Landslides induced by the April 2007 Aysén Fjord earthquake

Recent Landslides
Landslides (2010) 7:483–492
DOI 10.1007/s10346-010-0203-2
Received: 4 July 2009
Accepted: 3 February 2010
Published online: 13 March 2010
© Springer-Verlag 2010
Sergio A. Sepúlveda . Alejandra Serey . Marisol Lara . Andrés Pavez . Sofía Rebolledo
Landslides induced by the April 2007 Aysén Fjord
earthquake, Chilean Patagonia
Abstract On the 21st of April 2007, the Aysén Fjord earthquake
(Mw 6.2) in southern Chile (45.3° S, 73.0° W) triggered hundreds of
landslides in the epicentral area along the fjord coast and
surroundings. Some of these landslides induced large tsunami
waves within the fjord causing fatalities and damaging several
salmon farms, the most important economic activity of the area.
The landslides included rock slides and avalanches, rock falls,
shallow soil and soil–rock slides, and debris flows. The earthquake
was the climax of a seismic swarm that began 3 months earlier. The
seismicity is associated with tectonic activity along the Liquiñe–
Ofqui fault zone (LOFZ), a major structural feature of the region.
The earthquake-induced landslides were mapped and classified
from field observations and remote sensing analysis. The landslide
areas and epicentral distances are within the expected range for the
earthquake magnitude according to worldwide data, while the
position of landslides on the slopes strongly suggests topographic
amplification effects in triggering the failures. The location of the
landslides is also clearly related to some of the main fault branches
of the LOFZ. The seismic event has configured a new situation of
seismic and landslide hazard in the Aysén region and along the
LOFZ, where the presence of towns and economic infrastructure
along the coasts of several fjords constitutes a potential risk that
was not considered before this seismic event.
Keywords Landslide . Earthquake . Patagonia
Introduction
On January 22, 2007, an unusual seismic swarm of shallow
earthquakes (focal depths less than 10 km) began in the Aysén
Fjord (45.3° S, 73.0° W, Figs. 1 and 2), an EW to NW trending fjord
that is the main maritime access route to the towns of Puerto
Chacabuco and Puerto Aysén (Fig. 2). These towns connect by land
to the regional capital Coyhaique, located about 80 km to the east.
The fjord hosted many salmon farms, the most important economic
activity of the region, together with tourism. The major event
occurred on April 21, when a Mw 6.2 earthquake struck the region.
The shaking triggered tens of landslides along the fjord coast in and
near the epicentral area (Sepúlveda and Serey 2009; Naranjo et al.
2009; Fig. 3) and several more in the surrounding mountainous
areas. The mass displacement into the fjord produced waves that
crossed it in a few minutes, causing geomorphological changes in the
coastline and severe damage to the salmon farms. The waves’ impact
left marks of stripped vegetation along the shoreline, with run-ups
up to several tens of meters (Naranjo et al. 2009).
The landslide-induced tsunamis and some debris flows caused
ten fatalities, about half of which are still missing (Naranjo et al.
2009). Some landslides also occurred on hills around the town of
Puerto Aysén, about 30 km east of the epicenter, causing some
damage to houses located along the foothills.
The seismic swarm has been related to the reactivation of
movement along the Liquiñe–Ofqui fault zone (LOFZ), a major
regional (about 1,200 km long) structure of NNE trend that crosses
the Aysén Fjord (Cembrano and Hervé 1993; Fig. 1). Seismic activity
has been interpreted as partially related with magmatic activity,
which is not rare considering that there is a strong spatial
correlation between the LOFZ and major stratovolcanoes as well as
tens of small monogenetic volcanoes (López-Escobar et al. 1995;
Lahsen et al. 1997; Stern 2004). The LOFZ has also an important
control on the regional geomorphology, with several fjords and
valleys related to the fault trace. The April 21 earthquake is the first
major seismic event clearly recorded in the LOFZ, which reveals
that the fault is active and therefore may be the source of new
important earthquakes in the future. An earthquake reported in
1927 in the Aysén Fjord area may have also been related to the fault
activity (Naranjo et al. 2009). The Aysén swarm coincides with
further recent activity along the LOFZ, such as a ML 5.7 earthquake
on December 31, 2006 at the northern extreme of the fault zone in
the Alto Bio-Bio region (~38° S), a seismic swarm near the Comau
Fjord (~42.1° S) in May 2008, and the eruption of the Chaitén
Volcano (42.8° S) also in 2008 (Servicio Sismológico 2007a, 2008;
Lara 2009). Previous seismicity has also been reported north of the
Comau Fjord by Lange et al. (2008). This new information on the
activity of the fault and the presence of towns and economic
infrastructure along the coasts of the mentioned fjords reveals a
new hazard related to earthquakes and associated landslides that
requires study and evaluation.
