GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 10, 10.1029/2002GL014868, 2002
Tsunami earthquakes as transient phenomena
Tetsuzo Seno
Earthquake Research Institute, University of Tokyo, Tokyo, Japan
Received 6 February 2002; revised 1 April 2002; accepted 11 April 2002; published 24 May 2002.
[1] Recent modeling of tsunami waveforms caused by tsunami
earthquakes indicates that fault slip occurred along the subduction
boundary close to the trench axis. On the other hand, it is well
known that seismicity is generally very low within ca. 50 km from
the trench axis, and this has been attributed to the stable frictional
sliding properties of the subducted unconsolidated sediments. This
makes tsunamigenic slip near the trench difficult to explain. In
order to solve this dilemma, I propose that transition from stable
sliding to nearly zero friction, with a velocity-weakening property,
occurs as a result of areal increase of zones of elevated fluid
pressure, which may make fairly rapid seismic slip possible
following the breakage of asperities at normal seismogenic depths.
This transition can be identified as seismic reflections with
negative polarities, which may help to rate hazards for a coming
large tsunami earthquake.
INDEX TERMS: 7209 Seismology:
Earthquake dynamics and mechanics; 7223 Seismology: Seismic
hazard assessment and prediction; 7230 Seismology: Seismicity
and seismotectonics; 8120 Tectonophysics: Dynamics of
lithosphere and mantle—general; 8123 Tectonophysics:
Dynamics, seismotectonics
1. Introduction
[2] For a variety of reasons, and disaster mitigation in particular,
it is important to understand the occurrence of tsunami earthquakes, for which tsunamis are larger than expected given their
surface wave magnitudes. Previous studies have elucidated how
tsunami earthquakes occur and proposed various models for their
occurrence. Tsunami earthquakes are generally characterized by
slow faulting, manifest as a deficiency of short period components
of seismic radiation relative to long periods [Kanamori, 1972;
Pelayo and Wiens, 1992; Kanamori and Kikuchi, 1993; Polet and
Kanamori, 2000]. However, it is also known that slow slip alone is
insufficient to explain the small magnitudes compaired with
anomalously large tsunamis produced by some tsunami earthquakes, such as the 1896 Sanriku and 1946 Aleutian earthquakes
[Fukao, 1979; Kanamori and Kikuchi, 1993]. Upheaval by highangle splay faulting off the main thrust within a weak accretionary
prism [Fukao, 1979], landslides caused by shaking [Kanamori and
Kikuchi, 1993], and larger seismic slip due to low rigidity of
sediments along the shallow thrust zone [Geist and Bilek, 2001],
have been proposed as mechanisms for the required additional
uplift.
[3] On the other hand, recent modeling of tsunami waveforms
of tsunami earthquakes based on dislocation theory has revealed
that fault slip in several cases occurred very close to the trench axis
[Satake, 1994; Piatanesi et al., 1996; Tanioka and Satake, 1996;
Johnson and Satake, 1997; Satake and Tanioka, 1999]. Tsunami
source areas of historical tsunami earthquakes also show similar
features [Ishibashi, 1983, 1986]. These make the involvement of
splay faults unlikely. The low rigidity (10 GPa) of sediments in
the shallow portion of the boundary can produce larger slip given
the same seismic moment [Geist and Bilek, 2001], however, this
Copyright 2002 by the American Geophysical Union.
0094-8276/02/2002GL014868$05.00
produces the same order of stress drop (1 MPa) as ordinary
earthquakes, and how such a stress can be maintained in stable
sliding sediments remains unresolved. Furthermore faulting close
to the trench axis must occur of very low dips, which makes the
uplift less efficient. To compensate this, Seno [2000] proposed that
soft sediments near the toe of the trench slope are offscraped by a
sudden horizontal movement of the backstop over the decollement
(the detachment surface between offscraped and subducted sediments), causing an inelastic uplift at the toe. This effect was
demonstrated to be potentially effective by numerical simulations
of tsunamis of the 1896 tsunami earthquake off the Sanriku coast
of northern Honshu [Tanioka and Seno, 2001]; the required fault
slip of 10 m without sediment deformation was reduced to 6 m.
