Download Where and why do large shallow slab earthquakes occur?

Where and why do large shallow slab earthquakes occur?
Tetsuzo Seno and Masaki Yoshida
Earthquake Research Institute, University of Tokyo, Tokyo, Japan
(submitted to J. Geophys. Res., on Jan. 18, 2001)
Abstract. It has been believed that large earthquakes seldom occur within the shallow portion (20-60 km
depth) of a subducting slab, because the differential stress there is generally expected to be low between
bending at trench-outer rise area and unbending at intermediate-depth. However, there are several regions
where large earthquakes (M≧7.0), including three events early in 2001, have occurred in this portion.
Searching such events from published individual studies and the Harvard University centroid moment tensor
catalogue, we find twenty events in E. Hokkaido, Kyushu-SW. Japan, S. Mariana, Manila, Sumatra,
Vanuatu, N. Chile, C. Peru, El Salvador, Mexico, N. Cascadia and Alaska. Slab stresses revealed from the
mechanism solutions of those large events and nearby smaller events are down-dip tensional. However,
young ages of the subducting oceanic plates degrade a possibility of slab pull forces being a major cause for
those large events. Except for E. Hokkaido, Manila, Sumatra and S. Vanuatu, there are pieces of evidence for
a horizontal stress gradient within the upper plate in the across-arc direction. We infer that mantle drag forces
operating beneath the upper plate produce the stress gradient, and drive the upper plate seaward. The upper
plate driven by this drag would suck a young oceanic plate, resulting in extra-tension in the shallow portion
of the slab, which might be a cause for the large shallow slab earthquakes.
1. Introduction
It has been believed that large earthquakes seldom occur in the shallow portion of a subducting slab.
However, early in 2001, fairly large shallow slab earthquakes succeeded on Jan. 13 (the El Salvador
earthquake, Mw 7.6), on Feb. 28 (the Nisqually earthquake, Mw 6.8) in Washington, U.S. and on March 24
(the Geiyo earthquake, Mw 6.8) in Southwest Japan. Even though they were events within slabs, they
caused severe damage on land areas because their hypocenters were relatively shallow. We are forced to take
into account these shallow slab events for mitigation of disasters, hazard assessments of constructions, and
then it would be important to understand where and why such large shallow slab earthquakes occur.
We define here "shallow" portion of a slab by the depth range of 20-60 km. Events shallower than 20 km
are excluded because they are located very close to a trench, which makes it difficult to discriminate them
from trench - outer rise events. We also exclude events deeper than 60 km, since they are belonging to
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so-called intermediate-depth earthquakes.
Trench - outer-rise areas are generally characterized by the occurrence of earthquakes with horizontal
T(P)-axes in the shallow (deep) portion (Seno and Yamanaka, 1998), consistent with the stresses produced by
bending of an oceanic plate prior to subduction. Double seismic zones at intermediate-depth show usually
down-dip compression (tension) in the upper (lower) zone (Hasegawa et al., 1978), consistent with
unbending stresses (Engdahl and Scholz, 1977; Kawakatsu, 1986). Between bending at the trench - outer rise
and unbending at the intermediate depth, a slab is usually expected to be in a low stress, which explains the
scarcity of large events there.
Although they are scarce, historically some large earthquakes are known to have occurred in the shallow
portion of slabs. The 1931 Oaxaca earthquake (Mw 7.7) in Mexico (Singh et al., 1985), the 1949 Olympia
earthquake (M 7.1) in Cascadia (Baker and Langston, 1987), and the 1970 Peru earthquake (Mw 7.9) (Abe,
1972) are such examples. Along with the recent large shallow slab events in 2001 mentioned earlier,
occurrence of such events might not be so seldom as generally thought. In order to clarify how seldom they
occur and in what tectonic regimes they tend to occur, we search for large shallow slab earthquakes
systematically in this study.
We particularly focus on stress state and age of the subducting slab and also stress state of the upper plate
where those large shallow events occurred. Seno and Yamanaka (1998) showed that there is a relation
between a stress in the shallow portion of a slab and a back-arc stress such that a tensional (compressional)
slab tends to have a compressional (tensional) back-arc. Some arcs have tensional slabs with tensional
back-arcs, violating this rule. We will show that these are the regions where large slab earthquakes tend to
occur. They are also characterized by an across-arc horizontal stress gradient in the upper plate. Nakamura and
Uyeda (1980) first noted this kind of a stress gradient in several arcs. Wang (2001) recently examined a stress
state from a fore-arc to a back-arc in detail and concluded that the inner part of a fore-arc often has a low
stress and there would be no significant change in stress state between fore- and back-arcs. In this study, we
depict trajectories of σHmax (maximum horizontal stress) in the upper plate where large shallow slab events
have occurred, in order to examine whether a stress gradient exists or not. Changes of σHmax directions are
observed in most of the regions, implying existence of a stress gradient there. We finally discuss
implications of the stress gradient for a role played by convection beneath the upper plate on the cause for
large shallow slab earthquakes.
2. Large shallow slab earthquakes
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We search for large shallow slab events from the Harvard centroid moment tensor (HCMT) catalogue for
the period during 1977-2001 and from individual studies for the period prior to 1977. We restrict events with
magnitudes larger than or equal to 7.0 and centroid or focal depths between 20 and 60 km. Moment
magnitudes are used when available; if not, surface-wave magnitudes or their equivalents are used. We judge
whether they occurred within a slab or not mainly based on their focal mechanisms, making references to
relative plate motions and focal mechanisms of nearby smaller events; details of the procedure for each event
are described in the following sub-sections.
Table 1 lists the large shallow slab earthquakes thus selected. We include in Table 1 two recent events
with moment magnitudes smaller than 7; they are the Feb. 28, 2001 Nisqually earthquake (Mw 6.8) and the
March 24, 2001 Geiyo earthquake (Mw 6.8). This is because they occurred very close to historical events in
the same region and are useful to clarify the faulting associated with those former events. Out of the twenty
earthquakes listed in Table 1, fourteen events occurred during 1977-2001. This rate of occurrence of large
shallow slab events (ca. 0.5 event per year) suggests that many slab events might have been omitted during
the period prior to 1977, for which no routine determination of focal mechanisms like HCMT was available.
Figure 1 shows the focal mechanisms (lower hemisphere) plotted at the epicenter of those events with
event numbers (Table 1); for the three historical events near Japan (Events 2, 4 and 5), only their epicenters
are plotted because reliable mechanisms are not available. Sources of the mechanism solutions are described
in the caption of Table 1. The mechanism solutions in Figure 1 show both normal and reverse fault types.
In the following sub-sections, we cite evidence for regarding those events as fracture within the shallow
portion of a slab. We also show in more detail the stress state of the subducting slab for each region using
published focal mechanism solutions of nearby smaller events. P- and T-axes of nearby smaller events
(Mw>=5.8) using HCMT are plotted in cross-sections (Appendix, Figure A1). We also delineate σHmax
directions in the upper plate for each region. When σHmax trajectories are available, we use them directly;
these are for Kyushu-SW. Japan and Alaska. In other cases, we draw trajectories using published stress
indicators, such as focal mechanisms, Quaternary active faults, dikes, monogenic volcanic centers, and
in-situ stress measurements. Data sources are described in each sub-section. We call a stress state
compression (tension) if a horizontal stress is larger (smaller) than a vertical stress.
