50 50
− 68.60 50
− 68.75 50
IX
IX
VIII
Reference
Lat.
(°)
Long.
(°)
INPRES (on-line
catalog) [29]
NEIC
ISS a
BCIS (Bureau Central
International de
Sismologie) a
Gutenberg and
Ritcher (1945) a
− 31.60
− 68.60 30
− 31.50
− 31.50
− 31.10
− 68.60 30
− 68.60 –
− 67.90 –
− 31.50
− 67.50 35
CALTECH Pasadena
Bulletin (1945) a
Jesuit Seismological
Asoc. Bulletin
(1944) a
NEIC
ISS a
Gutenberg and
Ritcher (1945) a
VIII
1952 Earthquake
Depth
(km)
MM intensities for the 1944 earthquake [43] represent the Maximum
Modified Mercalli intensity that would correspond to each hypocentral
location. We used the epicentral location from INPRES [29] for the
1944 and the 1952 events in our study (Figs. 1b and 2).
a
References from reports in the ISC archive data.
information alone it is difficult to estimate source
parameters for this large earthquake.
All of the reports are consistent describing a 6- to
8-km-long fault scarp forming with the east side up on
the La Laja reverse fault after the 1944 earthquake
(Fig. 2). The La Laja fault has been interpreted as a
splay or branch of the larger eastern Precordillera
thrust fault system of N–NE trend and west vergence,
composed of several segments between 31°S and
32.30°S [16,20] (Fig. 2a). The surface rupture which
appeared after the occurrence of the 1944 event
showed a dip-slip displacement of 0.30 m immediately
after the earthquake, which increased to 0.60 m in the
following days [30,33]. Harrington [33] also observed
a horizontal right-lateral strike-slip displacement of
0.25 m on the same segment. Based on scaling relationships, the expected rupture and average displacement for an earthquake of magnitude ∼7.0 might be of
40 to 50 km and ∼ 1.4 m, respectively [44].
The 1944 earthquake was well recorded at teleseismic distances and the P-wave first motions are
consistent with a thrust focal mechanism but neither
plane is well constrained. All of the teleseismic P-wave
records we obtained show a compressional first arrival.
We also used horizontal seismic records which are
plotted in Fig. 4a to be consistent with the convention
that a downward motion on the seismogram corresponds
to a P-wave dilatation. Local and regional components
from stations in Mendoza (MEN) and Buenos Aires
(BAA) in Argentina and Huancayo (HUA) in Perú
(Table 1) show dilatational first motions. More information reported in the LPA bulletin from the three
component seismic station in La Plata (LPA), Argentina,
located at 34.91°S and 57.93°W, also indicates a
dilatational P-wave arrival [45]. These data from closer
stations to the 1944 epicenter (Fig. 3a and Table 1) are
consistent with a thrust focal solution (Fig. 4a). Because
we have so few records compared to modern events and
of the importance of determining the focal mechanism
and depth, we have used several approaches to constrain
the source parameters.
Previous depth-phase analysis results using the
vertical component of the stations with epicentral
distances between 50° and 87° indicate a thrust focal
mechanism of fault planes oriented in a north–south
direction and a shallow focal depth b 12 km, approximately [40].
We have performed a grid-search for a range of strike,
dip and rake values by inverting the data at a series of
fixed focal depths for the source time function using Pwaves recorded on the vertical component at stations
PAS, TUC, WES, BRK and CHR. All of these records
are observed at teleseismic distances and have clear first
arrivals (Fig. 4a). We carefully examined the predicted
phases for each waveform modeling, the normalized
error between observed and synthetic amplitudes, and
the source time function results in the grid search. We
started exploring fault planes with dip between 0° and
90°, rake between 30° and 150°, using a grid spacing of
10° and a fixed strike for different focal depths. The best
results occur for a dip of 30–40° to the east and a rake of
100–130°. We then fixed the rake and varied the dip
between 0° and 90°, and the strike between 340° and
100°, in steps of 10°. Acceptable solutions have a strike
between 20° and 50° and a dip of ∼ 30–40°. For
simplicity, we present amplitude-misfit error results of
modeling for the best depth (11 km) using a fixed strike
P. Alvarado, S. Beck / Earth and Planetary Science Letters 243 (2006) 615–631
623
of N45°E in Fig. 5a and rake of 110° in Fig. 5b,
respectively. Our preferred solution (source time function mostly positive and best comparison between
observed and synthetic waveforms with normalized
error 0.53) has a strike of N45°E, a dip of 35° to the
southeast, and a rake of 110° as shown in Fig. 4b.