This paper gives a full description of the landslides induced by
the 2007 Aysén earthquake and their characteristics, distribution,
mechanics, and possible relationships with seismic parameters.
Further details on the earthquake and tsunami effects are given by
Naranjo et al. (2009).
Study area
The study area (Fig. 2) is located in the Andes Main Range, which,
in the region, forms a middle altitude range, up to around 2,000 m
above sea level near the Aysén fjord. The geomorphology of this
region is characterized by a primary tectonic control superimposed
by glacial modeling. This combination results in an abrupt relief of
U-shaped valleys, the orientations of which follow the LOFZ
tectonic trend. The slopes of the fjord and valleys are commonly
steep with angles over 30° and are covered by shallow volcanic soils
formed by pyroclasts from the nearby volcanoes. Landslides,
alluvial fans, volcanic deposits, glacial till deposits, and fluvial
erosion are the most recent geomorphological features.
The geology of the region is dominated by the North Patagonian
Batholith (NPB), where the Aysén Fjord is located, together with a
Cretaceous metasedimentary and metavolcanic unit (Traiguén
Formation), Pleistocene–Holocene volcanic units, and a meta-
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Fig. 1 Location map and tectonic setting of the study area, showing the regional traces of the LOFZ and location of the 2007 earthquake (after Cembrano et al. 2002 and
Sepúlveda and Serey 2009)
morphic Paleozoic basement (Niemeyer et al. 1984; Cembrano et al.
2002). The NPB is composed of three NNE trending belts of dioritic
to granitic composition, among which the two lateral belts are
Mesozoic and a central belt is Miocene (Cembrano et al. 2002). In
the Aysén Fjord region, the plutonic complex mainly consists of
foliated gabbros, diorites, quartz diorites, granodiorites, and
tonalites, with some dyke swarms and gneissic bands (Bartholomew
1984). To the northwest of the epicentral area, an important extension
is covered by volcanic deposits of the Macá–Cay volcanic complex
(Niemeyer et al. 1984).
The most relevant tectonic structure is the LOFZ (Fig. 1), a
major NNE right-lateral strike-slip structure that accommodates
the parallel component of the oblique subduction of the Nazca
Plate beneath the South American Plate (Cembrano and Hervé
1993; Lavenu and Cembrano 1994; Cembrano et al. 2002; Cembrano
et al. 2007). The coincidence of plutonic belts and recent volcanoes
along the fault suggests that the structure has exerted a control on
magma ascent and emplacement (Cembrano and Hervé 1993).
Several monogenetic cones are located a few kilometers north
and south of the Aysén Fjord, in the Cuervo and Pescado valleys,
respectively (Lahsen et al. 1997; Universidad de Chile, unpublished report). In the 2007 epicentral area, several NNE faults and
shear zones are recognized (Bartholomew 1984; Universidad de
Chile, unpublished report; Vargas et al. 2009, Fig. 2) in fjords
and valleys observing similar orientations, suggesting structural
control.
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The April 21, 2007 earthquake is the first recorded large
magnitude earthquake clearly related to the LOFZ, although an
earthquake in the region in 1927 reported by Naranjo et al. (2009)
may also have been related to the fault. Furthermore, there have
been records of seismic activity along the fault zone since the late
1980s, with reports related to volcanic activity in the northern tip of
the fault zone (Barrientos and Acevedo-Aránguiz 1992), but mainly
in the Palena region (42–43° S), where activity with a magnitude up
to Mw 3.8 was registered and described by Lange et al. (2008). In
the same area, according to the Chilean Seismological Survey, near
the Comau Fjord and the town of Hornopirén, a seismic swarm
with shock magnitude up to ML 5.3 commenced in May 2008,
continuing at a lower frequency and intensity until now.
The climate in the region is cold and rainy, with an average
annual rainfall of about 2,600 mm in Puerto Aysén, although it can
reach up to 4,000 mm in the fjordland. The average monthly
rainfall in Puerto Aysén varies from less than 150 mm in February
up to 250–300 mm between May and August. Given this large
amount of rainfall, the soils are usually close to saturation, and a
dense rainforest covers most of the area.