3. Slab and upper plate stresses
3.1. Eastern Hokkaido
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In and around Hokkaido, the Pacific plate is subducting beneath the Okhotsk plate with a rate of 79 mm/yr
in the WNW direction (Seno et al., 1996). Off the east coast of Hokkaido, a large shallow earthquake
occurred on Oct. 4, 1994 (the Hokkaido-toho-oki earthquake, Mw 8.3), having a reverse fault type mechanism
solution different from typical underthrust types in this region (Event 1 in Figure 1a, HCMT; see also
Kikuchi and Kanamori, 1995; Tanioka et al., 1995). The T-axis of the mechanism solution is dipping to the
NE, subparallel to the slab dip. The hypocentral depth of this event was determined to be 33 km, along with
the aftershock depths extending between 0-80 km, by the microearthquake network of Hokkaido University
(Katsumata et al., 1995). Kikuchi and Kanamori (1995) and Tanioka et al. (1995) determined centroid depths
of this event as 56 km and 50 km, respectively, from the body-wave analysis, which are shallower than the
68 km depth of HCMT. The mechanism solution along with the nearly vertical aftershock distribution leads
to the interpretation that this event was fracture within the shallow portion of the slab. The age of the
subducting plate near the epicenter is 123 Ma (Nakanishi et al., 1992). T-axes of nearby smaller events show
that the slab is dominantly down-dip tension (Figure A1), consistent with the results of previous studies
(Suzuki et al., 1983; Umino et al., 1984; Kosuga et al., 1996).
Figure 2 shows the σHmax stress trajectories in Hokkaido. The small bars indicate P-axes of the focal
mechanisms of shallow earthquakes occurring in the crust (Moriya, 1986; Kosuga, 1999), which were used
to draw the trajectories. The stress regime is mostly of strike-slip fault type with E-W σHmax , except for the
Hidaka area and its north where reverse fault types are dominant and Oshima Peninsula where σHmax is
directed in the NW. The reverse faulting in the Hidaka area is probably representing the collision of the Kuril
fore-arc sliver to western Hokkaido (Kimura, 1981; Seno, 1985; Tsumura et al., 1999). The NW σHmax in
Oshima Peninsula may be due to a stress disturbance at the arc-arc junction. Except for these areas, the σHmax
direction is oblique to the arc.
3.2. Kyushu-Southwest Japan
The Philippine Sea plate is subducting along the Nankai Trough beneath SW. Japan and Kyushu with
rates of 40-50 mm/yr in the WNW direction (Seno et al., 1993). The age of the subducting plate is 15-30
Ma along the Nankai Trough (Okino et al., 1994) and 50 Ma at the junction with the Ryukyu Trench near
Kyushu (Seno, 1988). To select historical slab events in this region, we use the Catalogue of Major Slab
Earthquakes near Japan (Association for the Development of Earthquake Prediction, 2000), in addition to
individual studies. We restrict events since 1890 around when the Japan Meteorological Agency started
systematic instrumental observations over Japan.
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Three large (M>=7) earthquakes are listed; they are the 1899 Kii-Yamato earthquake (Event 2, M 7.0), the
1905 Geiyo earthquake (Event 4, M 7.2) and the 1931 Hyuganada earthquake (Event 5, M 7.1). The
Kii-Yamato earthquake (Event 2) occurred beneath the southeast coast of Kii Peninsula (Figure 3). Nakamura
(1997) inferred that this event occurred within the Philippine Sea slab at the 40-50 km depth, based on the
fact that severe damage was localized at the epicenter, but strong shaking was felt in a wide area, which is a
characteristic feature to the slab events in this area. The 1905 Geiyo earthquake (Event 4) occurred northwest
of Shikoku. Close to the epicenter of this earthquake, a large earthquake occurred on March 24, 2001 (the
2001 Geiyo earthquake, Event 3, Mw 6.8) at the depth of 47 km, having a normal fault type mechanism with
a W dipping T-axis (Figure 1a, HCMT). This event is regarded as fracture within the Philippine Sea slab
because its focal mechanism is similar to those of smaller slab events in this region as mentioned later. We
suppose that the 1905 Geiyo earthquake was a similar slab event to the 2001 event, since the iso-intensity
map and the damage pattern are similar for both events, and estimate its depth around 50 km. The 1931
Hyuganada earthquake (Event 5) occurred under the sea between Kyushu and Shikoku at the depth of 40 km.
Ichikawa (1971) suggested a normal fault type mechanism for this event. Near this event, two normal fault
type earthquakes with W dipping T-axes occurred on August 6, 1984 (Mw 6.9, depth = 29 km) and on March
18, 1987 (Mw 6.6, depth = 38 km) (HCMT). We infer that the 1931 event had a similar focal mechanism to
these smaller events and was fracture within the slab.
Beneath SW. Japan, microearthquake activities are dipping to the NNW with a low angle to the depth of
60-80 km (Ukawa, 1982; Nakamura et al., 1997; Matsumura, 1997), and beneath Kyushu, they are dipping
to the W with a high angle to the depth of 180 km (Shimizu et al., 2000). Beneath SW. Japan, the T-axes of
small events within the slab are sub-horizontal and subparallel to the E-W strike of the slab (Ukawa, 1982;
Matsumura, 1997). Moderate-size slab events beneath Kyushu have W-dipping T-axes along the slab (Seno,
1999). The stress state of the slab from Kyushu to SW. Japan is thus regarded as tension in the E-W
direction along the slab. The T-axes plotted in Figure A1 also support this.
Figure 3 shows the σHmax trajectories of the upper plate from Kyushu to SW. Japan (Seno, 1999). In S.
Kyushu, the back-arc has a stress state of strike-slip fault type with NE σHmax . The fore-arc has a stress state
of reverse fault type with NW σHmax . However, the latter σHmax direction was derived from the dike-volcano
data in Nakamura and Uyeda (1980) and the fault type is not well constrained. CMT solutions from recent
small and moderate-size events in the Ryukyu fore-arc show normal faulting with along-arc T-axes (Kubo
and Fukuyama, 2001). If this stress regime extends to SE. Kyushu, the stress state of the fore-arc might be
of normal fault type.
In N. Kyushu and SW. Japan, σHmax is directed in the E-W. In SW. Japan and the eastern part of N.
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Kyushu, strike-slip faulting is dominant, and in the westernmost part of N. Kyushu, normal faulting is
dominant (Seno, 1999).
3.3. Southern Mariana
The Pacific plate is subducting beneath the Philippine Sea plate along the Mariana Trench. However, due
to the opening of the Mariana Trough, convergence direction of the Pacific plate beneath the Mariana fore-arc
differs from that beneath the Philippine Sea plate (Seno et al., 1993). With the velocity between the Mariana
fore-arc and Eurasia determined by GPS (Kotake, 2000) and the NUVEL1 Eurasia-Pacific velocity (DeMets et
al., 1990), the convergence of the Pacific plate is calculated to be in the WNW direction with a rate of 76
mm/yr near Guam (Figure 4). The age of the subducting plate near Guam is 164 Ma (Nakanishi et al.,
1992). On August 8, 1993, a large earthquake (Event 6, Mw 7.7) occurred south of Guam, having a reverse
fault type mechanism with a nodal plane shallowly dipping to the NNW and an auxiliary plane dipping
steeply to the SSE (Figure 1a). The HCMT centroid depth is 59 km and Tanioka et al. (1995) determined the
source depth ranging 40-50 km from a body wave analysis. Because the NNW-SSE slip vector of this event
is much different from the plate convergence direction described above, we regard this event as fracture within
the subducting Pacific slab. The seismicity in the southernmost part of the Mariana arc where the 1993 event
occurred is shallower than 200 km (Eguchi, 1984). Mechanism solutions of nearby smaller events are
similar to that of the 1993 event (Figure A1; see also Eguchi, 1984), indicating that the short slab beneath
the southern Mariana arc is down-dip tensional.
Although data for stress state of the upper plate are scarce because the area is mostly submarine, the
back-arc spreading in the Mariana Trough manifests that the back-arc is tensional with along-arc σHmax
(Figure 4). In the fore-arc, Martines et al. (2000) depicted normal faults striking perpendicular to the arc,
using side-scan sonar images. This indicates that σHmax is in the across-arc direction in the fore-arc (Figure 4).