Another constrain used to test the focal mechanism
consisted of the S-wave amplitude ratio and polarity
observed at station SJP (Puerto Rico Observatory). The
back-azimuth from SJP to the earthquake is 182.5° (Fig.
3a and Table 1), hence, the S waves are almost naturally
rotated to SV and SH waves. Both SV and SH phases
have impulsive arrivals with opposite polarities at SJP
(Fig. 6). At SJP, the ratio of the initial pulse of the SV to
SH is 0.52 with SV polarity up (north) and SH polarity
down (west). Most of the focal mechanisms that
produced the correct SV/SH ratio and P-wave first
motion polarities have a thrust focal mechanism with a
strike of 350–85°, a dip of 20–65° to the east, and a rake
of 70–140°, including our preferred focal mechanism
solution (Fig. 4a).
Fig. 5. Example of the grid-search results for the focal mechanism
parameters of the 1944 earthquake using the multistation P-wave
inversion. (a) Map of the normalized errors between synthetic and
observed seismograms for each inversion varying the dip and rake of
the fault plane in steps of 10°, for a fixed strike of N45°E and a source
depth of 11 km. (b) Same as (a) but varying the dip and the strike of the
fault plane in steps of 10°, for a fixed rake of 110°. (c) Amplitudemisfit errors between observed and synthetic seismograms as a
function of fixed focal depths from the P-wave inversion assuming our
preferred focal mechanism of strike 45°, dip 35° and rake 110°. Error
bar in the focal depth is based on acceptable synthetic fits to the
observed teleseismic P-waveform data.
Fig. 6. Plot of the SH and SV components recorded at station SJP for
the 1944 earthquake. The S waveforms are naturally rotated for this
station (Fig. 3a).
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P. Alvarado, S. Beck / Earth and Planetary Science Letters 243 (2006) 615–631
We evaluated the best focal mechanism at a series
of fixed focal depths using P-waveform inversions
(Fig. 5c). The normalized error versus focal depth
curve shows a minimum at a depth of 11 km, although
acceptable synthetic P-waveforms occur in the depth
range between 9 km and 13 km (Figs. 2b and 5c). The
resultant source time function indicates one simple
pulse with duration of 10 s (Fig. 4b). For comparison,
we present the results for a focal depth of 20 km in
Fig. 4c which show a higher amplitude misfit, a more
negative source time function (corresponding to a
smaller seismic moment and a smaller moment magnitude) and synthetic P-waveforms more inconsistent
with the observed waveform data.
We also investigated the behavior of the source time
function with increasing depth. We find that at depths
greater than 13 km, the source time function begins to
exhibit the periodic ringing indicative of a depth
overestimation [46] (Fig. 7). In fact, at the best depth
(11 km), the seismic moment is concentrated toward the
beginning of the deconvolved source time function
(Fig. 4b). It seems likely that the local residents did
feel one single event of short duration. We concluded
that the majority of the seismic moment release
occurred at shallow depths with the best point source
at 11 km, although we cannot rule out small amounts
of moment release further down-dip.
The source time function for the 1944 earthquake has
one simple pulse with duration of 10 s, suggesting that
most of the moment release occurred near the epicenter.