Earthquake-induced landslides
The Aysén Fjord seismic swarm
The 2007 Aysén Fjord seismic swarm began on January 22, 2007.
Between this date and the main shock on April 21, thousands of
Fig. 2 Shaded relief map with the inventory of landslides triggered during the 2007 earthquake. Trace of faults of the LOFZ after Universidad de Chile (unpublished
report) and Vargas et al. (submitted)
earthquakes were registered in the area by the Seismological
Survey of the University of Chile (Servicio Sismológico 2007b;
Barrientos et al. 2007; Vargas et al., submitted). After May 2007, the
activity significantly decreased, recording just a few tremors per
month until present. Four foreshocks exceeding 5.0 in magnitude
preceded the main shock (Vargas et al, submitted). The main focal
mechanism is strike-slip with a north–south nearly vertical plane
solution, which fits the LOFZ orientation (Global CMT Catalog
2008). The intensities of these foreshocks were approximately
around IV to V (modified Mercalli intensity) in Puerto Aysén and
Puerto Chacabuco towns. The focal depth of the tremors is around
or less than 10 km. Analysis of seismic activity prior to the main
shock (Barrientos et al. 2007) revealed a concentration of
hypocenters along an 8-km NNE–SSW vertical plane crossing the
fjord and the development of a later north–south concentration,
also crossing the fjord. The location of seismic activity coincides
with several faults of the LOFZ, as described in detail by Vargas et
al. (submitted).
The main shock (Mw 6.2, Ms 6.3; Global CMT Catalog 2008;
NEIC 2008) occurred at 13:50 hours local time on April 21. The
reported intensity was VII in Puerto Aysén and Puerto Chacabuco.
No proper instrumental information on aftershocks was
registered immediately after the main shock, as most instruments
of the local network were damaged or destroyed by the tsunami
and had to be replaced. For the same reason, precise localization of
the main shock by the local network could not been obtained.
From May onward, the earthquakes are of low magnitudes (equal
or less than Mb 5.1; NEIC 2008), and the intensities in the nearby
towns have not been higher than IV.
Landslide inventory and classification
The 2007 earthquake triggered different types of landslides, as
shown in Figs. 2 and 3 and Table 1. They were recognized, mapped,
and classified during a short field reconnaissance some few days
after the main shock, followed by mapping and preliminary
classification by spectral analysis of satellite images (Pavez et al.
2007), interpretation of air photographs at different scales
(1,20:000 to 1:70,000) from both before and after the earthquake,
detailed field mapping by boat, helicopter, and truck during field
campaigns in January and March 2008, and checked during short
field trips in February and March 2009. The mapping was
complemented by comparison with the preliminary landslide
inventory by Naranjo et al. (unpublished report) and the study in
the Cuervo River basin by Universidad de Chile (unpublished
report). The appearance of the landslides in the epicentral area
around the fjord is shown in Fig. 4.
Following the indications by Keefer (1984) with some adaptations to the area of study, the earthquake-induced landslides were
classified as follows (Fig. 2):
Shallow soil slides These are shallow translational failures within
the volcanic soil cover or some glacial till deposits (Fig. 3a),
usually less than 2 m deep. They occurred on fjord slopes as well
as in the interior valleys. The largest of these kinds of slides (area
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Fig. 3 Examples of the different types of earthquake-induced landslides in the Aysén Fjord area. a Shallow soil slides. b Shallow soil–rock slide. c Rock slide in front of
Mentirosa Island. d Rock slide and avalanche in Punta Cola area. e Rock falls. f Debris flow
of about 150,000 m2) occurred on the slope of one of the
monogenetic cones of the Pescado River Valley (Fig. 2, Table 1).
