03. 2011年4月13日 18:58:17: cqRnZH2CUM
論文もあった
http://profile.usgs.gov/professional/mypage.php?name=rstein
http://www.usgsprojects.org/TohokuQuake2011/Toda_etal_EPS_2011_with_summary.pdf
Submitted 7 April 2011 to the Tohoku Earthquake Special Issue of Earth Planets Space
Using the 2011 M=9.0 Tohoku earthquake to test the Coulomb
stress triggering hypothesis and to calculate
faults brought closer to failure
Shinji Toda1, Jian Lin2, and Ross S. Stein3
1Disaster Prevention Research Institute, Kyoto University, Japan
toda@rcep.dpri.kyoto-u.ac.jp
2Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA
jlin@whoi.edu
3U.S. Geological Survey, Menlo Park, California, USA
rstein@usgs.gov
Plain English Summary
Most seismologists assume that once a major earthquake and its expected aftershocks do
their damage, the fault will remain quiet until stresses in Earth’s crust have time to
rebuild, typically over hundreds or thousands of years. But research over the past two
decades has shown that earthquakes interact in ways never before imagined. A major
shock does relieves stress―and thus the likelihood of a second major tremor―but only
in some areas. The probability of a succeeding earthquake adjacent to the section of the
fault that ruptured or on a nearby but different fault can jump by as much as a factor of
ten.
At the heart of this hypothesis―known as Coulomb stress triggering―is the realization
that faults are responsive to subtle stresses they acquire as neighboring faults shift,
deforming the Earth’s crust. Drawing on records of past tremors and novel calculations of
fault behavior, stress relieved during an earthquake does not simply dissipate; instead it
moves down the fault and concentrates in sites nearby, promoting subsequent tremors.
Indeed, studies of about two dozen faults since 1992 have convinced many of us that
aftershocks can be triggered even when the stress is increased by as little as one-tenth the
pressure required to inflate a car tire, or about 0.3 bar. While the frequency of aftershocks
decays rapidly with time after the mainshock, their magnitude does not decay at all, and
so large late aftershocks are an unexpected consequence of a great earthquake.
To calculate the Coulomb stress transfer, one needs to know the distribution of slip on the
mainshock rupture, and the geometry, friction, and sense of slip on surrounding faults.
The 11 March 2011 Tohoku earthquake provides an unprecedented test of the extent to
which Coulomb stress transfer governs the triggering of aftershocks, and by extension,
subsequent mainshocks on surrounding faults. Thanks to Japan’s superb monitoring
networks, the earthquake is the best-recorded great event the world has ever known.
During 11-31 March 2011, there were almost 200 aftershocks for which the slip and
geometry on the causative fault is known (they possess “focal mechanisms”), and so the
Coulomb stress change imparted by the mainshock rupture can be resolved on the
aftershock fault planes to test whether they were, in fact, brought closer to failure,
consistent with the theory.
The distribution of fault slip in the mainshock is nevertheless uncertain; many “source
models” for the mainshock have been developed during the past 3 weeks from seismic,
geodetic, and tsunami observations. Here we show that among the six source models we
tested, there is a mean 47% gain in positively stressed aftershock mechanisms over that
for the background earthquakes (shocks that struck before the Tohoku mainshock), which
serve as the control group. This is a strong confirmation of the Coulomb hypothesis.
Among all the tested models, that of Wei and Sladen [2011]
not only produced the largest
gain, 63%, but the largest yet found for any earthquake we or others have tested. Further,
a value of friction (0.4), midway between ‘teflon and rough cohesive faults, is found to fit
the aftershock set best.
With this test as confirmation, we use the best-performing model and best value of
friction to calculate the stress transferred to all major faults that surround the mainshock.