3.4. Manila
The South China Sea is subducting beneath Luzon with a rate of ca. 50 mm/yr in the ESE direction (Yu
et al., 1999). On Dec. 11, 1999, a large earthquake (Event 7, Mw 7.2) with a normal fault type mechanism
occurred east of the Manila Trench northwest of Manila at the depth of 35 km (HCMT), with an E dipping
T-axis. The age of the subducting plate is 22 Ma near the event (Taylor and Hayes, 1980). The seismicity
associated with the slab is shallower than 250 km along the Manila Trench (Seno and Kurita, 1978;
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Cardwell et al., 1980). Mechanism solutions of nearby smaller events show T-axes more or less similar to
that of the large event (Figure A1; see also Cardwell et al., 1980), indicating that the slab is down-dip
tensional and the 1999 event was fracture within the slab.
The stress state of the upper plate is characterized by across-arc compression as evidenced by the NW
striking left-lateral strike-slip Philippine fault. However, the notion of fore- and back-arcs in the Philippines
is obscured due to the subduction from both sides of the archipelago, and σHmax trajectories are not drawn in
this region.
3.5. Sumatra
The Australian plate is subducting beneath southern Sumatra with a rate of 70 mm/yr in the NNE
direction (DeMets et al., 1990). The age of the subducting plate is 66 Ma (Liu et al., 1983). On June 4,
2000, a large earthquake (Event 8, Mw 7.8) occurred off southern Sumatra at the depth of 44 km (HCMT),
having a strike-slip fault type mechanism with a NE dipping T-axis. The seismicity associated with the slab
is shallower than 200 km in this region (Slancova et al., 2000; Newcomb and McCann, 1987). Mechanism
solutions of nearby smaller events show similar T-axes to this event (Figure A1; see also Slancova et al.,
2000), indicating that the slab is down-dip tensional and the 2000 event was fracture within the slab. The
stress state of the upper plate is characterized by N-S σHmax as evidenced by the NW striking right-lateral
strike-slip Semanko fault; the σHmax direction is thus oblique to the arc trend. We do not draw σHmax
trajectories in this region due to the paucity of data.
3.6. Vanuatu
Beneath the New Hebrides fore-arc, the Australian plate is subducting with rates of 100-150 mm/yr in the
NEE direction (Louat and Pelletier, 1989). The arc changes its strike to the nearly E-W direction at the
southern end, where the plate boundary becomes a transform fault in the east as the Hunter Fracture zone.
Two large earthquakes with focal mechanisms different from the underthrust-type occurred in this region: one
on July 13, 1994 (Event 9, Mw 7.1) in the central part of the arc at the depth of 25 km (HCMT), and the
other on July 6, 1981 (Event 10, Mw 7.5) near the southern corner at the depth of 58 km (HCMT). The ages
of the subducting plate are 52 Ma at Event 9 (Weissel et al., 1982) and 35 Ma at Event 10 (Malahoff et al.,
1982). The seismicity associated with the slab is steeply dipping to the east and shallower than 300 km,
which becomes shallower than 150 km to the south (Coudert et al., 1981). T-axes of the mechanism
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solutions of the large events are oblique to the slab dip. Mechanism solution of nearby deeper smaller events
are down-dip tensional (Figure A1; see also Coudert et al., 1981). Although the T-axes of the large events
are not similar to those of the smaller events, we regard the large events as fracture within the slab, because
their mechanism solutions are much different from the underthrust type.
West of the epicentral location of Event 9, the d'Entrecasteaux Ridge is colliding with the New Hebrides
fore-arc. The back-arc in this place is in compression as revealed by the thrust type HCMT solutions (Figure
5; Charvis and Pelletier, 1989). Further to the east, the North Fiji Basin is currently opening with the N-S
striking ridge axis, as evidenced by the magnetic anomaly lineations, diffuse seismic activities, and
hydrothermal features (Malahoff et al., 1982; Hamburger and Isacks, 1988; Auzende et al., 1988; Tanahashi
et al., 1994). Figure 5 shows the σHmax trajectories in the New Hebrides - N. Fiji Basin region, drawn from
the spreading axis and the focal mechanisms in Charvis and Pelletier (1989). The σHmax direction changes
from the N-S in the center of the North Fiji Basin to the E-W in the New Hebrides arc.
The place where Event 10 is located does not seem to be a typical subduction zone and the stress state of
the upper plate above this event is difficult to depict.
3.7. Northern Chile
The Nazca plate is subducting beneath the S. American plate in N. Chile with a rate of 84 mm/yr in the
NEE direction (DeMets et al., 1990). Here on Feb. 23, 1965, a large earthquake (Event 11, Mw 7.0) having a
normal fault type mechanism with an E dipping T-axis occurred at the depth of 60 km (Malgrange and
Madariaga, 1983). The age of the subducting plate at Event 11 is 48 Ma (Herron, 1972).
The seismicity associated with the slab beneath S. America is split into the intermediate-depth and deeper
ones (Barazangi and Isacks, 1976). In N. Chile, intermediate depth earthquakes are distributed down to the
300 km depth with a dip angle of 20-25° (Barazangi and Isacks, 1976). Smaller events mostly have E
dipping T-axes similar to the large event (Figure A1; see also Malgrange and Madariaga, 1983; Comte and
Suarez, 1995), and focal mechanism solutions of intermediate-depth earthquakes also show that the slab is
down-dip tensional (Stauder, 1973; Comte and Suarez, 1995). We then regard the large event as fracture
within the slab. However, there are a few smaller events with down-dip P-axes (Figure A1), and seismic
activities revealed by local microearthquake networks show more complexities such as double seismic zones
(Comte and Suarez, 1994; Comte et al., 1999). Beneath N. Chile, though the slab stress shows a tendency
of down-dip tension, its magnitude might not be large. The stress state of the upper plate will be described in
the next sub-section along with Central Peru.
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3.8. Central Peru
On May 31, 1970, a large earthquake (Event 12, Mw 7.9) occurred off the coast of C. Peru at the depth of
43 km, having a normal fault type mechanism with an E dipping T-axis (Abe, 1972). The age of the
subducting plate at Event 12 is 44 Ma (Herron, 1972). In north - central Peru, the slab seismicity shows
flattening at the 100-150 km depth range (Hasegawa and Sacks, 1981). Mechanism solutions of nearby
smaller events in the shallow and intermediate-depths show generally down-dip T-axes similar to that of the
large event (Figure A1; see also Stauder, 1975; Hasegawa and Sacks, 1981), and we regard the large event as
fracture within the slab. However, some earthquakes with down-dip P-axes have also been found (Stauder,
1975; Isacks and Barazangi, 1977). This indicates that the tensional stress of the slab might not be large, as
in N. Chile.
Figure 6 shows the σHmax trajectories in S. America drawn from the World Stress Map data (Zoback, 1992;
Assumpcao, 1992). The sub-Andes is characterized by reverse faulting with E-W σHmax (See also Stauder,
1975; Suarez et al., 1983). The high Andes, Altiplano, Cordillera Blanca and Pacific lowland are
characterized by normal faulting mostly with N-S T-axes (see also Sebrier et al., 1985; Suarez et al., 1983).
The normal faulting can be explained by the high topography and thicker crust in the high Andes-Altiplano
and Cordillera Blanca (Dalmayrac and Molnar, 1981; Froidevaux and Isacks, 1984; Coblentz and Richardson,
1996; Meijer et al., 1997; Liu and Yang, 2000), and the whole Andes area, if it had a normal crustal
thickness, must turn to be in compression, similarly to the sub-Andes. The Quaternary normal faults in the
Pacific coast (Sebrier et al., 1985) remain as an enigma because the crustal thickness there is not large.
Because three reverse fault type earthquakes with E-W P-axes occurred along the coast of Peru and N. Chile
(Figure 6), we guess that the present stress state along the coast is E-W compression.