Shallow soil and rock slides These are the most common type of
landslides induced by the earthquake. They are shallow (generally
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less than 2–3 m deep) and small (up to few hundred thousands of
square meters in surface area) translational slides that involved
the whole soil cover and possibly the uppermost weathered layer
of bedrock. The exposed shear surface is commonly fresh rock
and, in many cases, showing a clear shear plane that indicates
Table 1 Location and type of largest areas of landsliding during the 2007 earthquake
Site
Acantilada Bay
Punta Cola
Mentirosa Island
Fernández Creek
Frío Creek
Marta River
Acantilada Bay
Fernández Creek
Marta River
Pescado River
Coordinates
45°23.83′ S
45°22.78′ S
45°24.05′ S
45°24.02′ S
45°23.92′ S
45°20.33′ S
45°22.80′ S
45°23.42′ S
45°20.94′ S
45°25.44′ S
72°53.15′ W
72°59.90′ W
72°58.09′ W
72°55.08′ W
72°56.67′ W
73°0,26′ W
72°51.00′ W
72°54.29′ W
72°58.87′ W
73°6.09′ W
Landslide Type
Soil-rock slide complex
Rock slide and avalanche
Soil-rock slide complex
Debris flow
Soil-rock slide complex
Soil-rock slide complex
Debris flow
Rock slide
Soil-rock slide
Soil slide
Area (m2 )
1,193,691
990,833
943,055
497,311
469,311
464,935
461,859
304,303
246,407
143,989
Landslide areas include deposits. Site locations in Figs. 2 and 4
structural control by joints subparallel to the topographic surface,
possibly related to unloading after the glacial retreat (Fig. 3b).
They were more concentrated around the fjord and, in many
cases, are clustered forming landslide complexes (Figs. 2 and 4).
Rock slides and avalanches These correspond to the largest
landslides and are restricted to the fjord slopes and a couple of
slides in the Marta Valley and the Frío and Fernández creeks
(Figs. 2 and 4). They were triggered on steep slopes, with a
massive, mainly translational movement of rock with no clear
control on shear by joint surfaces but tended to occur in very
fractured rock around shear or fault zones (Sepúlveda and Serey
2009). Many of them tended to evolve into rock avalanches during
their transit, but most were quickly deposited in the fjord and
Fig. 4 Spot image of the epicentral
area, the most affected by the
landslides
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would thus be classified either as disrupted rock slides or rock
avalanches following Keefer (1984), depending on their travel
distance. Their thickness varies from 5 to 20 m on average but may
locally reach dozens of meters close to the scarps.
The most significant rock slides that caused the tsunami are
those in front of Mentirosa Island and in a ravine next to Punta
Cola, both at the northern shore of the fjord (Figs. 2 and 3c, d).
The estimated volumes of these landslides are 8 and 12 million
cubic meters, respectively (Sepúlveda and Serey 2009). The slide
in Punta Cola was triggered on a lateral valley slope and
continued as an avalanche along the valley until reaching the
fjord, with a run-out of about 1 km from the slide toe. The flow
stripped vegetation from the opposite slope in front of the
landslide and also downstream, with heights reaching several
dozens of meters. The deposit in the ravine, which extends to the
Rock falls These occurred on steep rock slopes, in some cases in
the form of rock wedges that slid along joints and then fell, or
more commonly by the detachment of rock blocks from
subvertical cliffs. Some important rock falls were triggered along
a
1000000
100000
Area affected by landslides (km2)
Fig. 5 Comparison with curves of
Keefer (1984) and Rodriguez et al.
(1999) of maximum landslide area and
epicentral distance for worldwide
earthquake-triggered landslides. a
Maximum landslide area. b Maximum
epicentral distance for disrupted landslides. c Maximum epicentral distance
for flows
fjord shore, has variable thickness up to around 20 m and rests on
top of glacial and alluvial deposits, reaching over 80 m above sea
level. These evidences suggest a flow mechanism of a stream of
rock fragments at very high velocities, which may be classified as
a rock or debris avalanche, rather than a debris flow as described
by Naranjo et al. (unpublished report). The avalanche destabilized the lateral ravine slopes inducing soil and rock slides. This
rock slide and avalanche, together with the rock slides in front of
Mentirosa Island, are the main triggers of the tsunami waves
(Sepúlveda and Serey 2009).
10000
Keefer 1984
Rodriguez et al. 1999
Aysen EQ 21/04/07
1000
100
10
1
4
4,5
5
5,5
6
6,5
7
7,5
8
8,5
9
Magnitude Mw
b
1000
Maximum epicentral distance (km)
Disrupted Landslides
Keefer 1984
AYSEN EQ 21/04/07
100
10
1
0,1
4
4,5
5
5,5
6
6,5
Magnitude Mw
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7
7,5
8
8,5
9
the northern shoreline of the fjord, while others are recognized in
the river valleys (Figs. 2 and 3e).
