IMAGING THE SLAB AND FOREARC ABOVE THE 2010 MAULE EARTHQUAKE IN
CENTRAL CHILE
by
Mallory Morell
A Prepublication Manuscript Submitted to the Faculty of the
DEPARTMENT OF GEOSCIENCES
In Partial Fulfillment of the Requirements
for the Degree of
MASTER OF SCIENCE
In the Graduate College
THE UNIVERSITY OF ARIZONA
2012
1
Imaging the Slab and Forearc Above the 2010 Maule Earthquake in Central Chile
Mallory Morell, Susan Beck, George Zandt
Abstract
The Mw=8.8 Maule earthquake that occurred off the coast of Chile on February 27, 2010 is one
of the largest megathrust earthquakes ever to be recorded and ruptured ~600 km of the plate
boundary. This segment of the Nazca-South America plate boundary is an ideal region to
investigate the processes related to the structure of the forearc wedge, continental Moho and
subducting slab because of all the seismic data recorded following the Maule earthquake. Within
weeks of the Maule earthquake, international teams joined Chilean seismologists to install an
array of seismic stations between 33°- 38.5ºS, from the coast to the foothills of the Andes to
produce the International Maule Aftershock Deployment (IMAD) data set. These arrays were
deployed ~6-10 months in order to capture and study the aftershocks in and around the rupture
zone, and to better understand crustal and mantle wedge structure. We calculated receiver
functions (RFs) from P and PP phases and made Common Conversion Point (CCP) stacks to
image the structures in the slab and forearc wedge.
In our trench parallel cross-sections near the Chile coast we image a strong P-to-S
conversion we interpret as the down-going oceanic Moho. In addition we see a arrival indicating
a decrease in velocity that we interpret as the base of the slab (oceanic LAB) indicating the
oceanic slab is ~50-60 km thick. Near the hypocenter of the Maule earthquake the slab arrival is
disrupted and is interpreted to be faulted or imbricated at a depth of ~40 km. In our trench
normal sections we see the oceanic Moho to a depth of ~70-80 km and the continental Moho at
~35 km in the forearc where it approaches the slab. Finally, we combined IMAD data with
previously collected CHARGE data to create a more complete CCP RF cross-section across the
entire Andes at ~36ºS. This is the first study that has imaged the continental Moho in the forearc
at this latitude (36º-37.5ºS) or to use RFs to image the rupture features from the Maule event.
Introduction
The Chilean subduction zone is one of the most active convergent plate boundaries on
Earth, producing large subduction zone megathrust events every couple of decades. Some
examples include the Mw=9.5 1960 event that was the largest instrumentally observed
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earthquake to date and devastated the Pacific rim with its resulting tsunami [Cifuentes, 1989;
Barrientos and Ward, 1990] and the more recent 2010 Mw=8.8 Maule earthquake that created a
local tsunami and whose shaking affected as many as 1.8 million people [USGS, 2012]. This
subduction system is also responsible for creating some of the tallest and most active volcanoes
on Earth, including the stratovolcano Cerro Azul whose peak is 3788 m high and was the source
of the world’s largest explosive eruption of the 20th century in 1932 that ejected 9.5 cu km of
dacitic tephra into the atmosphere [Siebert and Simkin, 2002-2012].
The Mw=8.8 Maule earthquake ruptured ~600 km of the subduction zone on the 27th of
February 2010 causing a damaging tsunami locally and 500 deaths [USGS, 2012]. Immediately
after this megathrust earthquake, a large international response was organized to deploy seismic
instruments in the aftershock region. The International Maule Aftershock Deployment (IMAD),
a joint project between the University of Chile, IRIS (Incorporated Research Institutions for
Seismology), IPGP (Institut de Physique du Globe de Paris), Caltech, GFZ (Deutsches
GeoForchungsZentrum), and the University of Liverpool, was comprised of 104 broadband
stations located at 99 sites deployed two weeks to 1 month after the mainshock with the goal of
recording aftershocks (Fig. 1). The best station coverage was achieved between April 1 and June
1, 2010 and the data were made available as soon as it could be archived. The station spacing
across this array was on average 30 km and covered the whole length of the rupture from 32.5º38.5ºS. Lange et al., [2012] and Rietbrock et al., [2012] have determined 20,205 and 30,000+
aftershock locations, respectively, using data collected by this international array. In addition,
Hayes et al. [in preparation, 2012b] have also relocated Maule earthquake aftershocks using the
IMAD data. All three studies show similar patterns with the deepest aftershocks occurring at
~50-55 km depth along the plate interface and a noticeable gap in aftershocks between ~35-40
km depth.
Our goal in this study was to image the subducting slab, forearc, and arc, and examine
how the subducting slab interacts with the continental Moho in the forearc using receiver
function analysis. We found the location and density of this array to be optimal for expanding
upon past receiver function studies that had also incorporated the southern line of the CHARGE
array [Fig. 1; Gilbert et al., 2006] and to compare with local source tomography studies
conducted over the 1960 rupture further south as discussed below [e.g. Haberland et al., 2006,
2009; Boehm et al., 2002; Yuan et al., 2006; Asch et al., 2006].
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Tectonic Setting
Between 32º and 39ºS along the south central Chilean continental margin, the Nazca plate
is converging obliquely with the South American plate at a rate of ~7.4 cm/yr with an azimuth of
~74º [DeMets et al., 2010]. This angle of convergence has caused a series of along-strike fault
systems, including the 1100 km long right-lateral Liquiñe-Ofqui Fault Zone that follows the
foothills of the Main Cordillera or active arc [Haberland et al., 2006]. The age of the subducting
plate in this region varies between 30-35 Ma, with age increasing to the north [Müller et al.,
1997]. The Juan Fernandez Ridge and its resultant flat slab region bound the 2010 Maule rupture
zone to the north and the Mocha, Valdivia, and Bio-Bio Fracture zones act as a southern
boundary. The southern boundary is also where the 1960 Mw=9.5 rupture nucleated and where
we find the anomalous Arauco Peninsula [Hicks et al., 2012].