To the east across the continent, several reverse and strike-slip faults are seen in the Proterozoic-Archean
shield area, and at the east coast, a few strike-slip and normal faults with N-S or E-W σHmax appear (Figure 6,
Assumpcao, 1992; Zoback, 1992). The tension at the east coast along with the E-W compression in the
sub-Andes indicates that there is a horizontal stress gradient across the S. American continent.
3.9. El Salvador
The Cocos plate is subducting beneath the Caribbean plate near El Salvador with a rate of 79 mm/yr in
the NNE direction (DeMets et al., 1990). On June 19, 1982 and January 13, 2001, two large earthquakes
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(Event 13, Mw 7.3 and Event 14, Mw 7.7) occurred off the coast of El Salvador, at the depths of 52 and 56
km, respectively (HCMT). Both events had normal fault type focal mechanisms with NE dipping T-axes
(HCMT). The age of the subducting plate off El Salvador is unknown, but would be older than 37 Ma,
judging from the magnetic anomalies further offshore (Herron, 1972). The seismicity associated with the
slab is shallower than 300 km, and has a steep dip angle of 60-70° below 50 km (Burbach et al., 1984).
Nearby smaller events have similar focal mechanisms to the large events (Figure A1, see also Burbach et al.,
1984), and we regard the large events as fracture within the slab.
The stress state of the upper plate between the coast and the volcanoes is of strike-slip fault type with NW
σHmax (World Stress Map, Zoback, 1992). Since the arc is elongated in the NW direction, the fore-arc is in
across-arc tension in this region. We do not draw stress trajectories in this region due to the paucity of data.
3.10. Mexico
The subduction zone off Mexico is divided into the Jalisco, Michoacan, Guerrero, and Oaxaca blocks from
north to south (Figure 7, inset). The Rivera plate is subducting beneath the Jalisco block with rates of 20-50
mm/yr in the NE direction (Kostoglodov and Bandy, 1995), and the Cocos plate beneath the other three
blocks with rates of 50-70 mm/yr in the NE direction (DeMets et al., 1990).
On Jan. 11, 1997, a large earthquake (Event 17, Mw 7.1) occurred beneath the Michoacan block at the
depth of 40 km (HCMT), and on Sept. 30, 1999, another large earthquake (Event 15, Mw 7.4) occurred
beneath the Oaxaca block at the depth of 47 km (HCMT). Both events had normal fault type mechanisms
with NNE dipping T-axes. Historically similar large earthquakes occurred beneath the Michoacan block in
1858 (Singh et al., 1996), and beneath the Oaxaca block in 1931 (Event 16, Singh et al., 1985). The 1931
event had a normal fault type mechanism similar to the recent large events (Singh et al., 1985). The age of
the subducting slab at these events are 10-17 Ma (Klitgord and Mammerickx, 1982).
The seismicity associated with the slab is shallower than 100 km, and has a steep dip angle (~50°) beneath
the Jalisco block and a gentle dip angle (20-30°) beneath the Michoacan and Oaxaca blocks. Beneath the
Guerrero block, the slab is flattened at 40-70 km depth range. Smaller earthquakes show T-axes more or less
dipping to the NE or horizontal along the slab, similar to the large events (Figure A1; see also Suarez et al.,
1990; Pardo and Suarez, 1995). Although the stress axes of the smaller events in Figure A1 are somewhat
disturbed and a few earthquakes with down-dip P-axes have also been found within the slab beneath central S.
Mexico (S. K. Singh, personal comm., 2000), we regard the large events as fracture within the slab. At
depths larger than 60 km, several large earthquakes have occurred within the slab (Mikumo, 2000). They
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have normal fault type focal mechanisms with down-dip or horizontal T-axes. The 1980 Huajuapan de Leon
earthquake (Mw 7.0) at the depth of 65 km exceptionally had a S dipping T-axis (Yamamoto et al., 1984).
Figure 7 shows the σHmax trajectories in the Jalisco-Michoacan blocks (Seno and Singh, 2001). The stress
state of the upper plate is mostly of normal fault type. Along the Trans Mexican Volcanic Belt (TMVB), the
σHmax direction is subparallel to the belt as seen in the strikes of active normal faults (Sutter, 1991; 1992)
and focal mechanism solutions (J. F. Pacheco, personal communication, 2000). Borehole breakout data show
that the σHmax direction changes to the NS in the northeast of TMVB (Sutter, 1991). To the south of TMVB,
alignment of monogenic volcanic centers (Hasenaka, 1994) and strikes of normal faults in the Colima rift
and the other two rifts (Allan, 1986; Sutter, 1991) indicate that the σHmax direction changes to the across-arc
one (Figure 7). A few normal fault type small earthquakes within the overriding plate south of TMVB
(Singh and Pardo, 1993; J. F. Pacheco, personal communication, 2000) are consistent with the across-arc
σHmax in the fore-arc. The stress trajectories of reverse fault type near the coast in Figure 7 are inferred from
the coastal mountains such as Sierra Madre del Sur. In some places, a normal fault type stress state may
reach the coast as seen in the Colima Rift.
3.11. Northern Cascadia
The Juan de Fuca plate is subducting beneath the North American plate in Cascadia with rates of 35-45
mm/yr in the NE direction (Riddihough, 1984; Wilson, 1993). On April 13, 1949, a large earthquake (Event
19, M 7.1) occurred in the Puge Sound area, western Washington, at the depth of 54 km. This event had a
normal fault type mechanism with a SE dipping T-axis (Baker and Langston, 1987). Recently on Feb. 28,
2001, the Nisqually earthquake (Event 18, Mw 6.8) occurred 20 km northeast of the 1949 event at the depth
of 47 km (HCMT), having a similar focal mechanism to the 1949 event, but had an E dipping T-axis
(HCMT). The age of the Juan de Fuca plate off Washington is 10 Ma (Wilson, 1993).
The seismicity associated with the slab is shallower than 100 km (Crosson and Owens, 1987; Ma et al.,
1996). For smaller events, one solution available from HCMT shows a similar focal mechanism to the
larger events (Figure A1). One moderate-size event on April 29, 1965 (M 6.5) at the depth of 59 km also had
a similar focal mechanism (Algermissen and Harding, 1965). Microearthquakes occurring within the slab
beneath western Washington show a slight tendency of E dipping T-axes (Ma et al., 1996). These all indicate
that the large events were fracture within the slab and the slab is down-dip tensional beneath the northern part
of Cascadia.
Small earthquakes within the upper plate in Puge Sound west of the volcanic front have N-S horizontal
11
P-axes with T-axes distributed on a great circle in a vertical plane with an E-W strike (Ma et al., 1996). This
indicates that the stress state of the inner fore-arc is nearly neutral in the across-arc direction with the
along-arc σHmax (see Wang et al., 1995 for details). P-axes parallel to the arc have also been obtained for
small earthquakes in the upper plate in the Vancouver island (Wang et al., 1995). We do not draw σHmax
trajectories in this region, because the back-arc stress is complex (Zoback, 1992).
3.12. Alaska
The Pacific plate is subducting beneath Alaska with a rate of ca. 60 mm/yr in the NNW direction (DeMets
et al., 1990). The age of the subducting plate is around 55 Ma (Pitman et al., 1974). On December 6, 1999,
a large earthquake (Event 20, Mw 7.0), having a strike-slip fault type mechanism solution with a NW
dipping T-axis, occurred in the Kodiak Island at the depth of 54 km (HCMT). Hansen and Ratchkovski
(2001) relocated this event at the 36 km depth, along with many aftershocks, using a joint hypocentral
determination method. The focal mechanism solution along with the nearly vertical distribution of the
relocated aftershocks indicates that this event was fracture within the slab. Nearby small events show similar
T-axes to the large event (Figure A1).