We find that large sections of the Japan trench megathrust to the north of the mainshock
(the offshore ‘Sanriku-Hokobu’ region 200 km northeast of Sendai), and to the south of
the mainshock (the ‘Off-Boso’ region 100 km east of Tokyo), were also brought ≥0.3
bars closer to failure. East of the megathrust, tensional (‘outer rise normal’) faults are
bought 1-15 bars closer to failure; large earthquakes on these could trigger a tsunami. The
‘Kanto fragment’ that we believe is wedged 50-70 km beneath Tokyo, and the Itoigawa-
Shizuoka Tectonic Line (‘ISTL’) 100 km west of Tokyo, were also brought ≥0.3 bars
closer to failure. Based on our other studies, these stress increases are large enough to
increase the likelihood of triggering significant aftershocks or subsequent mainshocks.
Submitted to Tohoku Earthquake Special Issue of Earth Planets Space 7 April 2011
Using the 2011 M=9.0 Tohoku earthquake to test the Coulomb
stress triggering hypothesis and to calculate
faults brought closer to failure
Shinji Toda1, Jian Lin2, and Ross S. Stein3
1Disaster Prevention Research Institute, Kyoto University, Japan
2Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA
3U.S. Geological Survey, Menlo Park, California, USA
Abstract
The 11 March 2011 Tohoku earthquake provides an unprecedented test of the extent to
which Coulomb stress transfer governs the triggering of aftershocks, and by extension,
subsequent mainshocks on surrounding faults. During 11-31 March 2011, there were
177 aftershocks with NIED F-net focal mechanisms, and so the Coulomb stress change
imparted by the rupture can be resolved on the aftershock nodal planes to test whether
they were, in fact, brought closer to failure. Many source models for the mainshock have
been inverted from seismic, geodetic, and tsunami observations. Here we show that
among the six tested source models, there is a mean 47% gain in positively stressed
aftershock mechanisms over that for the background (1997-10 March 2011) earthquakes,
which serve as the control group. An aftershock fault friction of 0.4 is found to fit the
data better than 0.0 or 0.8, and among all the tested models, Wei and Sladen [2011]
produced the largest gain, 63%. We also calculate that at least 5 of the seven large,
exotic, or remote aftershocks were brought ≥0.3 bars closer to failure. With these tests as
confirmation, we calculate that large sections of the Japan trench megathrust, the Kanto
fragment beneath Tokyo, and the Itoigawa-Shizuoka Tectonic Line, were also brought
≥0.3 bars closer to failure.
Submitted to Tohoku Earthquake Special Issue of Earth Planets Space page 2
1. Introduction
The M=9.0 Tohoku-chiho Taiheiyo (hereafter, ‘Tohoku’) earthquake is unprecedented in
size in Japan’s long recorded history of earthquakes, although some foresaw its
possibility [Kanamori et al., 2006]. The earthquake struck on the Japan trench
megathrust, which accommodates about 80 mm/yr of convergence. Thanks to Japan’s
superb monitoring networks, the earthquake is the best-recorded great event the world
has ever known. Seismic, geodetic and tsunami observations not only permit detailed
source inversions for the distribution of slip on the megathrust surface, but also provide
an unparalleled set of aftershocks, which the NIED has used to calculate focal
mechanisms for M≥3.5 events recorded by F-net. Further, the extensive geomorphic,
geodetic and paleoseismic analysis of Japan’s inland faults enables us to consider how
the Tohoku earthquake might have changed the conditions for failure on surrounding
faults.
2. The Coulomb Stress Triggering Hypothesis
An earthquake fault rupture permanently deforms the surrounding crust, changing the
stress on nearby faults as a function of their location, geometry and sense of slip (rake).
The Coulomb stress change is defined as ΔCFF = Δτ + μΔσ, where τ is the shear stress on
the fault (positive in the inferred direction of slip), σ is the normal stress (positive for
fault unclamping), and μ is the apparent friction coefficient. Failure is promoted if ΔCFF
is positive and inhibited if negative; both increased shear and unclamping of faults are
taken to promote failure, with the role of unclamping modulated by fault friction. Despite
some discord [Felzer and Brodsky, 2006; Richards-Dinger et al., 2010], most
investigations of Coulomb stress triggering [Harris, 1998; Stein, 1999; King and Cocco,
2000; Freed, 2005] find that it plays an important role in the production of aftershocks
and subsequent mainshocks on surrounding faults.