Assuming a rupture velocity of 2.5 km s− 1, and a 10-s
duration, we estimate that most of the seismic moment
was released within approximately 25 km of the
epicenter. Depending on if the earthquake had a unilateral or bilateral rupture, the fault length should be
between 25 km and 50 km, respectively. We determined
a seismic moment M0 = 3.01 × 1019 N m which gives a
moment magnitude Mw = 7.0 (using the equation
Mw = 0.67 × log M0 − 6.0). We can calculate the average
displacement (D) using M0 = μDA. For this purpose, we
used two possible fault areas (12 km × 25 km for a
unilateral rupture and 12 km × 50 km for a bilateral
rupture) and a rigidity μ of 3 × 1010 N m− 2, and obtained
an average slip of 3.3 m and 1.7 m, respectively.
According to Wells and Coppersmith [44], an
empirical scaling relationship between the moment
magnitude and the surface rupture length (SRL) is
given by Mw = 5.08 + 1.16 × log (SRL). This relationship
predicts a surface rupture length of 45 km for the 1944
event rather than the 6–8 km observed. However, there
is a large amount of scatter in this relationship for
continental shallow thrust or reverse fault earthquakes
Fig. 7. Example of the source time functions for the 15 January 1944
San Juan earthquake deconvolved from the simultaneous inversion of
vertical components at stations TUC, BRK, WES, PAS and CHR
(Table 1) at depths from 1 to 40 km. Note the increase in ringing of the
latter part of the source time function at greater depths.
especially with magnitudes near 7.0. There are many
examples of intraplate earthquakes related to blind
thrusts not reaching the surface despite magnitudes
between 6.5 and 7.0 [47]. Another empirical regression
curve indicates a maximum displacement of 2.5 m for a
magnitude Mw 7.0 [44], which is similar to our estimate
based on the seismic moment and fault areas assumed
above.
We have observed that the teleseismic P-waveform
inversion is not sensitive to the different epicentral
locations reported for the 1944 earthquake (Table 2).
However, the hypocentral location determined using
local and regional seismic data by INPRES [29]
(Table 2) is consistent with the east dipping fault-plane
P. Alvarado, S. Beck / Earth and Planetary Science Letters 243 (2006) 615–631
geometry suggested by the observed surface rupture
(Fig. 2), and by our focal mechanism solution (Fig. 4a).
Our results are consistent with the 1944 earthquake
rupturing upward mostly or in part on the La Laja fault.
In fact, the fault parameters measured on the 1944 fault
scarp by Perucca and Paredes [36] yield a strike of
N45°E, a dip between 25° and 45° to the southeast and
a rake of 90°, as measured in Neogene strata and a
Quaternary terrace. These parameters are comparable
to those observed by INPRES [48] and Costa et al.
[26]. Our solution is also consistent with the sense of
coseismic displacement vectors (vertical thrust dip-slip
and horizontal right-lateral strike-slip) observed on the
La Laja fault after the 1944 earthquake [30,33]. Hence,
we suggest the southeast-dipping nodal plane (strike
N45°E, dip 35°, rake 110°) is the fault plane that
ruptured and the northwest-dipping plane (strike 201°,
dip 57°, rake 76°) is the auxiliary plane in the focal
mechanism solution.
Besides the clear 6–8-km-long coseismic rupture
[30,32,33], Castellanos [30] describes another less welldeveloped break further south in the vicinity of Ullum
related to the 1944 earthquake. It was observed in soft
sediments of ∼ 1.5-m vertical displacement with orientation along the strike of the La Laja fault (Fig. 2a).
Harrington [33] also pointed out that the La Laja fault
might have ruptured for more than 20 km during the
1944 event but the presence of sediments and
agricultural sites makes its trace confusing. In addition,
Smalley et al. [16] have observed a LANDSAT
lineament of N45°E-trending near La Laja fault that
continues to the south ending at the Villicum-Zonda
segment in the eastern Precordillera fault (Fig. 2a).
Taken together this suggests that the La Laja fault might
be 40 to 50 km long and capable of a magnitude 7.0
earthquake (Fig. 2).
