Geology
doi: 10.1130/G34326C.1
2013;41;e309 Geology
Richard W. Allmendinger, Gabriel González, José Cembrano, Felipe Aron and Gonzalo Yáñez
8.8 2010 Maule Chile earthquake: COMMENT
w
M Splay fault slip during the
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GSA, employment. Individual scientists are hereby granted permission, without fees or further requests to
Copyright not claimed on content prepared wholly by U.S. government employees within scope of their
Notes
© 2013 Geological Society of America
on November 21, 2013 geology.gsapubs.org Downloaded from on November 21, 2013 geology.gsapubs.org Downloaded from
GEOLOGY FORUM
|
December 2013
|
www.gsapubs.org e309
Richard W. Allmendinger
1
*, Gabriel González
2,4
,
José Cembrano
3,4
, Felipe Aron
1
, and Gonzalo Yáñez
3,4
1
Department of Earth & Atmospheric Sciences, Cornell University,
Ithaca, New York 14853, USA
2
Departamento de Ciencias Geológicas, Universidad Católica del
Norte, Avenida Angamos 0610, Antofagasta, Chile
3
Departamento de Ingeniería Estructural y Geotécnica, Pontificia
Universidad Católica de Chile, Vicuña Mackenna 4860, Macul,
Santiago, Chile
4
National Research Center for Integrated Natural Disaster Management,
Chile
Over the past decade, the Isla Santa María (south-central Chile)
has provided one of the most exquisitely detailed records of deformation
and tectonically controlled sea-level change anywhere along the Andean
margin (Bookhagen et al., 2006; Melnick et al., 2006, 2009, 2012). This
work has been combined with extensive offshore suites of data, especially
seismic reflection profiles and local and regional geophysical studies,
to provide an unparalleled view of the offshore portion of the southern
Chilean forearc. Melnick et al. (2012) recently postulated the activity of a
splay thrust fault, the Santa María Fault System (SMFS), during the 2010
Maule earthquake. This work is timely given current interest in the role of
splay faults in contributing to total moment release during great subduction
zone earthquakes, local tsunami hazard, and rupture segmentation. Given
the importance, one must ask whether their evidence is so compelling as to
require the activity of a splay thrust, especially considering that their field
observations show only normal fault activity.
As described in their paper, Melnick et al. (2012) document
exclusively normal fault ruptures and tilting of the island. No direct
observational evidence of reverse fault activity of the SMFS, either on-
or offshore, during or after the Maule event, is provided: there are no
well-documented crustal reverse fault focal mechanisms and no surface
features indicative of reverse faulting. The inference that the SMFS was
active during the Maule event is based on a previous hypothesis that
surface extensional features on the Isla Santa María represent stretching
in a broad anticline located at the tip of the SMFS (Melnick et al., 2006).
Additionally, they show shallow seismicity with reverse focal mechanisms
on the SMFS during the interseismic period preceding the Maule event
(Bohm et al., 2002).
One of the notable characteristics of the Maule earthquake was that
the largest aftershocks were produced by crustal normal faults in both the
upper and lower plate. The best known of these, the Pichilemu sequence,
which began 12 days after the main event, cut the entire crust of the
forearc as shown by local and regional seismic network data and modeling
of geodetic data (Farías et al., 2011; Ryder et al., 2012; Aron et al., 2013).
Curiously, Melnick et al. (2012) show the Pichilemu structure as a fault of
“unconstrained kinematics” in their figure 1A.
In the northern Chile forearc, where there is substantially better
exposure than in the onshore portion overlying the Maule rupture, we
have documented numerous cases of reverse fault reactivation of crustal
normal faults (Allmendinger and González, 2010). Melnick et al. (2006)
also show numerous examples of reactivated faults interpreted on their
seismic profiles. Given the distribution of slip during the Maule rupture,
the northeast strike and relatively steep dip of the SMFS make it almost
ideally oriented for co/post-seismic reactivation as a normal fault
(Aron et al., 2013). Thus, it would appear to us that a simpler and more
straightforward interpretation of the Isla Santa María normal fault ruptures
associated with the Maule event is that they are the surface expression of
extensional reactivation of SMFS, which is active as a reverse fault during
interseismic periods but, like most of the rest of the forearc, is stretched
during coseismic rebound.