3. Test Design
Submitted to Tohoku Earthquake Special Issue of Earth Planets Space page 3
While there is abundant evidence that aftershocks locate in regions of calculated stress
increase (the stress triggering lobes), a stricter test of the Coulomb hypothesis is whether
the nodal planes of the aftershocks are promoted for failure. By resolving the Coulomb
stress change on aftershock nodal planes in their rake directions, one need not make any
assumptions about the aftershock fault geometry or the regional stress. But while the
shear stress on the two orthogonal nodal planes is the same, the unclamping stress is
different. So, except for the special case of zero fault friction, the Coulomb stress
imparted to the two nodal planes will differ, and except under unusual circumstances we
do not know which of the two planes slipped. Thus to conduct this test, we resolve the
Coulomb stress change on both nodal planes of each mechanism, and use both resulting
stress changes in the statistical sample. This means that because of the nodal plane
ambiguity, even if all aftershocks were brought closer to failure we would never find a
100% accord.
But even if we found that a majority of aftershock nodal planes were brought closer to
failure by the mainshock rupture, this would not necessarily demonstrate that the
mainshock stress transfer was responsible. It is possible that shocks that occurred before
the mainshock rupture in the same region would yield a similar percentage, principally
because faults that slip during aftershocks might share the same geometry as those that
slip normally. Therefore, the percentage of positively stressed aftershocks must be
normalized by the positively stressed background shocks. For the ‘background,’ we use
shocks that occurred before the mainshock. In testing terminology, the aftershocks
comprise the test group, and the background shocks the control group. What is important
is the percentage gain in positively stressed aftershocks relative to the background
shocks, with the background shocks selected to match the geographic area and depth
range as the aftershocks. This procedure, first used by Hardebeck et al. [1998], is
analogous to pharmaceutical testing, in which to prove efficacy, the drug given to the test
group must outperform a placebo administered to the control group.
Submitted to Tohoku Earthquake Special Issue of Earth Planets Space page 4
It has also proven very difficult to discriminate among possible values of fault friction in
Coulomb stress transfer, and so a mid-value of 0.4 is most commonly adopted. On
creeping faults, low friction has been found to fit best, whereas on young thrust and
normal faults, a high value of friction may be superior [Parsons et al., 1999; Toda and
Stein, 2002]. Because of the quality of the test data for the Tohoku earthquake, we test
three values of fault friction (0.0, 0.4, and 0.8) for each source model, seeking the value
that produces the highest gain.
To graphically represent the results of the test with some economy, we plot the most
positive stress change of each pair of nodal planes in Fig. 1, with the convention that
where aftershocks overlap, the most positively-stressed shocks are plotted atop. This
schema is used for both the aftershocks (Fig. 1a) and the background shocks (Fig. 1b);
this ‘positive bias’ is only used in the figure; stress on both planes are used in all
calculations, as summarized in Table 1. We performed all calculations using
Coulomb 3.2 (http://www.coulombstress.org).
4. Aftershock Data and Source Models
We use the 177 NIED F-net aftershock focal mechanisms between 11 and 31 March
2011; all are M≥3.5 events within the depth range of 0-80 km, bounded by 34.5-41.0°N
latitude and 137-145°E longitude (Fig. 1a). For the background earthquakes, we
restricted our search area and depths to the same range as the aftershocks, and sought a
dataset about five times larger than the aftershocks for statistical confidence; we used 840
M≥4.5 NIED F-net focal mechanisms during 17 February 1997-10 March 2011 (Fig. 1b).
We tested six source models of the 11 March 2011 mainshock that span the range of the
assumed megathrust dips (9-15°) and datasets used in the source inversions (teleseismic
P, SH, body, and long period surface waves; static GPS displacements; and tsunamigrams
and coastal tide gauge records), as noted in Table 1. First, we use the combined statistics
for all models to test the Coulomb hypothesis, and to find the best value of fault friction.