Figure 8 shows the σHmax trajectories in western Alaska (Nakamura et al., 1980). From the fore-arc to the
rear side of the volcanic front, σHmax is in the across-arc direction, with a reverse fault type stress state in the
fore-arc, and with a strike-slip type in the rear side of the volcanic front. Further to the NW, σHmax is in the
along-arc direction with a normal fault type stress state.
4. Discussion
4.1. Slab stresses
As we have shown in the previous sub-sections, T-axes of the large shallow slab events are more or less
in the down-dip direction of the slab. Nearby smaller events have generally similar T-axes. This indicates
that the slab is down-dip tensional in these regions. However, this does not necessarily mean that a slab pull
force is dominant there. For N. Cascadia, SW. Japan, and Mexico, ages of the subducting slab are younger
than 20 Ma. Since it is known that until 20 Ma, negative buoyancy of the slab does not overcome the
positive buoyancy of the oceanic crust-harzburgite layer (Davies, 1992), slab pull forces in these areas would
be negligible. In other regions, slabs are older than 20 Ma. However, they are not so old except for E.
12
Hokkaido and S. Mariana where the old Pacific plate is subducting (Table 1). We therefore believe that the
slab pull force is not a major cause for the occurrence of these large shallow slab events.
4.2. Horizontal stress gradient in the upper plate
We have seen changes in σHmax direction in many of the regions where large shallow slab events have
occurred. Nakamura and Uyeda (1980) compiled several areas which have similar σHmax direction changes
using Quaternary active faults, earthquake mechanisms, dikes and volcanic center alignments, and suggested
existence of a horizontal stress gradient in the upper plate. Recently Wang (2001) showed that stress state of
the fore-arc is in tension in many arcs due to a low coupling at the thrust zone and a gravitational collapse of
the fore-arc topography, and there is not a significant stress gradient between the fore-arc and the back-arc.
We here re-examine this issue, i.e., whether there is a stress gradient in the upper plate or not, by
considering possible causes for the observed σHmax direction change.
There are two possible sources for stress gradient; one is a crust-plate structural variation, and the other is
shear traction at the base of the upper plate. The former is sometimes referred to as a gravitational potential
energy force in recent literature (e.g., Sandiford and Coblentz, 1994; Jones et al., 1996). In this case, a
differential force (a horizontal stress minus a vertical stress integrated over the plate thickness) at one point
with respect to a reference point can be calculated by an integral of ∆ρ(z)z over the plate thickness, where
∆ρ(z) is density difference between these two points and z is the depth (Artyushkov, 1973; Parsons and
Richer, 1980; Fleitout and Froidevaux, 1982). For a continental plate, crustal thickness change is the most
important factor to affect the differential force, but change in the mantle structure also contributes to this.
This kind of the differential force is the origin for the extension in High Himalayas-S. Tibet and
Altiplano-Cordillera Blanca which have a very thick crust.
In arcs, it produces more tension, in general to the landward, in the thicker crust - hotter plate area
(Froidevaux et al., 1988; Seno, 1999). Figures 9a and b illustrate the differential stress change across the arc
for this case. The level of σxx - σzz at the aseismic front, i.e., the landward edge of the thrust zone (Yoshii,
1975), is determined by the coupling at the thrust zone and the crustal thickness variation across the outer
fore-arc (Seno and Yamanaka, 1998; Seno, 1999; Wang, 2001). We assume the differential stresses, σxx - σzz
and σyy - σzz , are negative in this figure. From the aseismic front toward the back-arc, they change gradually
due to a crust-plate structural change by an amount of less than a few hundred bars (Froidevaux et al., 1988;
Seno, 1999). Because the plate thickness does not change much, the magnitude of the horizontal stresses
itself neither changes much (Figure 9a). Thus the horizontal stress gradient is small for both x- and
13
y-directions, producing no exchange in σHmax direction.
In contrast to this, σHmax changes its direction in many arcs as we have seen in S. Kyushu, S. Mariana,
Vanuatu-N. Fiji Basin, Mexico, and Alaska. In these regions, the back-arc side has along-arc σHmax and the
fore-arc side has across-arc σHmax generally. However, the boundary between these two stress regimes varies
from a region to another region; for example in Mexico the boundary is located in the fore side of the
volcanic front, and in Alaska it is in the rear side. Wang (2001) and Kubo and Fukuyama (2001) argued that
the arc-parallel tension in the fore-arc may result from its seaward motion due to the back-arc spreading
behind, producing the apparent rotation of the σHmax direction. However, this explanation might not be
applicable in general since back-arc spreading does not occur in most of the arcs cited above and, even if a
trench retreat occurs, the entire arc would suffer from the same strain due to the seaward migration, not only
the fore-arc. Also note that arc's curvatures are small for Kyushu, Mexico, and Alaska, which makes the
effect of the seaward migration to be small. We therefore believe that the σHmax direction change implies a
horizontal stress gradient in the across-arc direction as shown in Figure 9c. Since such a stress gradient
cannot be produced by interaction between plates or crust-plate structural change, drag forces operating at the
base of the upper plate would be a most likely cause (Figure 9d), as Seno (1999) suggested for S. Kyushu.
The drag forces would increase σxx seaward, and produce the desired σHmax direction change.
Even in some regions without a σHmax direction change, we can recognize a horizontal stress gradient. In
S. America, the sub-Andes is more compressional than the east coast as we have seen in Figure 6. Further to
the east, there is a spreading center in the Atlantic. We infer that mantle drag forces are similarly operating at
the base of the S. American continent, in a much larger scale than shown in Figure 9d. In N. Kyushu, σHmax
is directed in the E-W, and the eastern part has a strike-slip fault type stress and the western part has a normal
fault type (Figure 3). Since the crustal thickness does not change between these two areas, a plausible cause
for the change in fault type would be a horizontal stress gradient, i.e., more compression in the east than in
the west, consistent with the σHmax direction change in S. Kyushu (Seno, 1999).
The tectonic situation where large shallow slab events tend to occur is thus summarized in Figures 10a
and b. The drag forces at the upper plate base cause tension in the rear side and compression in the outer
fore-arc. This compression should be balanced with tension within the shallow portion of the slab (see Seno
and Yamanaka, 1998 for the force balance). The supposed drag forces would be due to flows within the
mantle wedge beneath the arc or continent. The expected flows might not be a simple secondary flow
associated with subduction (Hsui and Toksoz, 1981), which are common to all subduction zones (Nakamura
and Uyeda, 1980). We advocate convection currents associated with thermal or chemical density anomalies,
although their detailed nature remains unknown at present. In the case of S. America, the mantle convection
14
current originated from the accumulated heat beneath Gondowana would have caused the continent to drift
toward the west, overriding the Nazca plate (Figure 10b).
For Kyushu-SW. Japan and Cascadia where kinematics of the fore-arc is better known, we can depict
more realistic cartoons of the tectonic situation (Figures 11a and b). A mantle upwelling has been expected
in the South China Sea west of Kyushu from conductivity anomaly, seismic tomography, and OIB type
volcanism (see Seno, 1999 for details). Recent GPS data show that the fore-arc of Kyushu and SW. Japan are
migrating to the SE and NEE, respectively, with rates of more than 10 mm/yr relative to Eurasia (Ozawa et
al., 1999), probably dragged by the lateral flow from the upwelling (Figure 11a, Seno, 1999). SW. Japan is
then colliding with C. Honshu producing the E-W σHmax (Figure 3). Cascadia is likely to be a mirror image
of Kyushu-SW. Japan. There might be a mantle upwelling in the Basin and Range (Gough, 1984) and
paleomagnetic and GPS data show that the W. Oregon block has been migrating to the NW with respect to
N. America, resulting in the collision with the Canadian Coastal Mountains (Figure 11b, Wells et al.,
1998). The N-S σHmax in the Puge Sound area may be caused by this collision, which is similar to C. Japan.