The overriding plate above the 2010, Maule Earthquake rupture can be divided into three
trench-parallel geomorphic segments – the Coastal Cordillera, the Central Valley, and the Main
Cordillera or active arc. Where our cross sections intersect the active arc and extend into the
backarc, they cross a pair of historically active volcanoes, Descabezado Grande and Cerro Azul
[Siebert and Simkin, 2002-2012, Fig. 1].
Previous Geophysical Studies
The region between 32º-39ºS has been targeted in the past for geophysical studies in part
because of the transition from flat slab subduction to the north back to normal subduction, the
location of the 1960 epicenter just south of the region, and the gap in seismicity that has been
observed between 35ºS-37ºS [Campos et al., 2002]. There have been several arrays deployed
across Chile and western Argentina (e.g. CHARGE 2000-2002; ISSA2000 2000; TIPTEQ 20042005; temporary seismic installations between 39ºS-40ºS 2008-2009). The 8 stations in the
southern transect of the Chile Argentina Geophysical Experiment (CHARGE) array were the
only stations formerly deployed for any length of time within our study area. These stations were
spaced an average of 70 km apart, which allowed for the imaging of large scale structures such
as the continental Moho and crustal structures beneath the arc, but had limited resolution in the
forearc [Gilbert et al., 2006; Wagner et al., 2005; Heit et al., 2008].
A transect through the center of our study region at 36ºS was imaged using P-wave
receiver functions (RFs) and S-wave receiver functions (SRFs) by Gilbert et al. [2006] and Heit
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et al. [2008], respectively. They observed the continental Moho beneath the arc and in the
backarc at depths between 25-55 km. Heit et al. [2008] were also able to image the LAB
(Lithosphere-Asthenosphere Boundary) beneath the continental Moho in the backarc at depths
between 100-125 km. Neither study had resolution in the forearc, so neither was able to robustly
image the downgoing slab in this region. Using local travel time tomography, Wagner et al.
[2005] found low vp, low vs, and a high vp/vs directly beneath the arc at a depth of 50-100 km in
this region that was interpreted as localized pockets of melt.
South of the Maule aftershock region, at ~39ºS, receiver function studies by Asch et al.
[2006] and Yuan et al. [2006] and local travel time tomography studies by Haberland et al.
[2009] and Boehm et al. [2002] imaged the downgoing slab in great detail. Both receiver
function studies imaged the slab down to a depth of 110 km, closely following local seismicity.
They also observed the continental Moho reaching its maximum depth of 40-50 km beneath the
arc and shallowing to 25 km toward the west and 35 km toward the east. The local source
tomography studies observed the downgoing slab to a depth of 120 km, dipping 30ºE, and they
also observed a thickening of the continental crust under the arc to ~50 km and thinning to the
west. Haberland et al. [2009] noted that the mantle wedge is most likely not serpentinized
throughout, but instead is hydrated in patches. What is lacking for these studies is a clear image
of the continental Moho in the forearc and at 36ºS, a clear image of the downgoing slab.
Data and Methods
Data
IMAD stations contained 4 different sensor types: Guralp CMG-3ESP Seismometer (30s
– 50Hz), Nanometric Trillian 120P (120s – 10Hz), Guralp CMG40 (1Hz – 100Hz), and Guralp
CMG3T (120s – 50Hz). In addition to the IMAD experiment, we incorporated data from the
CHARGE portable broadband seismic experiment deployed in 2000-2002 as well as permanent
stations PEL from the Geoscope Global Network and GO05 from the Chilean National Network
(Fig. 1).
This IMAD deployment was designed with the primary goal of recording aftershocks
following the Mw=8.8 Maule earthquake. This made identifying teleseismic events challenging
because the aftershocks created “noise” that had to be filtered out, and in some cases made
teleseismic P- or PP waveforms unusable. P phases were chosen for large magnitude (Mw > 6)
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earthquakes at distances between 30º-95º and PP phases (Mw > 6.5) from 95º-180º. We were
able to use four events with well-recorded P phases and five events with well-recorded PP
phases. This resulted in high quality receiver functions, though it also limited our backazimuth
coverage. For the longer deployed CHARGE array we were able to use 66 events for P phases.
Methods
We carried out RF analysis on teleseismic P and PP phases using data recorded during
the IMAD experiment and data from other broadband stations in the region. Initially,
seismograms were bandpass filtered from 0.05 to 3 Hz, to help remove the thousands of local
aftershock events and anthropogenic noise. Then the horizontal components were rotated to the
radial and tangential components.
For the RF calculations we used an iterative time domain (pulse stripping) deconvolution
approach [Ligorria and Ammon, 1999], in which the vertical component is deconvolved from the
horizontal components in a 110 s time window, starting 10 s before the P or PP onset. This
process forms a series of Gaussian spikes that include P-S conversions from discontinuities.
Within the RF the amplitude and timing of the P-SV conversions can be directly related to the
discontinuity’s impedance contrast and depth below the station [e.g. Langston, 1977; Owens et
al., 1984]. We tested two different Gaussian values, 1 and 2.5, which act as low pass filters, (this
corresponds to low-pass filter corner frequencies of 0.5 and 1.2 Hz respectively). For the results
presented here, we show a Gaussian of 2.5; making our maximum resolution layers > 1 km.
We calculated unnormalized RF and all of the radial RFs were required to have a
variance reduction of at least 70%, meaning 70% of the signal is fit. Next, the RFs were visually
inspected to eliminate those of poor quality. Some qualities used to determine if a trace should be
excluded included a negative first arrival, large delay in the first arrival, and extreme ringing
along the trace. Of the 781 starting RFs for a Gaussian of 2.5, 376 unnormalized remained after
this quality control.
Once the RFs were calculated, we generated Common Conversion Point (CCP) receiver
function cross-sections migrated to depth [Dueker and Sheehan 1997; Gilbert and Sheehan
2004]. Stacking numerous RFs from various stations helped minimize the noise caused by near
surface effects that are specific to each station and not coherent between stations. Over 370 RF
traces (with a Gaussian of 2.5) were stacked to create our final cross-sections. The P and
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converted S waves are ray-traced through geographic bins using a constant vp and vp/vs. The rays
that went into each stack were determined by the bin spacing that in our case varied between 5
km and 15 km. We used a bin sharing of 2, meaning that the data from one bin in each direction
was averaged with the data from the center bin to calculate each stack. This helped eliminate
gaps in our cross-sections but also caused some lateral smearing. The exception to this bin
sharing value was Profile X2 where a bin sharing of 1.5 was used.