We then use the test to discriminate among the candidate models, with the expectation
Submitted to Tohoku Earthquake Special Issue of Earth Planets Space page 5
that the source model producing the highest gain in positively stressed aftershocks
relative to background shocks will also provide the best estimates of which major
surrounding fault systems have been brought closer to failure by the Tohoku earthquake.
5. Test Results and their Implications
The mean gain in the percentage of positively-stressed aftershocks with respect to the
background shocks for all six source models is 47%; in other words, 47% more
aftershocks are brought closer to failure than the control group. In a similar test,
Hardebeck et al. [1998] found a 37% gain for immediate aftershocks of the 1992 M=7.3
Landers, California, earthquake, and 46% gain after 3 years. Ma et al. [2005] found a
gain of 61% (70% for thrust and a 56% gain for strike-slip events) for the 1999 M=7.6
Chi-Chi, Taiwan, earthquake, using a 4-yr-long aftershock period. These studies tested
only a single source model, and were unable to discriminate among friction values. For
the Tohoku test, we find a fiction of 0.4 yields a 5-10% higher gain than for low or high
friction (Table 1). Since some aftershocks locate on or near the megathrust surface and
others locate in the outer rise or on continental faults (Fig. 1a), the 0.4 preference could
simply be due to averaging different fault strengths.
Among the candidate source models, Wei and Sladen [2011] (Figs. 1 and 2) has the
highest mean gain for all values of friction (55%); it has the highest gain of all (63%),
obtained for friction=0.4. We thus adopt this model and friction for the remaining tests,
and for calculations of stress transfer to surrounding faults.
We next examine the Coulomb stress triggering to the largest, most exotic, and remote
aftershocks. For this test we use the Global CMT catalog, which is more complete than
the NIED F-net catalog during the first day after the mainshock. The events include the
Mw=7.6 outer rise aftershock, several shallow coastal and offshore 5.4≤Mw≤5.8 normal
events, the thrust event near Nagano, and the two remote strike-slip events, in the Japan
Sea and at the base of Mt. Fuji (Fig. 2). We find that all but the Nagano thrust event
(aftershock #2 in Fig. 2 and Table 2) are brought ≥0.3 bars closer to failure on at least one
Submitted to Tohoku Earthquake Special Issue of Earth Planets Space page 6
nodal plane, and that four of the 7 are brought closer to failure on both nodal planes.
Thus, the Nagano event does not appear to be statically triggered.
6. Stress Changes Calculated on Major Faults in Central Japan
We calculate the Coulomb stress changes on all known major faults in their rake
directions in Fig. 3. Such a calculation is useful to develop insights as to which fault
systems are now more hazardous than they were before the Tohoku earthquake.
Nevertheless, we caution that the calculation idealizes stress transfer because smaller
faults need not share the same geometry as the major faults shown here, and the dip and
rake of even major faults are generally poorly known.
On the megathrust surface (which is rather unrealistically represented as a plane in all
Tohoku source models), there are large stress increases at depths greater than 35 km,
which could give rise to aftershocks, postseismic slip, and consequent coastal uplift. To
the north of the Tohoku rupture in the Sanriku-Hokubu-oki area, there are also large
stress increases at depths greater than 30-35 km (Fig. 3). This section hosted the 1994
M=7.5, 1901 M=7.4, 1931 M=7.6, and 1968 M=7.9 and 7.5 earthquakes (south to north
respectively, JMA magnitudes). To the south of the Tohoku rupture lies the Off-Boso
section (Fig. 3), which exhibits repeating earthquakes, aseismic slip transients, and
possibly uncoupled behavior [Nishimura et al., 2006; Ozawa et al., 2007]. Despite this,
there have been 4-5 M≥5 Off-Boso aftershocks of the Tohoku earthquake (Fig. 2), and so
it can also store elastic stress. Stress increases of about 10 bars are calculated along the
northern margin of the Off-Boso section of the Japan trench adjacent to the M=7.9
aftershock, with stress increases declining to 0.3 bars to the south. If this entire 100- by
180-km section were to rupture, it could host a M=8.1 earthquake, although no such
event is evident in the historical record [Grunewald and Stein, 2006].