Both regions have down-dip tensional stresses in the slab, which are indicated by the diverging solid arrows
in Figure 11, and thus belong to the tectonic regime shown in Figure 10a.
4.3. A cause for large shallow slab earthquakes
The fact that most large shallow slab earthquakes have occurred in the tectonic situation illustrated in
Figure 10 suggests a possible cause for these events. We have already degraded the possibility of a slab pull
force for their cause. The numerical simulations of mantle convection involving continental plates, which
are stable to convection, have been conducted (e.g., Lowman and Jarvis, 1993; Nakakuki and Honda, 2001).
After the breakup of a supercontinent, a continent drifts over an oceanic plate (Figure 12a). The streamlines
corresponding to an oceanic plate are deflected under the advancing continent, and the contour interval at the
shallow part of the subducting slab, marked by the arrow in Figure 12a, becomes narrower. We call this
phenomena "suction" by a continental plate. In this case, a continental plate sucks an oceanic plate, in
contrast to the opposite case proposed by Elsasser (1971). The numerical simulation by Nakakuki and Honda
(2001) involving a continent migrating seaward shows that the shallow portion of the slab is down-dip
tensional (Figure 12b). Taking into account the fact that large shallow slab events tend to occur in the
tectonic regime shown in Figure 10, we suppose that excess tension produced by the suction of an oceanic
plate by a continental plate is a cause for large shallow slab events.
15
4.4 Exceptional cases
In Manila, the back-arc is in the across-arc compression. In addition to the subduction of the South China
Sea beneath Luzon from the west, the Philippine Sea is subducting along the Philippine Trench from the
east. Thus the upper plate is sandwiched between the subducted slabs from both sides. In this case, an extra
slab pull force, if it exits, can be supported by a large compressional stress within the upper plate. We
cannot, however, find at present any reason for existence of such an extra slab pull force because the
subducting South China Sea is young. In Sumatra and E. Hokkaido, horizontal stress axes of the upper plate
are oblique to the arc trend, and it is difficult to discuss them in a simple two dimensional vertical
cross-section. Including S. Vanuatu, causes for the large shallow slab events in these areas remain unresolved
for future elaborate studies.
4.5 Relation to dehydration embrittlement
Foci of the large shallow slab events are generally greater than 30 km (Table 1). This implies that the
shear strength expected from the Coulomb-Navier fracture criterion is too large compared with the magnitude
of tectonic stresses (a few kbars at most). A weakening mechanism is necessary to produce these slab events,
similarly to produce intermediate-depth and deep earthquakes (See Kirby, 1995 for review). Dehydration
embrittlement (Raleigh and Paterson, 1965) is a viable mechanism for intermediate-depth events (Kirby et
al., 1996; Seno and Yamanaka, 1996; Peacock and Wang, 1999; Peacock, 2001; Seno et al., 2001). Then
this might also be applicable to shallow slab events. If we accept the dehydration embrittlement hypothesis,
there should be two types of origin for shallow slab events; one is due to dehydration of metamorphosed
oceanic crust, and the other is dehydration of serpentinized slab mantle. Fault dimensions of large shallow
slab events are on the order of tens km which exceed generally the oceanic crust thickness, and must involve
fracture of the slab mantle. This implies that the slab where such large events occur should be serpentinized
in the mantle portion.
In SW. Japan, large slab events have occurred in Kii Peninsula and near Kyushu (Figure 3). These are the
areas where Seno et al. (2001) inferred serpentinization of the Philippine Sea slab on the basis of seismic
evidence. For major aftershocks of the 2001 Geiyo earthquake, Seno (2001) preliminarily examined existence
of later phases traveling within the oceanic crust (see Ohkura, 2000 for a detailed study in this region), and
found no such later phases, which suggests that this event might have ruptured the slab mantle. For the
2001 Nisqually earthquake, Creager et al. (2001) examined the aftershock distribution and favored a fault
16
plane lying horizontally within the oceanic crust. Because these two recent events are rather small (Mw 6.8),
there remains an ambiguity whether these events have ruptured the mantle part of the slab or not. For many
large events listed in Table 1, however, there should be little doubt that their rupture has extended into the
mantle part, and thus serpentinization of the slab mantle must be a necessary condition for the occurrence of
such large shallow slab earthquakes.
5. Conclusions
We list (M>=7.0) earthquakes which occurred in the shallow portion (20-60 km depth) of slabs by
searching published individual studies and the Harvard University CMT catalogue. The regions where such
events are found are E. Hokkaido, Kyushu-SW. Japan, S. Mariana, Manila, Sumatra, Vanuatu, N. Chile, C.
Peru, El Salvador, Mexico, N. Cascadia, and Alaska.
These large events had focal mechanisms with down-dip T-axes, consistent with those of nearby smaller
events. However, most of the slabs are young and apparently have no large slab pull force. The upper plates
with these large shallow slab events are generally characterized by a horizontal stress gradient in the
across-arc direction. This stress gradient is best manifested by the σHmax direction change from the along-arc
to the across-arc from the back-arc side to the trench side. We infer that mantle drag forces beneath the upper
plate are operating in these regions, driving the upper plate oceanward. These drag forces beneath the upper
plate might have originated from small-scale or large-scale mantle convection currents. We propose that the
upper plate driven by the convection currents overrides an oceanic plate, and suck the slab to be in extra
tension, which may result in large shallow slab earthquakes.
The place where large shallow slab earthquakes are expected in the future would thus be a region which has
an across-arc stress gradient in the upper plate. Furthermore, if we accept the dehydration embrittlement
hypothesis for large shallow slab events, the slab mantle in this region must be hydrated to form serpentine.
Appendix
P- and T-axes of HCMT solutions (Mw>=5.8) are plotted in a cross-section along a great circle passing
through one of the large shallow events (Table 1) in the across-arc direction; events within a total width of
two hundred km are plotted. Only the width for section "b" is extended to 500 km, in order to cover the more
than one large slab events. The section is shown by the dotted line in Figure 1 with an alphabet
corresponding to each cross-section. The P- and T-axes are projected on the vertical plane; thus a shorter
17
length indicates more obliquity to the plane. The P- and T-axes with small dots are those of the large
shallow slab events in each section.
Acknowledgments. We thank Ruth Ludwin and Natalia Ratchkovski for providing us useful information
on the slab events in N. America, and Takeshi Mikumo, Krishna Singh, and Javier Pacheko for the slab
events and stress data in Mexico, and Masahiro Kosuga, and Kazuhiko Goto for the stress data in Japan, and
Masayuki Kikuchi for providing one of the authors (T.S.) a chance to visit Mexico, and Atsuki Kubo,
Katsuhiko Ishibashi, and Yoji Kobayashi for discussion.
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26
Figure Captions
Figure 1 Focal mechanisms of large shallow slab earthquakes listed in Table 1 are plotted with event
numbers (lower hemispheres in an equal area projection). For Events 2, 4 and 5, only epicenters are shown
because no reliable mechanism solutions are available for these events. The dotted lines labeled with
alphabets show sections along which P- and T-axes of smaller events from the Harvard centroid moment
tensor catalogue are plotted in Figure A1 (Appendix). (a) Western and South Pacific regions (b) North and
South American regions.
Figure 2 σHmax trajectories in Hokkaido. The gray and broken lines indicate σHmax directions for reverse and
strike-slip fault type stress regimes, respectively. The short bars are P-axes of the focal mechanisms of
shallow earthquakes occurring within the crust of the upper plate (Moriya, 1986; Kosuga, 1999), which are
used for drawing the stress trajectories. The thick and thin bars represent reverse and strike-slip fault type
mechanisms, respectively. The solid circle shows the epicenter of the 1994 slab event. The solid triangles
show active volcanoes. Convergence velocities are indicated by the arrows with numerals representing rates
in mm/yr.
