The role of subduction erosion on seismicity
Susan L. Bilek
Earth and Environmental Science Department, New Mexico Tech, Socorro, New Mexico 87801, USA
Subduction zones outline much of the Pacific
plate, producing many of the largest and most
destructive earthquakes recorded. Thus there is
considerable interest in understanding the tectonics and dominant processes associated with
these margins, as variations in these processes
may be a factor in the behavior of the earthquakes. These margins are usually classified as
either accretionary, with material actively added
to the overriding plate, or erosional, where material is removed from the base of the overriding
plate. Much of the early research of subduction
zone dynamics focused on accretionary margins; models for erosional margins have only
matured in the past 15 yr. However, seminal
papers that describe structural features of each
margin and quantify the relative amount of erosion in the global subduction zones suggest that
roughly 57% of worldwide subduction zones
are erosive instead of accretional (von Huene
and Scholl, 1991; Clift and Vannucchi, 2004).
Various models for erosive margins are contradictory. One class of models suggests that
fault friction is high, perhaps with the presence of a subducted topographic feature such
as a seamount, leading to the removal of material at the base of the overriding plate through
abrasion (e.g., Hilde, 1983; Adam and Reuther,
2000; Dominguez et al., 2000; Bangs et al.,
2006). Another model proposes a weaker fault
and suggests that the subduction erosion occurs
because of hydrofracturing of the upper plate,
allowing upper plate material to enter the subduction zone, thinning of the upper plate resulting in measurable subsidence, and fluid seeps
along the normal faults in the upper prism (von
Huene et al., 2004; Ranero et al., 2008). However, neither model has successfully predicted
all the structural features characteristic of erosive margins.
Wang et al. (2010, p. 431 in this issue of Geology) describe a model for erosive margins that
has significant implications for the seismicity
patterns at these margins. Their model is an
extension of the dynamic Coulomb wedge concept that has been used to successfully describe
features and seismicity at accretionary margins,
such as wedge geometry, activation of splay
faults, shallow afterslip, and very low frequency
earthquakes (Wang and Hu, 2006). They apply
the same concept of temporally varying fault
strength over the seismic cycle to subduction
erosion, using the steeper wedge slope and dip
geometries found for erosive margins relative to
accretionary prisms. During the period of time
between earthquakes, the middle prism is strong
relative to the underlying basal fault that has a
higher fluid pressure ratio. Updip of an earthquake, coseismic slip causes compression in the
prism and dilation in the subducting material.
As a result, the fluid pressure ratio increases in
the wedge and decreases in the underlying plate.
Thus, the basal fault strengthens as the overlying middle prism weakens, facilitating erosion
at the base of the prism. Erosion is temporally
limited to the coseismic rupture and the period
of rapid postseismic deformation shortly after
seismic slip, which is linked to the continued
readjustment of the stress state in the shallow
portion of the fault.
This model has implications for seismicity,
such as relating rare shear localization along a
plane in the shallowest portion of the subduction channel to long rupture times for earthquakes that occasionally occur in this updip
region. Seismic data support this claim. Tsunami earthquakes, those events that produce
large tsunami relative to their seismic moment
(Mo) and unusually long time to rupture, arise
from slip in the shallowest portion of subduction zones (e.g., Kanamori, 1972; Kanamori
and Kikuchi, 1993; Satake and Tanioka, 1999;
Polet and Kanamori, 2000; Abercrombie et
al., 2001; Bilek and Lay, 2002; Ammon et al.,
2006). These tsunami earthquakes are primarily located in erosive margins (Fig. 1A), suggesting that these events might be connected
to the processes described by Wang et al.
(2010). The largest events on record (those
with a magnitude greater than 9) tend to occur
only at accretionary margins, not at erosional
ones (Fig. 1A); likely related to differences in
geometry and friction conditions between the
Figure 1. Seismicity char18
Earthquake M>9
60
acteristics within eroTsunami
Earthquake
16
sional and accretionary
14
margins. A: Distribution
of erosional (thin red line)
40
12
and accretionary (thick
10
blue line) subduction
20
8
zones as defined by Clift
and Vannucchi (2004)
6
0
and Kopp et al. (2006).