Debris flows In many ravines, tributaries of the fjord (Fig. 3f), or
the major rivers such as the Cuervo and Marta rivers, debris flow
deposits generated by the debris of the formerly described
landslides can be observed. The combination of debris with
running water in the creeks, temporal damming of streams, and
some fluidization of the sliding masses during movement would
have allowed the formation of the flows. The presence of debris
flows was reported immediately after the main shock (Naranjo et
al., unpublished report), indicating that they were coseismic
(Fig. 2).
Following Keefer (1984), the observed earthquake-induced
landslides in the Aysén Fjord can be grouped as disrupted slides
(shallow slides, rock slides, rock avalanches, and rock falls) and
flows. No coherent slides, such as slumps, deep block slides, or
slow earth flows were observed.
It must be noted that some landslides were triggered along both
fjord shores by the foreshocks or with no direct trigger even hours
or days after some shocks. The Seismological Survey reported 14
landslides along the fjord until March 2007 (Servicio Sismológico
2007b), the location of which was preliminarily mapped by the
survey. The landslides are mainly shallow soil slides and rock falls,
and many of them were reactivated and enlarged during the main
shock. For this reasons, these landslides are included in the
inventory of Fig. 2.
Discussion: landslide distribution and position
As presented in Fig. 2, the landslides were distributed in the
epicentral area, with a larger number of landslides toward the east
and north of the epicenter, in general coinciding with the location
of major faults of the LOFZ mapped and described by Universidad
de Chile (unpublished report) and Vargas et al. (submitted). This is
particularly important for large rock slope failures, triggered on
c
1000
Flows
Maximum epicentral distance (km)
Fig. 5 (continued)
more susceptible slopes close to these faults where the rock mass
tends to have lower geotechnical quality (Sepúlveda and Serey
2009).
The total area affected by landsliding and the maximum
epicentral distances (about 17 km for flows and 42 km for
disrupted landslides) fit well beneath the upper bounding curves
proposed by Keefer (1984) and Rodriguez et al. (1999) from
statistical records of worldwide earthquake-induced landslides
(Fig. 5). This shows that the landslide geographical distribution is
within the expected for a M 6.2 earthquake in a mountainous
environment, with origin of flows more clustered around the
epicenter than disrupted slides.
The most common type of landslide is soil–rock slides, with 282
mapped events, followed by soil slides with 135, from the total of
538 landslides induced by the earthquake (Fig. 6). The total surface
area of landsliding reaches over 17 million square meters. Most
mapped landslides have surface areas below 100,000 m2 (Fig. 6),
while a minority occupies more than 100,000 m2. Nevertheless, all
landslide types show a similar relative distribution by area. Figure
6 shows a trend of smaller size with increasing distance, although
this pattern is less clear for shallow soil slides, which are more
randomly distributed in size independently of the epicentral
distance. It can also be seen that a majority of failures occurred
between 5 and 15 km from the epicenter, but it is difficult to be
more accurate due to the error of epicentral distances, up to 3 km,
caused by uncertainty on the epicenter location.
Field observations indicate certain controls on the landslide
position on the slopes. The crowns of the landslides are generally
in the uppermost part of the slope, which has been proposed as an
indication of landslide triggering related with topographic
amplification of the seismic waves (e.g., Densmore and Hovius
2000; Sepúlveda et al. 2005a; Meunier et al. 2008). Topographic
amplification is a site effect caused by the interaction of the
incoming seismic waves with certain geomorphological features, such as steep slopes in areas of strong topographic relief,
100
Keefer 1984
Rodriguez et al 1999
AYSEN EQ 21/04/07
10
1
0,1
5
5,5
6
6,5
7
7,5
8
8,5
9
9,5
Magnitude Mw
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Fig. 6 Number of earthquaketriggered landslides and their distribution. a Landslide area versus epicentral
distance. b Number of landslides by
area and type
which results in larger amplitudes of the ground motion toward
the ridge crests (e.g., Sepúlveda et al. 2005b; Brennan and
Madabhush 2009). This causes larger susceptibility to landsliding in the upper parts of the slopes. Meunier et al. (2008)
proposed a graphic method to represent the position of
landslides on the slopes, combining the normalized distance
of the landslide top to the ridge crest and the normalized
distance of the landslide toe to the nearest stream. This method
is applied in Fig. 7.