Based on the regional crustal seismicity around San
Juan, the 50-km focal depth reported by the ISS for the
1944 earthquake (Table 2), and the 1977 earthquake
source studies, several authors have suggested a deep
source (∼ 20–30 km) for the 1944 earthquake
[16,20,21,24,34,37]. Smalley et al. [16] investigated
the local seismicity in this area and found an active
planar structure dipping 35° to the northwest between 19
km and 35 km depth and more diffuse seismicity
between 5 km and 15 km beneath the eastern
Precordillera (Fig. 2). Thus, these authors speculated
that the 1944 event probably had a deep crustal source
located to the west of the eastern Precordillera, and may
have ruptured in multiple faults, like the 1977
earthquake in the Sierras Pampeanas [24,25] (Fig. 2a),
without generating a main surface rupture and inter-
625
preting the La Laja fault as a secondary feature
[16,34]. For focal depths greater than 15 km and fault
planes dipping 35° to the northwest, which are 22°
less inclined than our estimated dip (57°) for the
auxiliary fault plane, we find significantly worse fits
to the observed P-waveforms (Figs. 4b,c, 5 and 7). In
fact, we note that the dip is the best constrained
parameter in our focal mechanism grid-search with an
uncertainty of ± 5° (Fig. 5a, b). The simple source
time function (Fig. 4b) also indicates that the 1944
earthquake occurred most likely as a single event.
Hence, we suggest the 1944 earthquake is unlikely to
have ruptured on the deep west-dipping fault and as a
complex multiple event.
Siame et al. [20,21] consider the La Laja fault as part
of the N40°E-trending fault system that branches from
the main eastern Precordillera fault composed of several
segments. The same authors used structural geology,
geomorphology and exposure age dating analysis in this
area to suggest the 1944 earthquake may have ruptured at
depth on the Villicum–Las Tapias segment. According
to their study, this segment is a 65-km-long reverse fault,
oriented in a N20°E direction with a dip of 60° (±15°) to
the east at the surface but becoming much shallower at a
depth of 20–25 km [20,21] (Fig. 2). The strike N20°E is
consistent with the waveforms but not the high angle
(60°) dip. We note that if we assume this focal
mechanism with a high angle east dipping fault plane
for modeling, larger errors result between our observed
and synthetic P-waveforms (Fig. 5). These authors also
suggested a seismic source for the 1944 earthquake
located at 20–25 km depth but with the focus to the east
of the eastern Precordillera in a flat-lying part of the
Villicum–Las Tapias fault that did not produce any
surface rupture along this fault and distributed the
deformation in secondary fractures [20,21]. Our results
for an inversion of the P-waves with the moment release
at depths N 13 km (Figs. 4c, 5c and 7) do not produce the
best results. Our evidences indicate that it is more likely
that it ruptured on a shallower thrust fault with a
geometry similar to that observed for the La Laja fault.
However, it is possible that the 1944 event also produced
coseismic folding and faulting deformation and related
ground features. The 1971 (Mw 6.7) San Fernando,
California earthquake is another example of a shallow
(∼ 12 km) thrust event that ruptured spreading out along
several minor structures as it approached the surface
[50,51]. Given that we do not know the subsurface
geometry of active faults beneath the Tulum valley
(Fig. 2) from supporting industry oil-well or seismic
reflection data we cannot rule out rupture on the Villicum–Las Tapias segment. But if the seismic moment
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P. Alvarado, S. Beck / Earth and Planetary Science Letters 243 (2006) 615–631
release occurred at a shallow depth then we would expect
some surface rupture on the Villicum–Las Tapias fault
which is clearly not observed [20,21,26,52,53].