Figure 3 σHmax trajectories in Kyushu-SW. Japan (Seno, 1999). The gray, broken and dotted lines indicate
σHmax directions for reverse, strike-slip, and normal fault type stress regimes, respectively. The solid circles
show the epicenters of the 1899, 1905, 2001, and 1931 slab events. The solid triangles show active
volcanoes.
Figure 4 σHmax trajectories in S. Mariana drawn using geological normal fault strikes in Martinez et al.
(2000). The dotted lines indicate σHmax directions for a normal fault type stress regime. The solid circle
shows the epicenter of the 1993 slab event.
Figure 5 σHmax trajectories in Vanuatu - North Fiji Basin. Symbols for trajectory lines are the same as Figure
3. The short bars are P-axes of the focal mechanisms of shallow earthquakes occurring within the crust of the
upper plate (Charvis and Pelletier, 1989), which are used for drawing the stress trajectories. The thick, thin,
and thinnest bars represent reverse, strike-slip and normal fault type mechanisms, respectively. The solid
circles show the epicenters of the 1981 and 1994 slab events.
27
Figure 6 σHmax trajectories in S. America. Symbols for trajectory lines are the same as Figure 3. The short
bars are σHmax directions of the stress data within the crust of the upper plate (Assumpcao, 1992; Zoback,
1992), which are used for drawing the stress trajectories. The thick, thin, and thinnest bars represent reverse,
strike-slip and normal fault type stress regimes, respectively. The solid triangles show active volcanoes. The
solid circles show the epicenters of the 1970 and 1965 slab events.
Figure 7 σHmax trajectories in SW. Mexico (Seno and Singh, 2001). Symbols for trajectory lines are the
same as Figure 3. The small dots indicate monogenic volcanic centers (Hasenaka, 1994) and the thin lines
with ticks indicate active normal faults (Suter, 1992), which are used for drawing the stress trajectories. The
solid triangles and stars indicate active composite or silisic volcanoes and clusters of small monogenic
volcanoes, respectively. J. M. G and O in the inset right bottom denote the Jalisco, Michoacan, Guerrero,
and Oaxaca blocks. The solid circle shows the epicenter of the 1997 slab event.
Figure 8 σHmax trajectories in Alaska (Nakamura et al., 1980). Symbols for trajectory lines are the same as
Figure 3. The solid triangles show active volcanoes. The solid circle shows the epicenter of the 1999 slab
event.
Figure 9 Two types of stress gradients in the arc. The dotted lines in (b) and (d) indicate σHmax directions for a
normal fault type stress regime. (a) and (c) represent the change of the three principal stresses across the arc.
The differential stresses σxx - σzz and σyy - σzz at the aseismic front is determined by the coupling at the thrust
zone and the gravitational collapse of the topography (Seno and Yamanaka, 1998; Seno, 1999; Wang, 2001).
In both cases of (b) and (d), a weak coupling is assumed and σxx - σzz and σyy - σzz are assumed to be negative
at the aseismic front. In (a) and (b), there is no mantle drag at the upper plate base. Since the plate thickness
does not change much, neither of σxx or σyy changes much, and only σzz changes significantly, producing
more tension in the thicker crust area. In (c) and (d), there is mantle drag at the upper plate base producing a
change in the magnitude of σxx. This results in the change of the σHmax direction between x and y as shown
in (c) and (d).
Figure 10 Tectonic situations where large shallow slab earthquakes tend to occur. The stress gradient
observed in the upper plate implies mantle drag forces beneath it. The slab shows a down-dip tensional
stress, but its age is mostly young. (a) The arc is driven by a small scale convection current toward the
trench. (b) The continental plate is driven by a large scale convection current toward the trench.
28
Figure 11 Illustrations of the kinematics and dynamics occurring in (a) Kyushu - SW. Japan and (b) Cascadia
as examples of the tectonic situation shown in Figure 10a. The double arrows, converging and diverging,
show compressional and tensional stresses, respectively. The single arrows indicate motions of the arc,
upwelling plume or oceanic plate. (a) The mantle upwelling has been suspected in the South China Sea west
of Kyushu (Seno, 1999). Kyushu and SW. Japan migrate to the E and collide with C. Honshu, probably
dragged by the lateral flow from the upwelling. (b) The mantle upwelling has been suspected in the Basin
and Range (Gough, 1984). The W. Oregon fore-arc block migrates to the NW and collides with the Canadian
Coastal Mountains (Wells et al., 1998).
Figure 12 Numerical simulations of mantle convection with drifting continents. (a) The supercontinent
breakups (top) and each broken piece migrates toward the trench (bottom) (Lowman and Jarvis, 1993). T and
φ indicate the temperature contours and stream lines, respectively. As the continent is driven oceanward and
overrides the oceanic plate, streamline intervals in the shallow portion of the subducting slab become
narrower. We call this phenomena "sucking" by a continental plate, contrary to the sucking by an oceanic
plate proposed by Elsasser (1971). (b) The shallow part of the slab becomes tensional as the continent
migrates (Nakakuki and Honda, 2001).
Figure A1 P- and T-axes of HCMT solutions (Mw>=5.8) are plotted in a cross-section along a great circle
passing through a large shallow event in the across-arc direction; sections lines are shown in Figure 1.
29
Table 1. List of large shallow slab earthquakes
Region
Event
Date
Epicenter
°N
°E
Mw
Depth
(km)
Strike/Dip/Rake
(°)
Age
(Ma)
E. Hokkaido
1 Hokkaido-toho-ok
i
Kyushu-SW.
Japan
1994 10 04
43.42
146.81
8.3
33
158
41
24
123
2 Kii-Yamato*
1899 03 07
34.1
136.1
7.0
45
3 Geiyo
2001 03 24
34.13
132.71
6.8
47
181
57
-67
15-30
4 Geiyo*
1905 06 02
34.1
132.5
7.2
50
5 Hyuganada*
S. Mariana
1931 11 02
32.2
132.1
7.1
40
6 Guam
Manila
1993 08 08
12.98
144.80
7.7
45
238
24
82
164
7 Manila
Sumatra
1999 12 11
15.87
119.64
7.2
35
112
13
-169
22
8 Sumatra
Vanuatu
2000 06 04
-4.73
101.94
7.8
44
92
55
152
66
9 Vanuatu
1994 07 13
-16.50
167.35
7.1
25
272
42
02
52
10 Vanuatu
N. Chile
1981 07 06
-22.31
170.90
7.5
58
345
30
-179
35
11 Taltal
C. Peru
1965 02 23
-25.67
-70.79
7.0
60
16
86
-78
48
12 Peru
El Salvador
1970 05 31
-9.18
-78.82
7.9
43
160
37
-90
44
13 El Salvador
1982 06 19
12.65
-88.97
7.3
52
102
25
-106
>37
14 El Salvador
Mexico
2001 01 13
12.97
-89.13
7.7
56
121
35
-95
15 Oaxaca
1999 09 30
15.70
-96.96
7.4
47
102
42
-103
16 Oaxaca
1931 01 15
16.4
-96.3
7.7
40
90
34
-90
17 Michoacan
N. Cascadia
1997 01 11
18.34
-102.58
7.1
40
175
18
-28
18 Nisqually
2001 02 28
47.14
-122.53
6.8
47
176
17
-96
19 Olympia**
Alaska
1949 04 13
47.17
-122.62
7.1
54
14
82
-135
1999 12 06
57.35
-154.35
7.0
36
357
63
-180
20 Kodiak Island
10-17
10
55
*Hypocenters and magnitudes are from Utsu (1982), except for the depths of the 1899 and 1905 events which are estimated in
this study. Magnitudes are referring to the Japan Meterological Agency (JMA) magnitude. **Hypocenters and magnitudes are
from Baker and Langston(1987). Hypocenters for other events are from the Harvard University centroid moment tensor catalogue
(HCMT), except for the depths by individual studies mentioned in the text. Strike, dip and rake are from HCMT except for the
1993 Guam event: Tanioka et al. (1995), 1965 Taltal event: Malgrange and Madariaga (1983), 1970 Peru event: Abe (1972), 1931
Oaxaca event: Singh et al. (1985), and 1949 Olympia event: Baker and Langston (1987).