We freely admit that we cannot disprove the splay fault hypothesis
of Melnick et al. (2012), and furthermore, we readily recognize that splay
faulting during great subduction earthquakes can be an important process,
as with the reverse faults described by Plafker (1967) on Montague Island
following the 1964 Alaska earthquake. But, given the documentation of
normal fault surface ruptures, indisputable crustal normal faulting during
the Maule event elsewhere, and the complete lack of documentation
of co- or post-seismic reverse faulting on the SMFS, we suggest that
coseismic extensional reactivation of the SMFS is at least an equally
viable hypothesis. Thus, one would have to conclude that the evidence for
splay thrusting beneath the Isla Santa María during the Maule earthquake
is equivocal at best.
REFERENCES CITED
Allmendinger, R.W., and González, G.G., 2010, Neogene to Quaternary tectonics
of the coastal Cordillera, northern Chile: Tectonophysics, v. 495, p. 93–110,
doi:10.1016/j.tecto.2009.04.019.
Aron, F., Allmendinger, R., Cembrano, J., González, G., and Yáñez, G., 2013,
Permanent forearc extension and seismic segmentation: Insights from the
2010 Maule earthquake, Chile: Journal of Geophysical Research, v. 118,
p. 724–739, doi:10.1029/2012JB009339.
Bohm, M., Lüth, S., Echtler, H., Asch, G., Bataille, K., Bruhn, C., Rietbrock, A.,
and Wigger, P., 2002, The Southern Andes between 36 and 40 S latitude:
Seismicity and average seismic velocities: Tectonophysics, v. 356, no. 4,
p. 275–289, doi:10.1016/S0040-1951(02)00399-2.
Bookhagen, B., Echtler, P., Melnick, D., Strecker, R., and Spencer, J.Q.G., 2006,
Using uplifted Holocene beach berms for paleoseismic analysis on the
Santa María Island, south-central Chile: Geophysical Research Letters,
v. 33, L15302, doi:10.1029/2006GL026734.
Farías, M., Comte, D., Roecker, S., Carrizo, D., and Pardo, M., 2011, Crustal
extensional faulting triggered by the 2010 Chilean Earthquake: The Pichilemu
Seismic Sequence: Tectonics, v. 30, TC6010, doi:10.1029/2011TC002888.
Melnick, D., Bookhagen, B., Echtler, H.P., and Strecker, M.R., 2006, Coastal
deformation and great subduction earthquakes, Isla Santa Maria, Chile (37 ° S):
Geological Society ofAmerica Bulletin, v. 118, no. 11–12, p. 1463, doi:10.1130
/B25865.1.
Melnick, D., Bookhagen, B., Strecker, M.R., and Echtler, H.P., 2009, Segmentation
of megathrust rupture zones from fore-arc deformation patterns over hundreds
to millions of years, Arauco peninsula, Chile: Journal of Geophysical Research,
v. 114, no. B1, B01407, doi:10.1029/2008JB005788.
Melnick, D., Moreno, M., Motagh, M., Cisternas, M., and Wesson, R.L., 2012,
Splay fault slip during the M
w
8.8 2010 Maule Chile earthquake: Geology,
v. 40, p. 251–254, doi:10.1130/G32712.1.
Plafker, G., 1967, Surface faults on Montague Island associated with the 1964
Alaska earthquake: U.S. Geological Survey Professional Paper 534-G, 42 p.
Ryder, I., Rietbrock, A., Kelson, K., Bürgmann, R., Floyd, M., Socquet, A., Vigny,
C., and Carrizo, D., 2012, Large extensional aftershocks in the continental
forearc triggered by the 2010 Maule earthquake, Chile: Geophysical Journal
International, v. 188, no. 3, p. 879–890, doi:10.1111/j.1365-246X.2011.05321.x.
Splay fault slip during the M
w
8.8 2010 Maule Chile earthquake
*E-mail: rwa1@cornell.edu.