Our preferred solution is consistent with the geologic
model proposed by Ramos et al. [19] between the
Precordillera and the Sierra Pie de Palo (Fig. 2). These
authors inferred one major east-dipping thrust basement
fault extending up to 10–15 km depth beneath the
Tulum valley with several thrust branches as it becomes
shallower. We note that the crustal seismicity observed
in the same region by Smalley et al. between 5 km and
∼ 15 km in a diffuse distribution [16] may be related to
the same east-dipping thrust structures. Our interpretation, together with the already recognized deeper active
basement structure of a 35°-northwest dip by Smalley et
al. [16] (Fig. 2), can have direct implications in the
seismic risk around San Juan from both concealed and
exposed faults. A comparable example was evidenced in
the San Fernando Valley, California after the 1971 and
1994 earthquakes [51,54].
dip of 30–90° and a rake of 5–70°. We used these focal
mechanisms in the depth-phase analysis. The relative
depth phase amplitudes were well predicted when using
a roughly northeast fault plane of a steep dip of at least
50° with an important left-lateral strike-slip component
(Fig. 8a). We estimated a focal depth of 10–12 km from
the predicted depth-phase arrivals.
Teleseismic records at stations TUC, FLO, WES,
BRK and CHR (Fig. 3a and Table 1) were used in the
simultaneous inversion for the source time function
following the same methodology used for the 1944
event. In this grid search, we explored a wide range of
fault-plane focal mechanisms and focal depths. Fig. 9
3.2. The 11 June 1952 earthquake
After the 1944 earthquake San Juan was rebuilt
quickly using a modern building code for earthquakeresistant design [55]. Another damaging crustal earthquake occurred in 1952 [29]. However, the smaller
magnitude of this event, its location further away from
densely populated centers, and improved buildings
prevented widespread destruction.
The 1952 earthquake occurred on June 11 at
00:31:37 GMT. Its epicenter was located ∼ 15 km to
the S–SW of downtown San Juan, in the vicinity of the
Sierra Chica de Zonda (Figs. 1b and 2 and Table 2).
Almost 1000 houses collapsed or suffered severe
damage in Carpintería, Pocito, and Zonda localities,
among others, reaching a maximum MM intensity VIII
in this area [43] (Fig. 1b). Beyond this area, damage was
minor to moderate and mostly limited to old nonearthquake-proof design constructions as shown by our
interpolation for the MM intensities reported in
Argentina [43]. The mainshock caused 1 death in
Carpintería (Fig. 2) and 14 injured people [56,57]. The
reports indicate that the local residents felt 11 aftershocks during that night and point out the strongest one
at 03:00:33 GMT [57,58].
We used first motion P-waves (stations STL, HUA,
SJP, SLM, FLO, WES, TUC, BRK, CHR), P-diffracted
waves (stations STR, DBN), and the SV/SH ratio (0.49)
and positive polarities (SV north and SH east) observed
at station SJP as input to estimate the focal mechanism.
In general, possible solutions have a strike of 0–120°, a
Fig. 8. (a) Lower hemisphere P-wave first-motion focal mechanism
plot for the 11 June 1952 San Juan earthquake. First motion
convention as in Fig. 4a. The P-waveforms used are also plotted
showing 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 the
1952 San Juan earthquake. Station information is listed in Table 1.
P. Alvarado, S. Beck / Earth and Planetary Science Letters 243 (2006) 615–631
shows the results of the normalized errors between
observed and synthetic data obtained for each inversion.
In the first step, we varied the dip from 10° to 90° and
the rake from 0° to 120° in steps of 10°, respectively,
maintaining the strike at N40°E (Fig. 9a). Secondly, we
fixed the rake at 30° and varied the dip as before and the
strike from 340° through 90°, using a grid spacing of
10°. The minimum amplitude-misfit errors, best comparison between predicted and observed P-waveforms,
and source time function mainly positive, are found for a
focal mechanism with a fault plane of strike of 20–40°,
dip of 70–80° to the southeast, and rake of 20–40°
(Figs. 8b and 9a,b).
Fig. 9c shows the behavior of our preferred focal
mechanism solution (strike N40°E, dip 75°, rake 30°)
when varying the source depth from 1 km to 25 km, in
steps of 1 km. The best fit between observed and
synthetic data (amplitude-misfit error of 0.402) occurs at
a point source depth of 12 km, but acceptable fits result
between 10 km and 13 km focal depth, which is in
agreement with the depth-phase identification. In
addition, solutions with depths larger than 15 km
produce a ringing in the source time function indicative
of a depth overestimation [46].