The portions of the Sagami trough that last ruptured in the 1923 M=7.9 Kanto event
[Nyst et al., 2006] are calculated to experience Coulomb stress decreases of 0.2-0.5 bar
by the Tohoku earthquake, whereas the stress increased by 0.5-1.0 bar on the easternmost
Submitted to Tohoku Earthquake Special Issue of Earth Planets Space page 7
section. The stressed sections might have participated in the 31 December 1703 M~8.2
Genroku and 4 November 1677 M~8? tsunamigenic earthquakes [Grunewald and Stein,
2006]. The Suruga trough megathrust [Ando, 1975][Ishibashi, 1981], site of the 1854
M=8.4 event, was brought a 0.02-0.07 bars closer to failure; this is likely negligible, since
it is not much larger than the Coulomb stresses induced by the tides. The left-lateral
Itoigawa-Shizuoka Tectonic Line (ISTL), as well as many parallel faults to the west of it,
were brought 0.3-0.4 bar closer to failure. For a fault friction of 0.4, the major thrust
faults of Tohoku were brought more than 1 bar farther from failure. However, for a high
value of friction that might be more appropriate for youthful thrust continental thrusts
[Parsons et al., 1999], some faults are sufficiently unclamped for failure to be promoted
by 0.1-0.4 bar (see Fig. 3 inset). In contrast to the Tohoku thrusts, failure is promoted on
outer rise normal faults by 1.5-15 bars, depending on their proximity to the locus of high
slip. Numerous large Tohoku outer rise normal aftershocks have struck here since the
Tohoku mainshock, including the 11 March 2011 Mw=7.6 event (Fig. 2). The largest
outer rise event ever recorded in this region was the 3 March 1933 M=8.1 [Kanamori,
1971] quake that also caused tsunami damage to the Sanriku coast. The southern 2/3 of
the likely 1933 rupture zone was brought 2-15 bars closer to failure (not shown in Fig. 3).
The Kanto fragment was proposed by Toda et al. [2008] to explain the seismic
tomography, microseismicity, geodetic deformation and plate motion evolution of the
Philippine Sea and Pacific slab interaction beneath Tokyo. They argue that the fragment
broke off the Pacific slab and is wedged between the Pacific, Philippine Sea slabs and the
over-riding Eurasian plate. We calculate that the 60-80-km-deep base of the fragment that
is in contact with the Pacific slab has been brought 1-2 bars closer to failure, whereas the
upper surface, in contact with the Philippine Sea slab, has been brought 0.3 bars closer to
failure. Toda et al. [2008] argued that the intensity distribution of the destructive 1855
M~7.2 Ansei-Edo earthquake matched the 3 July 2005 M=6.0 earthquake at 83 km depth
[Grunewald and Stein, 2006], and so both quakes likely struck on the base of the
fragment. If the entire lower surface of the fragment were to rupture, a M=7.3 event could
strike at ~75 km depth beneath the highly-populated Kanto basin.
Submitted to Tohoku Earthquake Special Issue of Earth Planets Space page 8
7. Conclusions
We have tried to conduct the most rigorous test possible of the Coulomb failure
hypothesis that can be applied to the Tohoku earthquake less than one month after the
mainshock. The test is made feasible by the extraordinary quality and accessibility of the
Japanese seismic and geodetic monitoring networks, and by scientists openly sharing
their preliminary source models, for which we are grateful. We find that all source
models yield a at least a 43% gain in positively stressed aftershock nodal planes relative
to the nodal planes of background seismicity, and the best tested model yields a 63%
gain, the highest yet seen for any earthquake tested. When this model is used to examine
the seven remote, exotic, and largest aftershocks, at least one nodal plane of six of these
is brought >0.3 bar closer to failure, and so the remote aftershocks can have the same
static-stress origin as those on the rupture surface. Examining all major fault systems in
central Japan, large stress increases are calculated to the north and south and along the
base of the Tohoku megathrust rupture surface, on the easternmost sections of the Sagami
trough megathrust, along the ISTL and sub-parallel faults, and on the base of the Kanto
fragment that we believe that underlies Tokyo.