© 2013 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
doi:10.1130/G34326C.1
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Geology
Splay fault slip during the Mw 8.8 2010 Maule Chile earthquake: COMMENT
Richard W. Allmendinger, Gabriel González, José Cembrano, Felipe Aron and Gonzalo Yáñez
Geology 2013;41;e309
doi: 10.1130/G34326C.1
Email alerting services
click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles
cite this article
Subscribe
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Permission request
click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA
Copyright not claimed on content prepared wholly by U.S. government employees within scope of their
employment. Individual scientists are hereby granted permission, without fees or further requests to GSA,
to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make
unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and
science. This file may not be posted to any Web site, but authors may post the abstracts only of their
articles on their own or their organization's Web site providing the posting includes a reference to the
article's full citation. GSA provides this and other forums for the presentation of diverse opinions and
positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political
viewpoint. Opinions presented in this publication do not reflect official positions of the Society.
Notes
© 2013 Geological Society of America
Downloaded from geology.gsapubs.org on November 21, 2013
doi:10.1130/G34326C.1
Forum Comment
Splay fault slip during the Mw 8.8 2010 Maule Chile earthquake
Richard W. Allmendinger1*, Gabriel González2,4,
José Cembrano3,4, Felipe Aron1, and Gonzalo Yáñez3,4
1
Department of Earth & Atmospheric Sciences, Cornell University,
Ithaca, New York 14853, USA
2
Departamento de Ciencias Geológicas, Universidad Católica del
Norte, Avenida Angamos 0610, Antofagasta, Chile
3
Departamento de Ingeniería Estructural y Geotécnica, Pontificia
Universidad Católica de Chile, Vicuña Mackenna 4860, Macul,
Santiago, Chile
4
National Research Center for Integrated Natural Disaster Management,
Chile
Over the past decade, the Isla Santa María (south-central Chile)
has provided one of the most exquisitely detailed records of deformation
and tectonically controlled sea-level change anywhere along the Andean
margin (Bookhagen et al., 2006; Melnick et al., 2006, 2009, 2012). This
work has been combined with extensive offshore suites of data, especially
seismic reflection profiles and local and regional geophysical studies,
to provide an unparalleled view of the offshore portion of the southern
Chilean forearc. Melnick et al. (2012) recently postulated the activity of a
splay thrust fault, the Santa María Fault System (SMFS), during the 2010
Maule earthquake. This work is timely given current interest in the role of
splay faults in contributing to total moment release during great subduction
zone earthquakes, local tsunami hazard, and rupture segmentation. Given
the importance, one must ask whether their evidence is so compelling as to
require the activity of a splay thrust, especially considering that their field
observations show only normal fault activity.
As described in their paper, Melnick et al. (2012) document
exclusively normal fault ruptures and tilting of the island. No direct
observational evidence of reverse fault activity of the SMFS, either onor offshore, during or after the Maule event, is provided: there are no
well-documented crustal reverse fault focal mechanisms and no surface
features indicative of reverse faulting. The inference that the SMFS was
active during the Maule event is based on a previous hypothesis that
surface extensional features on the Isla Santa María represent stretching
in a broad anticline located at the tip of the SMFS (Melnick et al., 2006).
Additionally, they show shallow seismicity with reverse focal mechanisms
on the SMFS during the interseismic period preceding the Maule event
(Bohm et al., 2002).
One of the notable characteristics of the Maule earthquake was that
the largest aftershocks were produced by crustal normal faults in both the
upper and lower plate. The best known of these, the Pichilemu sequence,
which began 12 days after the main event, cut the entire crust of the
forearc as shown by local and regional seismic network data and modeling
of geodetic data (Farías et al., 2011; Ryder et al., 2012; Aron et al., 2013).
Curiously, Melnick et al. (2012) show the Pichilemu structure as a fault of
“unconstrained kinematics” in their figure 1A.
In the northern Chile forearc, where there is substantially better
exposure than in the onshore portion overlying the Maule rupture, we
have documented numerous cases of reverse fault reactivation of crustal
normal faults (Allmendinger and González, 2010). Melnick et al. (2006)
also show numerous examples of reactivated faults interpreted on their
seismic profiles. Given the distribution of slip during the Maule rupture,
the northeast strike and relatively steep dip of the SMFS make it almost
ideally oriented for co/post-seismic reactivation as a normal fault
(Aron et al., 2013). Thus, it would appear to us that a simpler and more
straightforward interpretation of the Isla Santa María normal fault ruptures
associated with the Maule event is that they are the surface expression of
extensional reactivation of SMFS, which is active as a reverse fault during
interseismic periods but, like most of the rest of the forearc, is stretched
during coseismic rebound.