Our best results (Fig. 8b) indicate a single pulse for
the source time function with a duration of 6 to 8 s and
seismic moment M0 = 2.2 × 1019 N m, which corresponds to a moment magnitude Mw = 6.8.
Although there are uncertainties in the epicentral
location of the 1952 earthquake, we can speculate on the
association of this event with the local geology.
Considering the epicentral information from INPRES,
NEIC and ISS (Table 2) and the maximum MM
intensities for this earthquake [43] (Fig. 1b), it is
possible that it is related to the structures in the Sierra
Chica de Zonda in the eastern Precordillera as suggested
by [26,48,52] (Fig. 2). This portion of the eastern
Precordillera has shown little crustal seismicity in a very
disperse pattern during local seismic deployments
[16,59]. However, neotectonic studies of the interaction
between the Precordillera and the Sierras Pampeanas by
Martinez and Perez [60] have identified important leftlateral horizontal displacements along active faults of
N–NE orientation in this sector of the Precordillera. The
NE striking plane in our focal mechanism solution is
consistent with this style of deformation.
Interestingly, both the 1944 and the 1952 earthquakes
seem to be related to the active eastern Precordillera
system and separated by 8 yr and a distance of about 35
km. Recent studies about earthquake interaction, based
on the Coulomb stress transference and seismicity rate
changes, have proposed that the occurrence of one
627
earthquake contributes to the next on an adjacent
segment [61,62]. It is likely that both earthquakes are
related to the interaction of the Precordillera and Sierras
Fig. 9. Example of the grid-search results for the focal mechanism
parameters of the 1952 earthquake using the multistation P-wave
inversion. (a) Map of the normalized errors between synthetic and
observed seismograms for each inversion varying the dip and the rake
of the fault plane, in steps of 10°, and fixing the strike at 40° and source
depth at 12 km. (b) Same as (a) but varying the dip and strike in steps
of 10°, while fixing the rake at 30°. (c) Amplitude-misfit errors
between observed and synthetic seismograms versus focal depths from
the P-wave inversion assuming our preferred focal mechanism of
strike 40°, dip 75° and rake 30°. Error bar in the focal depth is based on
acceptable synthetic fits to the observed teleseismic P-waveform data.
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P. Alvarado, S. Beck / Earth and Planetary Science Letters 243 (2006) 615–631
Pampeanas fault systems. Our constrains might help to
test this possibility which requires further study.
•
4. Conclusions
We have analyzed waveforms for the historical
earthquakes in 1944 and 1952 in San Juan, Argentina.
We have used a combination of several seismic
techniques to constrain the source parameters for both
crustal events. We obtained a thrust focal mechanism
(strike 45°, dip 35°, rake 110°), a seismic moment
M0 = 3.01 × 1019 N m, a moment magnitude Mw = 7.0
and a focal depth of 11 km for the 1944 earthquake. The
results for the 1952 event indicate an oblique focal
mechanism solution (strike 40°, dip 75°, rake 30°), with
M0 = 2.20 × 1019 N m, Mw = 6.8 and focal depth of
12 km. We found one single pulse for the source time
function of each event with duration of 10 s and 8 s,
respectively. Although it is difficult to make a formal
error analysis, we estimate the uncertainty in the strike,
dip and rake, and focal depth by trial-and-error
sensitivity tests using teleseismic P-waveform modeling. This gives uncertainties of about ± 15° in the strike
and rake, and ± 5° in the dip. The same analysis for the
focal 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 that
occurred on a northeast striking plane, dipping ∼ 35° to
the southeast. The 1952 earthquake, of a more oblique
focal mechanism, is consistent with the active N–NE
faults of the eastern Precordillera that show left-lateral
neotectonic displacements [60] in the Sierra Chica de
Zonda segment, but a particular association to the active
faults in the area is difficult (Fig. 2). We note that the
relocation study of some of the larger aftershocks
mentioned in the historical reports could help to
constrain the fault plane activated during each earthquake. Unfortunately, we do not have seismic records
for them.