The average number of good RFs at a station was 2 with the maximum of 7 at L006 and
U59B, and a minimum of 1 RF at 20 stations. Of the 104 stations installed, we were able to use
data from 87 stations. At the permanent station PEL we used 40 events and at GO05 we used 4
events. Despite the limited number of earthquakes we have a total of 109 stations with high
quality P and PP phases recorded.
For this study several different cross-sections and bin spacings were used: 1) for the
trench-parallel cross-sections (Profiles Y1-4 on Fig. 1), the bin centers are separated by 15 km in
both the NE-SW and NW-SE directions and the mesh was rotated 15º clockwise from north, 2)
trench perpendicular Profile X3 the bin centers are separated by 15 km in both the NE-SW and
NW-SE directions and the mesh rotated 15º counterclockwise from east, 3) trench perpendicular
Profile X2 differs only by having a bin spacing of 5 km in the NE-SW direction and 20 km in the
NW-SE directions, 4) for trench perpendicular Profile X1 the bin centers are also separated by 15
km in the N-S and E-W directions and the grid is not rotated, and 5) for Profile XCHRG the bin
centers are separated by 20 km in the N-S direction and 25 km in the E-W direction, have a bin
sharing of 2, and are also rotated 15º counterclockwise from east (Fig. 1). To migrate from time
to depth in the CCP cross-sections we used a vp = 6.5 km s-1 above 60 km, to insure crustal
features are not mapped artificially too deep by migrating them with mantle velocities, and vp =
8.0 km s-1 below that and a vp/vs of 1.75. This vp/vs value is the same as assumed by Campos et
al., [2002] in their earthquake location study across the region and is the average value of the
gradient calculated by Haberland et al., [2006, 2009] for the segment of the subduction zone just
south of our region. Changing the value assigned to the vp/vs does not change the pattern within
the CCP stacks, it would simply shift the depth of the discontinuities slightly (Fig. S1).
Amplitudes were rescaled to take into account the varying angles of incidence of the earthquake
arrivals by multiplying each trace by ir/i, where ir is a depth invariant reference angle (20º) and i
is the incident angle calculated from P slowness [Jones and Phinney, 1998].
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Modeling RF at Station PEL
As noted in many previous receiver function studies conducted in central and southern
Chile, most of the teleseismic earthquakes occur in Pacific rim subduction zones, making it
difficult to have the full range of backazimuths that would be ideal for this type of study [e.g.
Heit et al., 2008; Gilbert et al., 2006; Yuan et al., 2006]. South central Chile has only one active
subduction zone located to its east-southeast, the South Sandwich Islands, that were not active
with large events (>6.0) during the 10 months the stations were deployed. The earthquakes used
in our final dataset are located mainly to the west (Tonga Trench) and the northwest (Baja
California and Ecuador) of our region, with very few events from the east or southeast (Mid
Atlantic ridge, South Sandwich Islands). Events occurring at eastern azimuths are key to imaging
a slab dipping to the east, as indicated by studies done by Owens et al [1984] which illustrated
that waves incident from western azimuths for an eastward dipping slab produce the smallest
amplitude converted phases. In order to look in more detail at RFs from a broad range of
backazimuths, we incorporated data recorded at PEL, a permanent station from the Geoscope
Global Network. Station PEL has been operating since 1998, hence we identified 40 wellrecorded teleseismic earthquakes for a more detailed analysis. We calculated receiver functions
for PEL and then calculated synthetic RFs for different velocity structures to determine a more
detailed structure than we could obtain from the data recorded at the temporary stations. Figure
2 shows the RFs plotted as a function of backazimuth that shows a complex pattern of converted
phases. We use this suite of RFs and forward modeling to look at the sensitivity of the RFs to
velocity models with dipping layers (slab and Moho).
We calculated synthetic teleseismic waveforms with a full range of backazimuths starting
with a basic 3-layer over half space seismic velocity model containing continental crust,
continental Moho, oceanic slab, and oceanic Moho, using the program Ray3D [Owens et al.,
1984]. We then calculated RFs from the synthetic waveforms using the same iterative time
domain deconvolution approach with the same Gaussian value of 2.5 that was applied to the data
recorded at PEL. The RFs for both PEL and the synthetics were then binned and plotted by
backazimuth. Following the approach by Cassidy [1992], we started with a basic 3-layers over
half space model in order to constrain the orientation of large-scale features. As this region is
much more complex than a simple 3-layers over half space model, a more complex model was
needed to explain more of the prominent inter-crustal arrivals. We added inter-crustral layers one
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at a time and adjusted each layer’s depth until we developed a model that approximately fit the
RFs at PEL. We identified 2 inter-crustral layers above the continental Moho. This gave us a
final model with 5-layers over half space (Fig. 3).
Results
We first describe our results from the permanent station PEL and then go on to describe
our results in the forearc and across the arc in south central Chile.
Station PEL
Although forward modeling of RFs is non-unique, we can use RFs recorded at
PEL to test the sensitivity of dipping layers and event backazimuth. Figure 3 compares plots of
the synthetic RF traces distributed by backazimuth created from the final 5-layer model with RFs
calculated from data recorded at PEL. Although not every detail fits, the first order patterns
appear very similar. The 5-layer over half space model was built based on previous work in the
region. The slab contours (slab1.0) from Hayes et al. [2012a] indicate that the top of the slab is at
~70 km depth under PEL and these slab contours are consistent with oceanic Moho depths
estimated at 2 other IMAD stations (Fig. 4). We estimated the oceanic crustal thickness to be 10
km based on typical oceanic crustal thicknesses [Leeds et al., 1974] and the RF sections we have
stacked. We used a depth to the continental Moho of 42 km. Using these depths and thicknesses,
we created the initial 3-layer model. To define the intercrustal arrivals we observe in the PEL
RFs we looked to the topography and geology. There is a prominent positive arrival following
the direct P-arrival, which is followed by a very prominent negative arrival. The prominent
positive arrival might be explained by PEL’s location. Since PEL sits in the foothills of the
Andes, it is possible that there is a thin high-velocity impedance contrast dipping to the west,
produced by the base of a forearc sedimentary basin. The negative arrival that follows is very
prominent for a wide range of backazimuths, indicating it is relatively flat-laying. In our updated
model we added two inter-crustal discontinuities as shown in Figure 3b. Using our new 5-layer
model our synthetic RFs have similar patterns to the observed RF from PEL.