We freely admit that we cannot disprove the splay fault hypothesis
of Melnick et al. (2012), and furthermore, we readily recognize that splay
faulting during great subduction earthquakes can be an important process,
as with the reverse faults described by Plafker (1967) on Montague Island
following the 1964 Alaska earthquake. But, given the documentation of
normal fault surface ruptures, indisputable crustal normal faulting during
the Maule event elsewhere, and the complete lack of documentation
of co- or post-seismic reverse faulting on the SMFS, we suggest that
coseismic extensional reactivation of the SMFS is at least an equally
viable hypothesis. Thus, one would have to conclude that the evidence for
splay thrusting beneath the Isla Santa María during the Maule earthquake
is equivocal at best.
REFERENCES CITED
Allmendinger, R.W., and González, G.G., 2010, Neogene to Quaternary tectonics
of the coastal Cordillera, northern Chile: Tectonophysics, v. 495, p. 93–110,
doi:10.1016/j.tecto.2009.04.019.
Aron, F., Allmendinger, R., Cembrano, J., González, G., and Yáñez, G., 2013,
Permanent forearc extension and seismic segmentation: Insights from the
2010 Maule earthquake, Chile: Journal of Geophysical Research, v. 118,
p. 724–739, doi:10.1029/2012JB009339.
Bohm, M., Lüth, S., Echtler, H., Asch, G., Bataille, K., Bruhn, C., Rietbrock, A.,
and Wigger, P., 2002, The Southern Andes between 36 and 40 S latitude:
Seismicity and average seismic velocities: Tectonophysics, v. 356, no. 4,
p. 275–289, doi:10.1016/S0040-1951(02)00399-2.
Bookhagen, B., Echtler, P., Melnick, D., Strecker, R., and Spencer, J.Q.G., 2006,
Using uplifted Holocene beach berms for paleoseismic analysis on the
Santa María Island, south-central Chile: Geophysical Research Letters,
v. 33, L15302, doi:10.1029/2006GL026734.
Farías, M., Comte, D., Roecker, S., Carrizo, D., and Pardo, M., 2011, Crustal
extensional faulting triggered by the 2010 Chilean Earthquake: The Pichilemu
Seismic Sequence: Tectonics, v. 30, TC6010, doi:10.1029/2011TC002888.
Melnick, D., Bookhagen, B., Echtler, H.P., and Strecker, M.R., 2006, Coastal
deformation and great subduction earthquakes, Isla Santa Maria, Chile (37°S):
Geological Society ofAmerica Bulletin, v. 118, no. 11–12, p. 1463, doi:10.1130
/B25865.1.
Melnick, D., Bookhagen, B., Strecker, M.R., and Echtler, H.P., 2009, Segmentation
of megathrust rupture zones from fore-arc deformation patterns over hundreds
to millions of years, Arauco peninsula, Chile: Journal of Geophysical Research,
v. 114, no. B1, B01407, doi:10.1029/2008JB005788.
Melnick, D., Moreno, M., Motagh, M., Cisternas, M., and Wesson, R.L., 2012,
Splay fault slip during the Mw 8.8 2010 Maule Chile earthquake: Geology,
v. 40, p. 251–254, doi:10.1130/G32712.1.
Plafker, G., 1967, Surface faults on Montague Island associated with the 1964
Alaska earthquake: U.S. Geological Survey Professional Paper 534-G, 42 p.
Ryder, I., Rietbrock, A., Kelson, K., Bürgmann, R., Floyd, M., Socquet, A., Vigny,
C., and Carrizo, D., 2012, Large extensional aftershocks in the continental
forearc triggered by the 2010 Maule earthquake, Chile: Geophysical Journal
International, v. 188, no. 3, p. 879–890, doi:10.1111/j.1365-246X.2011.05321.x.
*E-mail: rwa1@cornell.edu.
© 2013 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
GEOLOGY FORUM | December 2013 | www.gsapubs.org
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