Figure 7a shows that the landslides triggered by the Aysén
earthquake are strongly clustered near ridge crests, which is
represented by a concentration of landslides close to the y-axis. An
important number of cases plot in the axis intersection, which
relates to those landslides that start at the ridge crest and whose
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Landslides 7 • (2010)
run-out extends down to the slope bottom. This is the case for
several failures in Aysén given their disrupted nature. This
observation is reinforced by the relative distribution of the
normalized distance to ridge crests, as shown in Fig. 7b. About
two thirds of the landslides start in the upper quarter of the slope,
while over 90% start in the upper half, which suggests that larger
ground motions due to topographic site effects influenced the
triggering of landslides during the earthquake. This is currently
being tested with geotechnical analysis by the authors (Serey et al.
2009).
Due to the concentration of landslides on the upper part of the
slopes, river incision is disregarded as a primary control on
earthquake-induced landslide distribution in the valleys. This
factor would be associated to landslides clustered at the hillslope
Fig. 7 Landslide relative position on the slopes. a Normalized distance of the landslide crowns to ridge tops against normalized distance of landslide toes to nearest
streams. b Distribution in percentage of ranges of normalized distance to ridge crests
toes, which is not the case. Lithology is neither a control in this
event, as the intrusive rocks covered by volcanic soil mantle are
quite similar throughout the affected area.
Other factors that may have influence on the distribution of
landslides are related with seismic effects on shaking in the near
field, such as the hanging wall and directivity effects. The
hanging-wall effect relates with larger ground motions on the
block above an inclined fault (the hanging-wall block) and is
common on earthquakes in thrust faults (e.g., Abrahamson and
Somerville 1996). Given that the LOFZ is a strike-slip fault and the
earthquake plane solution is nearly vertical, this effect is
negligible in this case. Directivity effects are related with the
rupture direction of the fault, tending to generate larger ground
motions toward this direction (Somerville et al. 1997; Somerville
2003). The clearly larger density of landslides toward the east and
north of the epicenter may suggest such kind of effect, but those
areas also correspond to areas of greater relief and faults, then
that distribution can be explained by geotechnical and topographic effects as previously described. The rupture direction of
this event is still a matter of research by seismologists; thus, the
directivity effect cannot be yet disregarded and must be further
analyzed in future research.
Conclusions
The April 21, 2007 shallow crustal earthquake (Mw 6.2) in the
Aysén Fjord, Chilean Patagonia, triggered over 500 landslides,
mainly shallow soil and rock slides and also rock falls, debris
flows, and rock avalanches. The landslide areas and epicentral
distances follow empirical relationships established by worldwide
data, while the position of the landslides on the slopes, close to
the ridge crests, strongly suggests the occurrence of topographic
amplification site effects, which would have played a role in
triggering the failures. The location of the landslides is also
clearly related to the main fault traces of the seismogenic
Liquiñe–Ofqui Fault Zone. This event has configured a new
situation of seismic and landslide hazard in the fjordland region,
where the presence of towns and economic infrastructure along
the fjord coasts constitutes a potential risk that has not been
considered before the 2007 earthquake. Since then, the geological
hazard of the Aysén fjord area has been assessed at a 1:50,000
scale basis by Universidad de Chile (unpublished report), while
the landslide susceptibility of the Aysén and Comau fjord areas
has been studied by Arenas et al. (2008) and Náquira et al. (2009),
respectively. Further investigations along other fjords are
necessary for a complete hazard assessment of the inhabited
zones of the Chilean fjordland.
Acknowledgments
This study was funded by Fondecyt Project 11070107, the
Millenium Nucleus Montessus de Ballore International Earthquake Research Center, and the Energía Austral Limitada
electricity company. Field work collaboration and support by G.
Orozco, R. Rauld, F. Bondoux, the National Forestry Corporation,
the National Emergency Office, and the Chilean Air Force are
greatly acknowledged. We also thank Energía Austral for allowing
publication of data obtained from baseline studies in the Cuervo
River basin, as well as M. Arenas, R. Hermanns, S. Löw, I.
Henderson, E. Anda, S. Saetre, P. Derch, G. Vargas, and L. Lara for
their fruitful discussions in the field, S. Barrientos for his helpful
insights on seismological issues, C. Padilla for helping with the
database, and J. Le Roux and B. Townley for their review of the
draft manuscript.
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S9_089
S. A. Sepúlveda ()) . A. Serey . M. Lara . S. Rebolledo
Departamento de Geología, Universidad de Chile,
Casilla 13518, Correo 21,
Santiago, Chile
e-mail: sesepulv@ing.uchile.cl
A. Pavez
Departamento de Geofísica, Universidad de Chile,
Blanco Encalada,
2002 Santiago, Chile