We can make several important observations from these results. The first observation is
that the slab arrival (top of the oceanic crust and oceanic Moho) are strongest at back azimuths
between 0 and 160° (northeast-east-southeast) where we have limited data recorded at the
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temporary stations. More earthquakes from the east would improve our image of the eastdipping slab. Second, many of our events are from backazimuth directions (200 to 350°) where
the signal from the slab disappears due to the angle of incidence between the P or PP phase with
the slab. Hence, our image of the slab in cross-sections perpendicular to the trench is limited in
resolution. The same is true for an east dipping continental Moho, the strongest conversions are
from stations between 0 and 180° and not from backazimuths of 200° to 350°
Chile Forearc
CCP RF stacks from the forearc region are shown in Figures 5 and 6. We show sections
both parallel (Fig. 5) and perpendicular (Fig. 6) to the trench in regions where we have the most
data to stack and can produce near complete cross-sections.
We show four trench-parallel CCP cross-sections in Figure 5 for the line locations shown
in Figure 1. The most prominent P-to-S conversion shows up as an increase in S-wave velocity
with depth (red arrival) seen as a continuous horizontal feature with a depth varying between 3560 km (Fig. 5). It also closely follows the seismicity, which clusters along a narrow band. We
interpret this feature as the oceanic Moho in the subducting slab. We would expect the oceanic
Moho to be an increase in velocity between the oceanic crust and mantle and the depth of the
conversions is at the right depth based on seismicity. In Figure 5 the white circles are Maule
aftershock events relocated by Hayes et al. [in preparation, 2012b] and the black circles are
events from 1970 to 2012 relocated by the NEIC [USGS, 2012]. The dashed black line indicates
the location of the slab calculated from the USGS Slab1.0 model [Hayes et al., 2012a] and the
white dashed line indicates our estimate of the location of the LAB (Fig. 5).
At a depth between 95-110 km there is another prominent horizontal conversion that
indicates a decrease in velocity with depth (blue) that we interpret as the oceanic lithosphereasthenosphere boundary (LAB). Based on the timing we conclude this conversion is a primary
arrival and not the second multiple of the oceanic Moho. This gives us a thickness of the oceanic
lithosphere of ~50-60 km which fits with the predicted thickness based on the 35-40 Ma age of
the subducted lithosphere [Leeds et al., 1974].
The subducting oceanic Moho varies slightly as you move from north to south along the
trench parallel sections (Fig. 5). Profile Y1-Y4 in Figure 5 runs sub-parallel to the down-dip
edge of the Maule earthquake rupture zone and profile Y1 likely includes the hypocenter where
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the mainshock rupture initiated (white star in Figure 5). At ~35.6º-36°S and at a depth of 35-40
km, there is a disruption in the positive arrival that we interpret as the oceanic Moho that can be
most prominently seen in Line Y1, but also appears in profiles Y2 and Y3. The disruption in Y1
looks as if the oceanic Moho has been uplifted ~10 km higher than the surrounding oceanic
Moho arrival. This same feature can be seen in profiles Y2 and Y3 at the same location and
depth, but instead of an offset of the oceanic Moho, it appears as an arrival above the Moho.
At the north end of our cross-sections (33.5°-34.5°S) there are positive arrivals directly
beneath the oceanic Moho. This would appear to be a discontinuity within the down going slab.
Along parts of our cross-sections there is a negative arrival above the positive oceanic
Moho arrival that might be the top of the oceanic crust suggesting in places that the thickness of
the oceanic crust is ~10 km depth. This negative arrival is discontinuous, hence we can’t be sure
if it consistently represents the top of the oceanic crust. At a depth of 40-50 km in the
subduction system it is not clear if we should see a decrease in seismic velocity at the top of the
oceanic crust since we are not sure what the material is above the slab. If the material above the
slab is continental crustal material then the oceanic crust would represent an increase in velocity
but if there is continental mantle material above the oceanic crust then we would expect to see a
decrease in velocity. If the material above the slab is hydrated mantle then we would not expect
to see the oceanic crust.
Along profile Y4, the slab shallows to the north, hence, we expect to see the oceanic
Moho arrival at a shallower depth than further south. In profile Y4 the oceanic Moho arrival is
more discontinuous than the other cross-sections and appears to have an abrupt change in depth
between 35º-35.5ºS consistent with the general trend of the slab contours from the USGS
[slab1.0 model, Hayes et al., 2012a]. We cannot be sure if this change in depth is as abrupt as it
appears or if we lack the data coverage to see a more gradational change.
Our most complete trench normal section is profile X2 in Figure 6. We identify the
oceanic Moho as a series of red arrivals that dip down to ~80 km. In general, we do not see the
top of the oceanic crust along the dipping slab in part because it may not have a distinct velocity
contrast to the over-riding forearc and or mantle material. There is a negative conversion at ~20
km just as the slab begins to descend into the mantle that is a possible candidate for the top of the
oceanic crust. The seismicity follows the prominent positive arrivals down to a depth of ~80 km.
The downgoing slab does not appear very continuous, which is likely due to the limited
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backazimuth directions of the events as was discussed in our synthetic modeling section. At the
bin spacing used (5 km), there is still some lateral streaking, making the downgoing plate appear
segmented. In the X2 profile, there is also a prominent positive arrival at ~35-40 km depth that
we interpret as the continental Moho under the foothills of the Andes. This discontinuity does not
reach the slab but the projection is consistent with the down dip limit of the Maule earthquake
rupture zone near a depth of 40 km [Lorito et al., 2011, Vigny et al., 2011].