[4] Above all, the large slip occurring along a shallow part of
the subduction boundary remains as the largest enigma regarding
tsunami earthquakes. It is known that seismicity is usually sparse
within a distance of ca. 50 km of the trench axis [Hirata et al.,
1983; Byrne et al., 1988; Scholz, 1998], and this has been
attributed to the stable frictional sliding of unconsolidated sediments subducted beneath the decollement and deeper zones [Marone and Scholz, 1988; Scholz, 1998; Hyndman et al., 1997]. This
stable sliding property is characterized by a positive a b value,
where a and b are coefficients in the rate and state dependent
friction laws [Dieterich, 1979; Ruina, 1983], and a fault zone with
this property operates as an absorbing barrier [Tse and Rice, 1986;
Boatwright and Cocco, 1996; Kato and Hirasawa, 1999] if seismic
faulting occurs at a nearby asperity, making the occurrence of
tsunami earthquakes difficult to explain. In this paper, I show first
that slip distributions associated with recent large earthquakes off
northern Honshu are consistent with the above stable sliding
frictional properties, then discuss a possibility of transient effective
normal stress and its effects on frictional stability as applied to the
occurrence of tsunami earthquakes.
2. Recent Large Earthquakes Off Northern
Honshu
[5] In the subduction zone off Sanriku, northern Honshu, close
to the rupture zone of the 1896 tsunami event, large (Ms > 7)
earthquakes have occurred recently in 1989, 1992, and 1994
(Figure 1, Table 1). Though these events had aftershock areas
extending very close to the trench axis, they were not tsunami
earthquakes (compare their magnitudes with those of the 1896
event in Table 1). The asperities ruptured in these recent events
[Nagai et al., 2001; Yamanaka et al., 2001] were generally located
in the deeper part of their aftershock zones (Figure 1, Hino et al.
[1999]; Tohoku University [1990]), indicating that only small
amounts of seismic slip occurred in the shallow portion near the
trench. Moreover, borehole strain meters on the coast and GPS
velocities throughout northern Honshu indicate that large amounts
of afterslip occurred with a time scale of 1 day to 1 year in the
aftershock zones of these events [Kawasaki et al., 1995; Heki et al.,
1997]. This behavior, i.e., small slip at the time of an earthquake
and large afterslip, is similar to that predicted by the numerical
simulation of frictional slip with a stable sliding zone in the
shallow portion of the subduction boundary [Kato and Hirasawa,
1999]. These are completely consistent with the idea that a shallow
subduction boundary constitutes a fault zone with positive a b
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SENO: TSUNAMI EARTHQUAKES AS TRANSIENT PHENOMENA
Figure 2. (a) Relative true-amplitude depth section of the seismic
reflection line in Northern Barbados [Shipley et al., 1994]. (b) Two
sections illustrating waveforms associated with the decollement in
Northern Barbados [Shipley et al., 1994]. Top is relatively high
amplitude, compound-negative polarity reflection; below is
example of transition from compound-negative polarity to positive
polarity.
Figure 1. Aftershock areas [Hino et al., 1996; Tohoku University,
1990] and asperities [Nagai et al., 2001; Yamanaka et al., 2001] of
recent earthquakes. Epicenters of the corresponding events are
indicated by stars. The fault zone of the 1896 tsunami earthquake
[Tanioka and Satake, 1996] is also shown. The aftershock areas for
the 1994, 1992, and 1989 events are lightly shaded, and their
asperities are darkly shaded. An asperity within the fault slip area
marked by the thin line is defined by the region whose seismic
moment amounts to a half of the total moment. The strong
reflections revealed by seismic surveys [Fujie et al., 2000] are
indicated by ‘‘F’’, with the low seismicity area shown by the
dashed line.
[e.g., Scholz, 1998]. Provided that these recent events shared the
fault zone with the 1896 event (Figure 1), it seems difficult to
explain the occurrence of the 1896 event by general factors such as
paucity of subducted sediments and so on (see Tanioka and Satake
[1996] and Polet and Kanamori [2000] for this kind of explanation). Note also that tsunami earthquakes are very rare; i.e., none
of the same subduction zone segments has experienced a tsunami
earthquake more than once during historic times.