4
Plate boundaries from
-20
2
Bird (2003). Earthquakes
0
with M > 9 (yellow stars;
-40
Stein and Okal, 2007)
have occurred in accreAccretionary
tionary margins, but tsu-60 Erosional
nami earthquakes (green
circles; Lay and Bilek,
80 100 120 140 160 180 -160 -140 -120 -100 -80 -60 -40 -20
2007) have occurred primarily in erosional margins. B: Statistics of moment-scaled rupture duration (time of rupture) of large catalogs of shallow earthquakes (depth
≤ 15 km) (Bilek et al., 2004; Bilek, 2007, 2009) within accretionary (blue) and erosional (red) margins. Rupture durations are scaled by the cube
root of Mo, normalized by the Mo of a moment magnitude (Mw) 6 event, to allow for comparison of earthquakes over the size range of Mw 5.5–7.5.
Black circles indicate the minimum and maximum values, and bars indicate the 5th, 25th, 50th, 75th, and 95th percentile for each region. Solid
black line indicates the lowest maximum value for erosional margins. Although the median values for all regions fall within a similar range, the
maximum values for events in the erosional margins tend to be higher than those for events in accretionary margins.
B
Alaska
Chile (South)
Kamchatka
Sumatra
Central America
Chile (North)
Izu-Bonin
Japan
Kurile
Marianas
Mexico
New Hebrides
Peru
Philippines
Tonga
Scaled Duration (s)
A
© 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
GEOLOGY,
May
2010
Geology,
May
2010;
v. 38; no. 5; p. 479–480; doi: 10.1130/focus052010.1.
479
two margins. Within the accretionary margins,
the region where earthquakes are possible
because of frictional behavior that allows for
slip (velocity weakening) extends further seaward than within erosive margins, allowing for
wider areas for coseismic rupture and leading
to much larger magnitude earthquakes.
Catalogs of subduction zone earthquake rupture durations (Bilek et al., 2004; Bilek, 2007,
2009) also suggest differences between types of
margins, although with additional complexity.
The maximum moment-scaled durations for the
shallowest earthquakes (upper 15 km) in erosive
margins tend to be larger than accretionary margins, with the exception of the Alaska-Aleutian
subduction zone (Fig. 1B). The discrepancy with
the Alaska-Aleutian subduction zone introduces
new questions about its accretionary classification and possible along-strike variations in its
nature. The longer source durations observed at
erosional margins might be indicative of similar
conditions to those required for tsunami earthquake generation. Median and 75th percentile
durations are similar across all regions, suggesting that the erosive margins produce fewer, but
longer-duration shallow earthquakes than accretionary margins. Thus, the model proposed for
the dynamics of erosive margins can be linked
to earthquake observations, and further advancement in our understanding of subduction zone
seismicity will come through continued theoretical development of models such as that proposed
by Wang et al., combined with additional seismic and geodetic observations of seismic cycle
deformation.
REFERENCES CITED
Abercrombie, R.E., Antolik, M., Felzer, K., and
Ekstrom, G., 2001, The 1994 Java tsunami earthquake: Slip over a subducting seamount: Journal
of Geophysical Research–Solid Earth, v. 106,
p. 6595–6607, doi: 10.1029/2000JB900403.
Adam, J., and Reuther, C.D., 2000, Crustal dynamics and active fault mechanics during subduction erosion: Application of frictional wedge
analysis on to the North Chilean Forearc: Tectonophysics, v. 321, p. 297–325, doi: 10.1016/
S0040-1951(00)00074-3.
480
Ammon, C.J., Kanamori, H., Lay, T., and Velasco,
A.A., 2006, The 17 July 2006 tsunami earthquake: Geophysical Research Letters, v. 33,
L24308, doi: 10.1029/2006GL028005.
Bangs, N.L.B., Gulick, S.P.S., and Shipley, T.H.,
2006, Seamount subduction erosion in the
Nankai Trough and its potential impact on the
seismogenic zone: Geology, v. 34, p. 701–704,
doi: 10.1130/G22451.1.
Bilek, S.L., 2009, Seismicity along the South American subduction zone: Large earthquakes, tsunamis, and subduction zone complexity: Tectonophysics, doi: 10.1016/j.tecto.2009.02.037
(in press).
Bilek, S.L., 2007, Using earthquake source durations
along the Sumatra-Andaman subduction system to examine fault-zone variations: Bulletin
of the Seismological Society of America, v. 97,
p. S62–S70, doi: 10.1785/0120050622.
Bilek, S.L., and Lay, T., 2002, Tsunami earthquakes
possibly widespread manifestations of frictional conditional stability: Geophysical Research Letters, v. 29, p. 1673, doi: 10.1029/
2002GL015215.