The trench perpendicular profiles X1 and X3 have less data so we used a bin spacing of
15 km and bin sharing of 2, which results in more lateral smearing, hence we do not see a clear
indication of the slab, however we do see the continental Moho and mid-crustal structure. The
line further to the north, profile X3, cuts far enough east to actually image below part of the
active arc, which includes a historically active volcanic complex with two stratovolcanoes (Cerro
Azul and Descabezado Grande). This line is just south of the CHARGE line that we will discuss
below, which cuts across the active arc into the backarc. Profile X3 allows us to image the
forearc of the same region in much greater detail. The most striking features in profile X3 are the
two strong positive arrivals under the arc and foothills at ~30 and ~50 km depths (Fig. 6). Under
the forearc we interpret a weaker positive arrival as the continental Moho at ~35 km depth.
The CCP RF stacks from the southern forearc, between 37º-38ºS, are shown in Profile X1
where we used an east-west orientation, with a 15 km bin spacing and bin sharing of 2. Hence
we see hints of the slab but there is significant lateral smearing. We observe a strong positive
arrival under the eastern part of the forearc that we interpret as the continental Moho at ~35 km
depth.
Continental Moho across the Arc and Forearc
We have combined RFs from the IMAD data with CHARGE data to calculate a more
complete CCP RF stack across the forearc, the active arc, and into the backarc as shown in
Figure 7. The most prominent feature on the cross-section is a strong increase in velocity in the
backarc interpreted as the continental Moho. This strong P-to-S conversion arrival was imaged
and identified as the Moho by Gilbert et al. [2006]. The continental Moho appears to bifurcate
under the arc and appears as a series of discontinuities suggesting a layered structure under the
arc between longitudes of 70° and 71°W. There is a strong conversion at ~30 km depth and a
deeper conversion at ~60 km. We do not image the down-going slab in part due to the bin
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sharing of 20 km, which would smear out dipping arrivals. There is no evidence in our sections
for the boundary with the continental LAB at a depth of ~100 km as described by Heit et al.
[2008] using S-wave RF.
Discussion
We have imaged the forearc between latitudes 32ºS and 39ºS using a relatively dense
array of IMAD broadband stations. Two features that have been identified through this RF study
include imaging of the subducting slab (interpreted as the oceanic Moho) and the continental
Moho in the upper plate in the forearc and arc. In this section, we discuss our results and
compare our interpretations with other studies in this region.
Imaging the subducting slab
We have imaged a P-to-S conversion in the depth range of 35-60 km in our trench parallel
cross-sections that is at a depth consistent with the down-going slab (Fig. 5). Our interpretation is
that it is most likely the oceanic Moho. In the discussion below we give two additional
possibilities and argue that the arrival is most likely the oceanic Moho. There are 3 possible
interpretations for the strong arrival we observed on the trench parallel cross-sections (Fig. 5).
1. Top of the oceanic crust. This interpretation would require low velocity continental crust
above the slab. Hence, there would be an increase in velocity from continental crust (6.46.9 km/s) to oceanic crust (7.2-7.5 km/s).
2. Oceanic crustal Moho. In this case the material above the oceanic crust has P-wave
velocities of in the range of 6.0-7.4 km/s consistent with some volume of hydrated and
serpentinized material and would not produce a strong increase in velocity at the top of
the oceanic crust. Hence, we are observing an increase from the oceanic crust (7.2-7.6
km/s) to the oceanic mantle (8.0-8.3 km/s).
3.
Base of the hydrated oceanic mantle within the slab. If the down-going slab is hydrated
into the upper mantle we might be seeing the increase in P-wave velocity from the
hydrated mantle (7.0-7.5 km/s) to the normal mantle (8.0-8.3 km/s).
Refraction studies done at 37-39°S, at the very end of the Maule rupture and further south in
the SPOC project give us some insight into the velocity structure [Sick et al., 2006]. At 38.25°S
the SPOC south line show P-wave velocities in the range of 6.8-7.0 km/s above the oceanic crust
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and similar velocities in the down-going oceanic crust suggesting that we should not see a
conversion at the top of the oceanic crust in the depth range of 25-45 km [Sick et al., 2006]. At
39°S seismic refraction data from the ISSA project show P-wave velocities above the oceanic
crust ~7.2-7.3 km/s with the oceanic crust having a slightly lower velocity of 6.9-7.0 km/s in the
depth range of 40-60 km/s [Boehm et al., 2002]. Haberland et al., [2009] also image the region
between 37-39°S using travel time tomography and identify the oceanic crust as a high vp/vs
layer and observe very little change in the P-wave velocity between the oceanic crust and the
upper plate between 25-45 km depth with the largest P-wave velocity increase at the oceanic
Moho.
Hicks et al. [2012] using Maule earthquake aftershocks and local travel time tomography
show a strong increase in P-wave velocity right below most of the aftershocks with velocities of
6.5-7.0 km/s above the deepest aftershocks at 30-50 km depth. Hicks et al. [2012] also see an
increase in vp/vs in the region of the deepest aftershocks. However, they do not image the
oceanic crust as a discrete layer, in part because they are using aftershocks, which may in large
part represent slip and deformation at the plate interface, and hence may not be ideal in imaging
the oceanic slab below the aftershocks.
Clearly the region at the down-dip edge of a major earthquake rupture zone (35-60 km) is
complex and processes such as fluid release are likely altering the material in the region. Based
on the studies sited above and extrapolating north we suggest that the strong arrival in our trench
parallel cross-sections is most likely the oceanic Moho and corresponds to case 2 above. Clearly
more work is needed to refine the details of this critical region of the subduction zone (Fig. 5).
Transect Across the Arc
The southern transect of the CHARGE array cuts through the center of our IMAD array
at 36ºS. The resolution in the forearc for previous RF studies conducted across this line had
sparse station coverage and hence, limited resolution [e.g. Gilbert et al., 2006; Heit et al., 2008].