Acknowledgements
We thank Volkan Sevilgen for technical prowess, Meredith Nettles for Global CMT
analyses, Yuji Yagi, Gavin Hayes, and Chen Ji for source modeling insights, and David
Shelly and Fred Pollitz for thoughtful and rapid reviews.
Submitted to Tohoku Earthquake Special Issue of Earth Planets Space page 9
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Figure captions
Figure 1. Stress imparted by the Wei and Sladen [2011] source model and the M=7.9
aftershock are resolved on both nodal planes of NIED F-net aftershock (a) and
background, or pre-Tohoku (b) focal mechanisms. Although both planes are used for the
calculations summarized in Table 1, the most positively-stressed of each pair of nodal
planes is shown here. The effect of the mainshock stress imparted to the mechanisms is
judged from the percent gain in positively stressed mechanisms after the mainshock (the
test group) relative to the background shocks (the control group). The results for
6 models are given in Table 1.
Figure 2. Focal mechanisms of the March 2011 Tohoku earthquake aftershocks from two
catalogs, with important or exotic aftershocks examined for stress transfer keyed to
Table 2. Notice that all aftershocks east of the mainshock are normal outer rise events,
but there is also a group of coastal normal events midway between Tokyo and Sendai.
The mainshock promotes failure of five of the seven events regardless of nodal plane or
friction; for the remaining two events (#1-2) only one of the nodal planes is promoted
(see Table 2).
Figure 3. Stress imparted by the Wei and Sladen [2011] source model and the M=7.9
aftershock to surrounding active faults, resolved in their inferred rake directions. Top
depth and bottom depths of most of the active faults are set to 0 and 15 km. Thrust faults
are assumed to dip 50°; outer rise normal faults are assumed to extend from 0 to 25 km
depth and dip 45°. While most active thrust faults in Tohoku are brought farther from
failure, parts of the Sagami trough megathrusts are brought closer to failure that last
ruptured in 1703, as well as the Kanto fragment [Toda et al., 2008] that likely last
ruptured in 1855.
Submitted to Tohoku Earthquake Special Issue of Earth Planets Space page 12
Table Captions
Table 1. Coulomb stress triggering test results. A representative sample of six inversion
models were tested on NIED F-net aftershocks and background shocks. The background
shocks were chosen to sample the same depth range and area as the Tohoku aftershocks.
All models showed gains of 43-55% over the control (right-hand column). It is
impossible to have positive stress change on 100% of nodal planes except for the special
case in which friction=0.0, because the Coulomb stress change to each nodal plane in a
pair is different, and we do not know which is the fault plane. The best value of receiver
fault friction among all models is seen to be about 0.4, and the best model among all
models is Wei and Sladen [2011].
Table 2. Coulomb stress imparted to the nodal planes of large, exotic, or more remote
aftershocks of the Tohoku earthquake. The earthquake number is keyed to Fig. 2. All but
aftershock #2 is brought ≥0.3 bars closer to failure on at least one nodal plane.
Table 1
Source model Ver. Moment Dip Slip Data % aftershocks with +ΔCFF % background with +ΔCFF Stress effect of mainshock (% gain) Model
(2011 citation) E29 dyn-cm (°) patches fric.=0.0 fric.=0.4 fric.=0.8 fric.=0.0 fric.=0.4 fric.=0.8 fric.=0.0 fric.=0.4 fric.=0.8 means
Yagi v.2 4.5 15 264 a 63 67 69 45 45.8 48.6 56.6 52.9 40.6 50.0
Hayes v.2 4.9 10.2 325 b 74 76 73 51.5 49.8 51.8 40.0 46.3 42.0 42.8
Shao et al v.3 5.7 10 190 b 64 63 61 45.5 41.2 43.4 43.7 52.6 40.9 45.7
Fuji & Satake v.1 2.8 14 25 c 77 81 82 55.7 55.8 56.6 38.2 45.2 44.9 42.8
Wei & Sladen v.1 4.5 9 350 b, d 73 74 74 50.2 45.5 47.5 45.4 62.6 55.8 54.6
Pollitz & Burgmann v.1 3.6 14 446 d, e 68 67 68 42.1 43.9 51.9 61.5 52.6 31.0 48.4
Friction means: 47.6 52.0 42.5 47.4
Notes:
1. a, teleseismic body waves; b, teleseismic P, SH, and long period surface waves; c, DART tsunamigrams and tide gauges; d, static GPS; e, smoothed from 15,876 patches
1. Stress effect of mainshock=[100*(% aftershocks positively stressed by mainshock/% background mechanisms positively stressed by mainshock)-100];
so if 0%, there is no effect of the rupture on aftershock mechanisms, and if 100%, aftershock mechanisms are promoted at twice the background rate.