(a)
90˚E
50˚N
100˚E
110˚E
120˚E
130˚E
140˚E
150˚E
160˚E
170˚E
180˚
1
Asia
a
40˚N
3
b
30˚N
5
20˚N
4
7
2
Pacific
6
d
c
10˚N
0˚
8
10˚S
e
9
f
20˚S
km
0
1000
2000
Australia
g
10
30˚S
Fig. 1a
(b)
160˚W
150˚W
140˚W
130˚W
120˚W
110˚W
100˚W
90˚W
80˚W
70˚W
60˚W
60˚N
20
n
50˚N
19
18
40˚N
m
N. America
30˚N
17
Pacific
20˚N
16
l
k
10˚N
15 14
0˚
j
S. America
13
12
10˚S
i
20˚S
km
0
30˚S
1000
2000
h
11
Fig. 1b
s V< s
Eurasian
plate
s
Okhotsk plate
H
< s < Vs
Hmin
Hmax
45°N
s
Hokkaido
s
s
s
s
s
l
s
1994
Japan
Sea
s
s
s
Hidaka
s
ch
41°
en
s
r
il T
ur
K
Oshima
Peninsula
79
79
Pacific plate
N. Honshu
140°
78
145°E
Fig. 2
s V< s
s
Vit
10°S
iaz
H
< s <Vs
s H< s V
Tre
Hmin
Hmax
nch
North Fiji Basin
D'Entrecasteaux
Ridge
New
100
l
1994
ench
s Tr
ride
Spreading Center
Heb
20°
1981
l
170°E
.
rF
Z.
nte
Hu
180°
Fig. 5
85°W
10 °
N
35°W
10 °
N
s
s
s
s
s
s
s
s
Cordillera Blanca
1970 l
Altiplano
ss
s
77
South America
s
s
s
s
-20°
-20°
1965 l
84
s
s
s
s
s
s
s
s
Atlantic
s
Pacific
s
s
s
s V< s
s
s
-50°
H
< s <Vs
s
Hmin
H
<s
Hmax
V
Fig. 6
s
c s
s
s
s <s
★ 小単成火山の集合
s H< s
成層火山
Mexico
s
s
V
単成火山の火口
Tra
ns
s
H
N
V
s
Mexican Volcanic Velt
c
c
Jal
isc
oB
c
loc
k
c
c
20°
cs
c s
s
Mic
hoac
M
.A
19
m
er
ica
Sierr
a
ch
lock
del S
ex
en
1997
Madr l
e
M
Tr
an B
o
ic
ur
Rivera Plate
TMVB
47
Cocos Plate
104°
J M
100 km
102°
18°
G O
100° W
Fig. 7
s V< s
s
140°W
H
70°N
< s < Vs
Hmin
s H< s
Hmax
Alaska
V
s
s
s
172°W
62°N
s
s
Bering Sea
s
s
s
s
s
s
s
s
s
s
54°N
s
s
s
s
s
s
s
s
s
s
s
s s
s s
s
s s
s
s
s
1999 l
58°N
58
Aleutian Trench
500 km
65
Fig. 8
(c)
s
X
Back-arc
s
zz
s
xx
yy
Stress magnitude
Stress magnitude
(a)
Aseismic Front
s
X
Back-arc
s
zz
xx
s
yy
Aseismic Front
(d)
(b)
s
s
Hmax
s
s
Asthenosphere
Z
Y
on
t
t
arc
on
Fr
re-
ic
is
Fo
m
Ax
eis
As
ter
Tr
en
ch
Fo
r
ne
In
Crust
Ou
X
s
arc
s
re-
Fr
Thrust
upling
Plate
Oceanic
Co
Weak
s
lca
is
ch
en
Tr
Continental Plate
s
Ax
ter
eis
Fo
m
re-
ic
Vo
Fr
arc
on
t
nic
reFo
r
ne
Crust
Ou
X
As
s
In
s
Y
arc
s
s
Vo
lca
nic
Fr
o
nt
Hmax
Continental Plate
Mantle Drag
Asthenosphere
Weak
st
ng Thru
Coupli
late
P
ic
n
Ocea
Z
Fig. 9
(a)
Compression
Tension
Oceanic plate
Arc
Tension
(b)
Compression
Tension
Oceanic plate
Continental plate
Tension
Fig. 10
(a)
pan
Ja
W.
Korea
- S
shu
C. Honshu
Kyu
E. China
Sea
Philippine Sea plate
(b)
Washington
Basin & Range
Juan de Fuca Plate
Fig. 11
E. Hokkaido (a,1)
depth [km]
0
NW
SE
0
100
100
200
200
NW
P-axis
300
T-axis
300
0
100
200
300
SE
0
100
200
300
Kyushu - sw. Japan (b,3)
depth [km]
0
W
0
E
100
W
E
100
P-axis
200
T-axis
200
0
100
200
300
0
100
200
300
Mariana (c,6)
depth [km]
0
NW
SE
0
100
100
200
200
NW
P-axis
300
T-axis
300
0
100
200
300
SE
0
100
200
300
Manila (d,7)
depth [km]
0
W
0
E
100
W
E
100
P-axis
200
T-axis
200
0
100
200
0
100
200
Sumatra (e,8)
depth [km]
0
SW
NE
100
0
SW
NE
100
P-axis
200
T-axis
200
0
100
200
300
0
100
200
300
Vanuatu (f,9)
depth [km]
0
SW
NE
100
0
SW
NE
100
200
200
P-axis
300
T-axis
300
0
100
200
300
horizontal distance [km]
0
100
200
300
horizontal distance [km]
Fig. A1
Vanuatu (g,10)
depth [km]
0
SW
0
NE
100
100
200
200
SW
P-axis
300
T-axis
300
0
100
200
300
NE
0
100
200
300
N. Chile (h,11)
depth [km]
0
W
0
E
100
W
E
100
P-axis
200
T-axis
200
0
100
200
0
100
200
Peru (i,12)
depth [km]
0
SW
0
NE
100
SW
NE
100
P-axis
200
T-axis
200
0
100
200
300
0
100
200
300
El Salvador (j,13,14)
depth [km]
0
SW
0
NE
100
SW
NE
100
P-axis
200
T-axis
200
0
100
200
0
100
200
Mexico (k,15,16)
depth [km]
0
SW
NE
100
0
SW
NE
100
P-axis
200
T-axis
200
0
100
200
300
400
0
100
200
300
400
Mexico (l,17)
depth [km]
0
SW
NE
100
0
SW
NE
100
P-axis
200
T-axis
200
0
100
200
300
horizontal distance [km]
0
100
200
300
horizontal distance [km]
Fig. A1 (continued-1)
depth [km]
0
0
SW
100
SW
100
P-axis
200
T-axis
200
0
100
200
300
0
100
200
300
El Salvador (j)
depth [km]
0
0
SW
100
100
200
200
SW
P-axis
300
T-axis
300
0
100
200
0
100
200
Cascadia (m,18,19)
depth [km]
0
W
0
E
100
W
E
100
P-axis
200
T-axis
200
0
100
200
300
0
100
200
300
Alaska (n,20)
depth [km]
0
NW
SE
100
0
NW
SE
100
P-axis
200
T-axis
200
0
100
200
300
horizontal distance [km]
0
100
200
300
horizontal distance [km]
Fig. A1 (continued-2)