Our higher density array of stations focused over the forearc allows us to extend upon the
observations made in previous studies. Gilbert et al. [2006] successfully imaged the continental
Moho under the backarc using RFs. We were also able to clearly image the continental Moho in
the backarc at ~40 km and the two positive arrivals at ~25-30 km and ~50 km depth under the
arc similar to Gilbert et al. [2006]. Gilbert et al. [2006] interpreted the two arrivals under the arc
14
as different parts of a magmatic system, with the shallower discontinuity marking the location of
the top of the lower crust and the deeper discontinuity is most likely the continental Moho under
the arc. A strong negative arrival in the RF occurs between these positive arrivals and may
indicate a region where basaltic material intrudes into the lower crust (Gilbert et al., 2006).
While we cannot be sure of the actual structure under the active arc it is clear that there is a
layered structure that does not extend all the way into the forearc or backarc, hence it is likely
due to magmatic processes. As noted the section samples a very active portion of the arc where
Cerro Azul had a major eruption in 1932.
Gilbert et al. [2006] also described a large coherent negative arrival beneath the crust that
they interpreted as the location of low-velocity material in the mantle that might be feeding the
shallower crustal magma chamber. In their cross-section this feature is very prominent and
continuous, across the entire section; within our section the negative arrival beneath the
continental Moho is still continuous beneath the backarc but with a much lower amplitude. The
negative arrival breaks up under the arc and ends as it approaches the location of the downgoing
slab at a depth of ~70 km.
The S-wave RF study by Heit et al. [2008] imaged a single continental Moho, with
depths shallowing to nearly 25 km in the eastern part of the backarc and reaching its maximum
thickness of 55 km under the arc. They also lose resolution under the volcanic arc, but see the
Moho reappear in the forearc at ~35 km. If we assume the deeper positive arrival under the arc is
the Moho, then these depths are consistent with the continental Moho in our sections, especially
in the forearc. They also noted the presence of the continental LAB in the backarc at a depth of
100-125 km. However, there is no evidence of this feature in our CCP RF sections.
Forearc – Southern lines
The forearc between 38º-40ºS has been imaged in some detail using seismic data from the
TIPTEC experiment [e.g. Haberland et al., 2006, 2009; Yuan et al., 2006]. Our southern-most
profile, X1 (Fig. 1), falls between 37º-38ºS at the northern end of these previous studies. This
region coincides most closely with the tomographic study by Haberland et al. [2009]. In the
study by Haberland et al. [2009], they imaged “patches” of hydration in the forearc wedge. In
our sections we see a large wedge of low-velocity between the slab and the crust. This could be
consistent with hydrated patches being present; simply the difference between the hydrated
15
patches and the rest of the mantle wedge is not extreme enough to create visible boundaries in
the RF stacks.
The extent of the contact zone is highly debatable from our study and the RF studies by
Yuan et al. [2006] and Asch et al. [2006]. They were unable to resolve the continental Moho
above the forearc wedge, but estimate the depth could be as shallow as 25 km where it is cut by
the Lanalhue fault zone. Our sections focus on the forearc, where we see the continental Moho
approaching the slab at ~35 km, which is much shallower than those noted by Haberland et al.
[2009] (~50 km) but deeper than the estimate made by Yuan et al. [2006]. Yuan et al. [2006]
estimated that the continental Moho was raised due to movement of the Lanalhue fault system,
but we do not see evidence for this because our cross-sections are too far north of the fault zone
to observe such effects.
Correlations with the 2010 Maule Rupture Zone
Our coverage in the forearc is unique from previous studies in that most of our profiles,
both along strike or normal to the trench, intersect with a portion of the rupture surface from the
Maule mainshock especially as defined by the aftershocks. In the along strike sections we
observe how the surface of the plate, in both the RF image and the USGS Slab1.0 model,
displays a long wavelength undulation that may have influenced the location of the rupture
interface. The most interesting correlation is that the rupture appears to have initiated where the
slab interface is located at the lowest point of a long wavelength downward flexure (Fig. 5). We
noticed in the along strike sections that where the rupture initiated there appears to be a
discontinuity in the relatively continuous slab. This disruption appears as small block of uplifted
slab that can be seen in the along strike sections further east as well, with the continuous slab
appearing underneath the anomalous discontinuity (Y1 and Y2 in Fig. 5). A high-velocity
feature was also imaged at approximately the same depth and location in the local source
tomography study also using aftershocks recorded by the IMAD array [Hicks et al., 2012]. At
36ºS they noted a high vp (7.0 km/s) and a high vp/vs (~1.89) region at a depth of 25 km along the
megathrust. They interpreted this as a topographic feature, possible subducted seamount on the
down-going plate. We suggest that we are imaging the same feature in our trench-parallel
sections (Fig. 5). We would expect a seamount to make the oceanic crust locally thicker, not
necessarily cause an offset in the oceanic Moho. Rather, we interpret this feature to be faulting or
16
imbrications in the slab near the point of maximum downward flexure of the slab in the along
strike direction, where along strike compression would be at a maximum in the upper part of the
slab. We further suggest that this feature might have acted as a local asperity that served as a
nucleation point for the Maule earthquake.
Conclusions
In this study, we used P and PP RFs to image the downgoing Nazca slab and forearc of
the south central Chilean subduction zone above the 2010 Mw=8.8 Maule rupture zone. In
addition, we extended the previous RF study across the entire arc and into the backarc. Our
major conclusions are as follows:
•
We imaged the downgoing oceanic lithospheric Moho on trench parallel (near the coast)
CCP RF cross-sections at 40-50 km depth. Above the oceanic Moho arrival is a
discontinuous negative arrival that likely corresponds to the top of the oceanic crust
indicating the crust is ~10 km thick, where it is present.
•
Our image of the subducted slab is disrupted by a section appear uplifted near the
hypocenter of the Maule earthquake at a depth of ~40 km. This may indicate that the slab
is faulted or imbricated in the region where the Maule earthquake initiated.
•
We image the base of the oceanic lithosphere as a decrease in seismic velocity indicating
that the lithospheric thickness is 50-60 km, consistent with the age of the downgoing
lithosphere.