Bilek, S.L., Lay, T., and Ruff, L.J., 2004, Radiated
seismic energy and earthquake source duration
variations from teleseismic source time functions for shallow subduction zone thrust earthquakes: Journal of Geophysical Research–
Solid Earth, v. 109, B09308, doi: 10.1029/
2004JB003039.
Bird, P., 2003, An updated digital model of plate boundaries: Geochemistry, Geophysics, Geosystems,
v. 4, p. 1027, doi: 10.1029/2001GC000252.
Clift, P., and Vannucchi, P., 2004, Controls on tectonic
accretion versus erosion in subduction zones:
Implications for the origin and recycling of
the continental crust: Reviews of Geophysics,
v. 42, RG2001, doi: 10.1029/2003RG000127.
Dominguez, S., Malavieille, J., and Lallemand, S.E.,
2000, Deformation of accretionary wedges
in response to seamount subduction: Insights
from sandbox experiments: Tectonics, v. 19,
p. 182–196, doi: 10.1029/1999TC900055.
Hilde, T.W.C., 1983, Sediment subduction versus
accretion around the Pacific: Tectonophysics,
v. 99, p. 381–397, doi: 10.1016/0040-1951(83)
90114-2.
Kanamori, H., 1972, Mechanism of tsunami earthquakes: Physics of the Earth and Planetary Interiors, v. 6, p. 246–259, doi: 10.1016/0031-9201
(72)90058-1.
Kanamori, H., and Kikuchi, M., 1993, The 1992 Nicaragua earthquake: A slow tsunami earthquake
associated with subducted sediments: Nature,
v. 361, p. 714–716, doi: 10.1038/361714a0.
Kopp, H., Flueh, E.R., Petersen, C.J., Weinrebe,
W., Wittwer, A., and Meramex, S., 2006, The
Java margin revisited: Evidence for subduction
erosion off Java: Earth and Planetary Science
Letters, v. 242, p. 130–142, doi: 10.1016/j
.epsl.2005.11.036.
Lay, T., and Bilek, S.L., 2007, Anomalous earthquake ruptures at shallow depths on subduction
zone megathrusts, in Dixon, T., and Moore, C.,
eds., The Seismogenic Zone of Subduction
Thrust Faults: New York, Columbia University
Press, p. 476–511.
Polet, J., and Kanamori, H., 2000, Shallow subduction zone earthquakes and their tsunamigenic
potential: Geophysical Journal International,
v. 142, p. 684–702, doi: 10.1046/j.1365-246x
.2000.00205.x.
Ranero, C.R., Grevemeyer, I., Sahling, H., Barckhausen, U., Hensen, C., Wallmann, K., Weinrebe, W., Vannucchi, P., von Huene, R., and
McIntosh, K., 2008, Hydrogeological system
of erosional convergent margins and its influence on tectonics and interplate seismogenesis:
Geochemistry, Geophysics, Geosystems, v. 9,
Q03S04, doi: 10.1029/2007GC001679.
Satake, K., and Tanioka, Y., 1999, Sources of tsunami
and tsunamigenic earthquakes in subduction
zones: Pure and Applied Geophysics, v. 154,
p. 467–483, doi: 10.1007/s000240050240.
Stein, S., and Okal, E.A., 2007, Ultralong period
seismic study of the December 2004 Indian
Ocean earthquake and implications for regional
tectonics and the subduction process: Bulletin
of the Seismological Society of America, v. 97,
p. S279–S295, doi: 10.1785/0120050617.
von Huene, R., and Scholl, D.W., 1991, Observations
at convergent margins concerning sediment
subduction, subduction erosion, and the growth
of continental crust: Reviews of Geophysics,
v. 29, p. 279–316, doi: 10.1029/91RG00969.
von Huene, R., Ranero, C.R., and Vannucchi, P.,
2004, Generic model of subduction erosion:
Geology, v. 32, p. 913–916, doi: 10.1130/
G20563.1.
Wang, K.L., and Hu, Y., 2006, Accretionary prisms
in subduction earthquake cycles: The theory of
dynamic Coulomb wedge: Journal of Geophysical Research–Solid Earth, v. 111, B06410, doi:
10.1029/2005JB004094.
Wang, K., Hu, Y., von Huene, R., and Kukowski,
N., 2010, Interplate earthquakes as a driver of
shallow subduction erosion: Geology, v. 38,
p. 431–434, doi: 10.1130/G30597.1.
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GEOLOGY, May 2010