2. 176 aftershocks used from NIED F-net database for 3/11/2011-4/1/2011 (all are M≥3.6, 0-80 km depth, andl lie within lat. 34.5-41.0°N, lon. 137-145°E
3. 840 background shocks from NIED F-net database for 1/1/1997-3/10/2011 (selection criteria: M≥4.5, 0-80 km depth, lat. 34.5-41.0°N, lon. 137-145°E)
4. M=7.9 aftershock included as a source (uniformly tapered with 5 patches from this study, based on Global CMT parameters and ARIA GPS; Mo=8.5E27 dyn-cm)
5. Full citations for each model will appear in the revised paper
Table 2
# Occurence time Lon. Lat. Depth Mw Mj strike dip rake normal shear Coulomb stress strike dip rake normal shear Coulomb stress
(local time) (km) NP1 NP1 NP1 stress stress μ=0.4 μ=0.8 NP2 NP2 NP2 stress stress μ=0.4 μ=0.8
mo/dy/yr hr:mn (°) (°) (°) (°) (°) (bar) (bar) (bar) (bar) (°) (°) (°) (bar) (bar) (bar) (bar)
1 3/11/11 15:26 144.6 38.27 21.1 7.6 7.5 182 42 -100 -9.3 2.2 -1.5 -5.2 15 49 -81 4.9 2.2 4.1 6.1
2 3/12/11 3:59 138.6 37.08 12.0 6.3 6.7 28 33 58 0.7 -0.3 0.0 0.3 244 63 109 0.1 -0.3 -0.3 -0.2
3 3/12/11 4:46 139.2 40.4 12.0 6.2 6.4 27 78 -177 1.9 0.5 1.2 2.0 296 87 -12 0.0 0.5 0.5 0.5
4 3/15/11 22:31 138.7 35.29 17.7 5.9 6.4 296 70 172 0.7 0.4 0.6 0.9 29 82 20 -0.1 0.4 0.3 0.3
5 3/19/11 18:56 140.6 36.85 12.0 5.8 6.1 146 44 -81 3.2 3.9 5.1 6.4 314 47 -98 3.3 3.9 5.2 6.5
6 3/22/11 7:12 140.8 37.09 12.0 5.7 6.0 15 37 -92 5.4 6.6 8.8 11.0 197 53 -89 9.0 6.6 10.2 13.8
7 3/22/11 7:36 140.8 37.05 12.4 5.4 5.8 182 42 -92 6.4 7.1 9.7 12.3 5 48 -88 8.3 7.1 10.4 13.8
Notes:
1. Wei & Sladen (2011) plus the Mw=7.9 aftershock model from this study are used as sources
2. Stress increases are red; stress decreases are blue
Fig. 3
0 100 km 200
Su ru g a t rough
Sanriku-
Off Boso
Outer
rise
normal
faults
Triple
Junction
Hokobu
ISTL
Japan trench
fragment
Kanto
Megathrust
(M=9.0)
M=7.9M=7.9
Sendai
Miyako
Iwaki
Tokyo
Nagoya
Kyoto
Fuji
Sagami tro ugh
−1 0 1
Coulomb stress change (bar)
on receiver faults
on megathrust source
Niigata
Friction = 0.4
Wei & Sladen (2011) source model
Friction = 0.8
Niigata
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