•
Along trench perpendicular CCP RF cross-sections we see the oceanic Moho in the
subducting slab down to depths of ~70-80 km.
•
We have a more complete CCP RF image across the entire Andes at ~36°S by combining
IMAD data and previously collected CHARGE data. Similar to previous studies [Gilbert
et. al., 2006] the crustal thickness is ~40 km in the backarc and increases to 60 km under
the arc. The arc has a strong midcrustal discontinuity (increase in velocity) at ~30 km
depth followed by a decrease in velocity above the Moho, yielding a strong layered
structure in the lower crust that may be indicative of magmatic processes like basaltic
underplating.
•
We observe the continental Moho at a depth of ~35 km in the forearc where it approaches
the downgoing slab. These depths as well as the subducting slab dipping at ~35º are also
17
supported by our forward modeling under the permanent station PEL at the northern end
of our array.
This is the first study using CCP RF stacks that has imaged the continental Moho in the
forearc at this latitude (36º-37.5ºS) or any of the rupture features from the Maule event. Future
work is still needed to resolve how much of the forearc wedge is hydrated and to better constrain
the uplifted section of the oceanic Moho seen near the point of nucleation of the Maule event.
Acknowledgements
This work was supported by NSF grant #1045597. Mallory Morell was also supported by
scholarships from ConocoPhillips and ChevronTexaco. The U.S. IRIS Community response to
collect IMAD data was made possible by NSF Rapid Response Research (RAPID) grant EAR1036349, EAR- 1036352 to the University of Florida and to IRIS respectively, and by the
availability of EarthScope Flexible Array (FA) instruments. Special thanks to the GSAT group at
the UA and all the IMAD field teams.
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Figure 1. Location map with IMAD broadband stations used in this study (squares) and cross
section lines shown in Figures 5, 6 and 7. Two different slip models for the Mw=8.8 Maule event
are outlined in blue and red. Coseismic slip model from Lorito et al. [2011] based on geodetic
and tsunami data is shown in blue and the coseismic slip model from Vigny et al. [2011] based
on GPS data is shown in red. Stations discussed in text are labeled in blue. Slab contours are the
slab1.0 model from NEIC [Hayes et al., 2012a]. Volcano locations (red triangles) were taken
from the Global Volcanism Program [Siebert and Simkins, 2002-2012].
22
Figure 2. Receiver functions stacked by backazimuth plotted as a function of time for station PEL.
Traces are sorted by backazimuth. Positive amplitudes are filled red and negative amplitudes are filled
blue.
23
A
)
B)
Figure 3. Modeling results for PEL. (a) Synthetic (left) and calculated (right) unstacked radial and
tangential receiver functions plotted as a function of time for station PEL. The synthetic receiver
functions are calculated with the 5-layer block model. Traces are sorted by backazimuth and structures
from the model are outlined and overlaid onto both the synthetic and calculated receiver functions
(shaded regions). (b) Velocity model of the 5-layer model (left) and its representative block model
showing east-west dips (right) of the major layers.
24
Figure 4. Stacked receiver functions from stations L006 and U59B, the stations with the most useable
data within IMAD deployment. Thick red line indicates the positive arrival we interpret as the oceanic
Moho below each station.
25
Figure 5. Common Conversion Point (CCP) cross-sections migrated to depth for trench parallel
geometry. (left side) and our interpretation (right side). Note how the shape of the oceanic plate changes,
becoming more “wavy” as the profiles move east away from the trench. Blue triangles are seismic
stations, white star is Maule hypocenter taken from the NEIC [USGS, 2012], white circles are relocated
aftershocks recorded during the IMAD experiment [Hayes et al., in preparation, 2012b], black circles are
all earthquakes on record in the NEIC from 1970-2012 [USGS, 2012], black dashed line outlines the
slab1.0 model [Hayes et al., 2012a], and white dashed line defines our estimate of the oceanic LAB. See
Figure 1 for line locations. The cartoon of CCP Stacks shows our interpretation with the prominent red
arrival the oceanic Moho. The black line is the slab1.0 model (Hayes et al., 2012a).
26
27
Figure 6. Common Conversion Point (CCP) cross-sections perpendicular to the trench (left side) and our
interpretation (right side). Note how the shape of the forearc wedge changes. In profiles X1 and X2,
there is a single continental Moho coming into a segmented slab, whereas in profile X3 the continental
Moho bifurcates, making the forearc wedge appear much more complicated. Blue triangles are seismic
stations, red triangles are volcanoes, white circles are relocated aftershocks recorded during the IMAD
experiment [Hayes et al., in preparation, 2012b], black circles are all earthquakes on record in the NEIC
from 1970-2012 [USGS, 2012], black dashed line outlines the slab1.0 model [Hayes et al., 2012a]. See
Figure 1 for line locations.
28
Figure 7. Trench normal CCP receiver function cross-section (upper) following the CHARGE line and a
cartoon of CCP cross-section (bottom) with our interpretation. Blue triangles are seismic stations, red
triangles are volcanoes, white circles are relocated aftershocks recorded during the IMAD experiment
[Hayes et al., in preparation, 2012b], black circles are all earthquakes on record in the NEIC from 19702012 [USGS, 2012] and the black dashed line is the slab1.0 model [Hayes et al., 2012a]. See Figure 1 for
line locations.