We find good agreement between the exposed La
Laja thrust fault and the seismogenic fault that produced
the 1944 Mw = 7.0 earthquake. This is mainly based on
the following reasons:
• The focal depth and geometry of the east-dipping
thrust fault plane solutions are consistent with the
parameters (strike, dip and rake) observed for the La
Laja fault.
• The east-dipping focal mechanism fault plane
solution is compatible with the sense of the slip
vectors (thrust and right-lateral) observed at the
•
•
•
surface on the La Laja fault scarp during the 1944
earthquake.
Given the shallow focal depth (∼ 11 km) of this large
earthquake, it is likely to have occurred on a fault
plane that ruptured to or near the surface.
Some reports of disruption at Ullum during the 1944
earthquake might be an indication of additional
activity along the same (La Laja) fault.
The La Laja fault has been predicted to continue to
the south for about 40 km until it meets the Villicum–
Zonda fault segment in the eastern Precordillera
[16,33,52], although the presence of unconsolidated
sediments and agriculture sites to the south of the La
Laja fault scarp make it difficult to map (Fig. 2a).
There were no other major faults that had any
coseismic rupture during the 1944 earthquake.
Our results for the 1944 earthquake show that the
style of deformation is similar to that observed in the
Sierras Pampeanas [13,19] where the basement blocks
are mainly uplifted by east-dipping faults (Fig. 2).
Although our source time function results predict the
activation of one main fault (one simple source with
one pulse of moment release), the 1944 earthquake
may have caused coseismic deformation in the
associated folding and terraces around the La Laja
fault [37,49,53]. Considering the deep west-dipping
active structure recognized by Smalley et al. in
previous seismic studies [16] and the activation of
an east-dipping thrust fault that might have reached
the surface during the 1944 earthquake (Fig. 2), there
is a high seismic hazard posed to San Juan from both
concealed and exposed faults related to the frontal part
of the eastern Precordillera.
The source characterization of the 1944 (Mw = 7.0)
and 1952 (Mw = 6.8) earthquakes completes the study of
the largest damaging crustal earthquakes during the last
century in the vicinity of San Juan. Both events exhibit a
simple source time function, a shorter duration and a
shallower focal depth than the most recent 1977
(Mw = 7.5) multiple shock earthquake in the Sierras
Pampeanas [25]. This backarc deformation is consistent
with the two décollement levels proposed by [15,63,64]
in the vicinity of San Juan. The crustal earthquakes in
1944 (this study) and 1977 [25] both showed predominantly thrust focal mechanisms (Fig. 2), but they
differed in their focal depths and in the complexity of
their coseismic ruptures. Since the occurrence of large
earthquakes is sporadic, the study of historical seismograms can help in identifying active structures that are
important in the mapping of the seismic hazard in this
area.
P. Alvarado, S. Beck / Earth and Planetary Science Letters 243 (2006) 615–631
Acknowledgments
We thank Jim Dewey, Keith Koper, Rick McKenzie,
Tony Monfret, Luis Rivera, John Ebel, Victor Cruz and
Brian Ferris, for helping us to obtain paper copies of the
historic earthquakes. Maureen Aspreen from the
International Seismological Centre (ISC) provided us
useful historic documents. We also thank Alberto Sáez
for helping us to obtain copies of old Argentinean
newspapers. We are grateful to Carlos Costa for
interesting discussions. We acknowledge Jim Dewey,
and two anonymous reviewers for their critical reviews
and suggestions to improve this paper. The material is
based on work partially funded by NSF grant EAR9811878 and awarded by LASPAU-University of
Harvard under Lewis A. Tyler Trustees' Fund Award
(2003), the National PERISHIP Dissertation Award
(2004), and CHEVRON-TEXACO Summer Scholarships 2004 and 2005 to P. Alvarado.
Maps were generated using Generic Mapping Tools
software [65]. We also used SAC2000 [66] to process
seismic data and used seismological data from the ISC,
and the USGS-NEIC Bulletins. Mauro Sáez helped us in
setting up the database for this project and with the
figures in this paper.
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