3. Transient Frictional Stability
[6] In subduction zones, fluids expelled from the subducting
plate play important roles in various phenomena such as earthTable 1. Magnitudes of Earthquakes off Sanriku Coast, N. Japan
Date
1989
1992
1994
1896
11
07
12
06
02
18
28
15
Ms
Mw
Mt
7.4
7.1
7.5
7.2
7.4
6.9
7.7
8.0 – 8.1
7.5
7.2
7.7
8.6
Ms is the surface wave magnitude determined by International
Seismological Centre; Mw is the moment magnitude from the Harvard
centroid moment tensor catalogue; Mt is the tsunami magnitude [K. Abe,
personal comm., 2001], except for the 1896 event of which parameters are
taken from Tanioka and Satake [1996] and Tanioka and Seno [2001].
quakes, volcanism, metamorphism and so on. Fluids released by
compaction of interstitial and fracture pore spaces or by dehydration of clay minerals migrate along the decollement - thrusts in the
accretionary prism. Recent seismic reflection and ocean drilling
surveys have revealed that reflection signals along the decollement
exhibit substantial spatial variation [Moore and Shipley, 1993;
Shipley et al., 1994]. For example, in some places the decollement
produces a negative polarity reflection, but not in others, and
reflection amplitudes also vary (Figure 2, Shipley et al. [1994]).
This is most easily explained by variations in impedance (velocity
x density) and thickness of the decollement. Negative polarity
reflections usually have their origin in a thin (10 m) layer
of lower impedance than surrounding layers, which is attributed
to pore space dilation by overpressured fluids [e.g., Moore
and Shipley, 1993]. The fluid pressures of such layers are
estimated to be 86 – 98% of the lithostatic pressure [Tobin
et al., 1994].
[7] The existence of elevated fluid pressures requires sealing
and fluid infiltration, which will dissipate with time unless an
effective seal can be continually maintained; thus high pore
pressures themselves must be an indication of transience. This
was confirmed by measurements in the holes of ODP Leg110
near the toe of the Barbados accretionary prism. Fisher and
Hounslow [1990], using shapes of geotherms and geochemical
anomalies, showed that flows through the horizontal conduits
had a transient nature with a time constant of decades to
thousands of years. This indicates the high and/or low fluid
pressures seen in Figure 2 may not be a permanent but rather
transient characteristic of the fault zone; a wide area along the
decollement might be covered by negative polarity reflections
temporarily.
[8] The effective normal stress in such an area would change
significantly as the pore fluid pressure is elevated to a substantial
fraction of the lithostatic pressure. The reduced effective stress
would substantially decrease the friction on the fault zone. In the
numerical studies of the mechanical behavior of a series of
asperities intervened by high pore pressure compartments, Lockner
SENO: TSUNAMI EARTHQUAKES AS TRANSIENT PHENOMENA
Figure 3. (a) A cross-section illustrating DSRs (Deep Strong
Reflectors) and the decollement beneath the Nankai accretionary
prism off Shikoku [simplified from Park et al., 2001]. The DSRs
and part of the decollement have negative seismic reflection
polarities which may indicate elevated pore fluid pressures. (b)
DSRs [Park et al., 2001], coseismic slip zones of the 1944
Tonankai and 1946 Nankai earthquakes [Ando, 1982], tsunami
source and inundation areas accompanied with the 1605 Keicho
earthquake [Ishibashi, 1983]. The seismic intensity at Kyoto at the
time of the 1605 event was negligible, but the tsunami disaster was
extensive to have killed more than two thousands of people
[Ishibashi, 1983].
and Byerlee [1995] showed that the system can be destabilized
when one of the locked asperities fails. Moreover, Saffer et al.
[2001] showed, in the double-direct shear experiment at room
humidity and temperature conditions, that smectite, which would
be the most abundant clay mineral in the decollement, starts to
have negative a b for the normal stress <30 MPa. This amount of
effective normal stress may be realized with the pore pressure of
90% of the lithostatic at 10 km depth, if the tectonic stress is not
large. The frictional property in such zones thus turns into the
weakly seismic regime of Boatwright and Cocco [1996].