29
Supplementary Tables/Figures
Table S1. Earthquakes recorded by the IMAD array and used in this study
Date Time Long Lat Depth Magnitude 4/4/10 22:40:43 -­‐115.278 32.297 4 7.2 4/6/10 22:15:01 97.048 2.383 31 7.8 6/12/10 19:26:50 91.936 7.881 35 7.5 6/16/10 3:16:27 136.543 -­‐2.174 18 7 7/23/10 23:15:10 123.259 6.776 640 7.5 8/12/10 11:54:15 -­‐77.306 -­‐1.266 206 7.1 9/3/10 16:35:47 171.83 -­‐43.522 12 7 10/21/10 17:53:13 -­‐109.159 24.69 10 6.7 30
Table S2. Earthquakes recorded by the CHARGE array and used in this study
Date Time Lat Long Depth Mag 12/4/00 4:43:09 14.88 -­‐93.94 33 12/12/00 5:26:45 6.01 -­‐82.68 10 12/18/00 1:19:21 -­‐21.18 -­‐179.12 628 1/13/01 17:33:32 13.05 -­‐88.66 60 2/13/01 14:22:05 13.67 -­‐88.94 10 2/18/01 13:04:53 -­‐47.46 32.39 10 2/28/01 18:50:13 13.28 -­‐88.83 65 4/7/01 23:17:37 -­‐27.55 -­‐176.34 33 4/13/01 15:33:53 -­‐59.72 -­‐25.59 26 4/28/01 4:49:53 -­‐18.06 -­‐176.94 351 5/20/01 4:21:43 18.82 -­‐104.45 33 6/3/01 2:41:57 -­‐29.67 -­‐178.63 178 6/26/01 12:33:52 -­‐4.07 -­‐104.47 10 8/6/01 3:52:59 -­‐55.54 -­‐123.42 10 8/21/01 6:52:06 -­‐36.81 -­‐179.57 33 8/25/01 2:02:02 7.63 -­‐82.77 24 9/2/01 10:06:51 -­‐54.36 -­‐137.02 10 10/2/01 0:48:18 -­‐16.18 -­‐173.82 106 10/17/01 11:29:09 19.35 -­‐64.93 33 10/21/01 0:29:21 -­‐37.14 178.98 18 11/9/01 0:47:55 9.64 -­‐82.3 10 11/28/01 14:32:32 15.57 -­‐93.11 84 12/7/01 19:27:34 -­‐44.22 168.82 10 1/1/02 10:39:06 -­‐55.21 -­‐129 10 1/16/02 23:09:52 15.5 -­‐93.13 80 3/9/02 12:27:11 -­‐56.02 -­‐27.33 118 31
6.1 6.1 6.6 7.7 6.6 6 6.1 6.2 6.2 6.9 6.3 7.2 6.1 6.7 7.1 6.1 6.3 6.2 6 6.7 6.1 6.4 5.8 6 6.4 6 Table S3. Earthquakes recorded at station PEL and used in this study
Date Time Lat Long Depth Mag 2/25/96 3:08:18 16.204 -­‐97.963 21 6/2/96 2:52:09 10.797 -­‐42.254 10 9/5/96 8:14:14 -­‐22.118 -­‐113.436 10 1/11/97 20:28:26 18.219 -­‐102.756 33 4/22/97 9:31:23 11.112 -­‐60.892 5 4/28/97 12:07:37 -­‐42.504 42.686 10 5/1/97 11:37:36 18.993 -­‐107.35 33 7/19/97 14:22:08 16.333 -­‐98.216 33 9/2/97 12:13:22 3.849 -­‐75.749 198 9/20/97 16:11:32 -­‐28.683 -­‐177.624 30 3/20/98 21:08:08 -­‐50.008 163.107 10 3/25/98 3:12:25 -­‐62.877 149.527 10 7/9/98 14:45:39 -­‐30.487 -­‐178.994 129 8/4/98 18:59:20 -­‐59.3 -­‐80.393 33 8/23/98 13:57:15 11.663 -­‐88.038 54 12/27/98 0:38:26 -­‐21.632 -­‐176.376 144 3/31/99 5:54:42 5.827 -­‐82.616 6/15/99 20:42:05 18.386 -­‐97.436 70 11/7/00 0:18:04 -­‐55.627 -­‐29.876 10 1/13/01 17:33:32 13.049 -­‐88.66 60 6/3/01 2:41:57 -­‐29.666 -­‐178.633 178 11/15/02 19:58:31 -­‐56.051 -­‐36.404 10 1/22/03 2:06:34 18.77 -­‐104.104 24 5/4/03 13:15:18 -­‐30.531 -­‐178.232 62 8/4/03 4:37:20 -­‐60.532 -­‐43.411 10 8/21/03 12:12:49 -­‐45.104 167.144 28 1/2/06 6:10:49 -­‐60.934 -­‐21.575 13 1/4/06 8:32:32 28.164 -­‐112.117 14 5/16/06 10:39:23 -­‐31.779 -­‐179.313 152 8/20/06 3:41:47 -­‐61.029 -­‐34.365 13 1/30/07 4:54:50 -­‐54.74 146.298 11 6/13/07 19:29:40 13.554 -­‐90.618 23 9/30/07 5:23:34 -­‐49.271 164.115 10 10/15/07 12:29:36 -­‐44.785 167.583 18 11/16/07 3:13:00 -­‐2.312 -­‐77.838 509 11/29/07 19:00:19 14.973 -­‐61.263 156 2/8/08 9:38:14 10.671 -­‐41.899 9 2/23/08 15:57:19 -­‐57.326 -­‐23.421 14 4/12/08 0:30:12 -­‐55.664 158.453 16 6/30/08 6:17:44 -­‐58.22 -­‐22.1 8 32
7.1 7 6.9 7.2 6.7 6.8 6.9 6.9 6.8 7 6.7 8.1 6.9 7.2 6.7 6.8 6.8 7 6.8 7.7 7.2 6.6 7.6 6.7 7.6 7.2 7.4 6.6 7.4 7 6.9 6.7 7.4 6.8 6.6 7.4 6.9 6.8 7.1 7 Figure S1. CCP RF stacks for the same section calculated using 3 different vp/vs ratios (1.7, 1.75
and 1.8). As was noted in the text, the features that we observe don’t change, the different vp/vs
ratio simply shifts the features vertically in the section.
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Figure S2. Ray density plots for the cross-sections shown in Figures 5, 6, and 7. Colors indicate the
number of traces that are stacked per bin. Under the forearc, as many as 50 traces are being stacked in
each bin, providing sufficient coverage to image the downgoing slab.
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Figure S3. A comparison of CCP RF stacks for profile X2 for normalized and unnormalized data. They
show similar results. The downgoing slab and continental Moho are observed in both sections. We
present the unnormalized RFs in this manuscript.
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