4. Mechanism of Tsunami Earthquakes
[9] Considering the peculiar nature of faulting associated with
tsunami earthquakes that it breaks the shallowest portion of the
subduction boundary and the possibility of transience of frictional
properties along such a part of the boundary, I propose that a
tsunami earthquake occurs when overpressured zones along the
shallow portion cover a considerable areal extent and asperities in
the seismogenic zone at depths break simultaneously, since the
zones may make fairly rapid seismic slip following the breakage of
asperities. The overpressured zones may not be necessarily connected with each other but be separated into compartments by
impermeable seals as discussed by Lockner and Byerlee [1995].
58 - 3
[10] Off Sanriku, northern Honshu, strong reflectors along the
plate interface, indicated by ‘‘F’’ in Figure 1, have been observed
by refraction-reflection surveys [Fujie et al., 2000] to the south of
the 1992 earthquake aftershock zone. The north-south dimension
of the reflectors is about 30 km, coincident with an area of
extremely low seismicity, which is indicated by the dashed line
in Figure 1. If this reflective zone stems from elevated fluid
pressures as Fujie et al. [2000] suggested and becomes pervasive
over the shallow portion of the plate boundary along the Japan
Trench, a tsunami earthquake, such as the 1896 event, may
eventually happen, in association with the breakage of asperities
in the deeper seismogenic zone, such as that of the 1992 event.
[11] In the Nankai Trough, great earthquakes have recurred
every 100 – 200 years. Among those great earthquakes, the 1605
Keicho earthquake is a unique tsunami earthquake, which had
small seismic intensities on land but caused disastrous tsunamis
(Figure 3b, Ishibashi [1983]). From the seismic reflection surveys,
Park et al. [2001] delineated reflections with negative polarities
beneath the Nankai accretionary prism 20 – 60 km landward of the
frontal thrust (Figures 3a, 3b), which are located deeper than the
negative polarity decollement near the frontal thrust [Moore and
Shipley, 1993]. These DSRs (Deep Strong Reflectors) are distributed seaward of the coseismic fault zone of the 1946 Nankai
earthquake [Ando, 1982] (Figure 3b). Park et al. [2001] interpreted
the DSRs as indicating elevated fluid pressures. A tsunami earthquake would happen when the sporadic DSRs shown in Figure 3b
become pervasive over a wider area along the trough, connected to
the negative polarity decollement in the shallow portion, and
simultaneously asperities in the deeper seismogenic zone break.
The 1605 Keicho earthquake might have been such an event. With
the fact that usual great earthquakes such as the 1946 Nankai
earthquake tend to rupture updip along the splay faults ca. 100 km
landward from the trough axis [Cummins and Kaneda, 2000;
Kuramoto et al., 2000], tsunami earthquakes in this region are
characterized by the slip close to the trough axis, beyond the splay
faults, due to the transience of the frictional property in the shallow
part of the subduction boundary.
5. Concluding Remarks
[12] The transient nature of the frictional property along the
positive a b portion of the subduction boundary proposed in this
study might be tested by repeated reflection surveys over a target
area. If possible, 3-D reflection surveys, such as the ones in the
Barbados and the Nankai Trough [Shipley et al., 1994; Kuramoto
et al., 2000] are desired. The transient nature of the frictional
property proposed in this study might also be applicable to
ordinary subduction zone or transform fault earthquakes. If so,
the hypothesis could be more easily tested for those events, which
have shorter reccurrence intervals than tsunami earthquakes.
[13] Acknowledgments. I thank John Townend and Kenji Satake for
critical review of the initial version of the manuscript, Eric Geist for useful
comments for the improvements, Toru Matsuzawa, Takashi Miyatake, and
Koichi Nakamura for discussion, and Toshihiro Igarashi, Jin-Oh Park,
Yoshiko Yamanaka, and Katsuyuki Abe for providing me their thesis,
preprints or data.
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T. Seno, Earthquake Research Institute, University of Tokyo, Tokyo,
Japan. ([email protected])