Comparison of source areas of M4.8±0.1 repeating earthquakes off
Kamaishi, NE Japan
- Are asperities persistent features ?
Tomomi Okada, Toru Matsuzawa, Akira Hasegawa
Research Center for Prediction of Earthquakes and Volcanic Eruptions,
Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
Fax: +81-22-264-3292
Phone: +81-22-225-1950
E-mail: okada@aob.geophys.tohoku.ac.jp
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Abstract
M4.8 +- 0.1 earthquakes have occurred regularly since 1957 with a
recurrence interval of 5.52 +- 0.68 yrs at the same location of the plate
boundary off Kamaishi, Iwate Prefecture.
The last event (M4.7)
occurred on 11/13/2001 16:44 (JST) and the previous one (M4.8) on
3/11/1995 10:30 (JST). Waveforms of these events are similar to each
other.
To examine the hypothesis that this characteristic earthquake
sequence is caused by repeating ruptures of an asperity surrounded by
creeping areas, we compared the rupture area of the 2001 event with that
of the 1995 event by a waveform inversion method. Estimated spatial
extents of the rupture area of the 2001 and 1995 events are almost the
same and are estimated to be about 1.5 x 1.5 km^2. The rupture area of
the 2001 event are mostly overlapped with that of the 1995 event,
although the ruptures of the two events were initiated from different
points respectively. The present observations clearly show that the 1995
and 2001 events are caused by the repeating ruptures of the same asperity
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patch, and support the hypothesis of persistent asperities.
Keyword: earthquake recurrence, repeating earthquakes, subduction
zone, interplate earthquake
1. Introduction
Earthquake recurrence is a basal problem for seismology and earth
sciences as well as long-term earthquake prediction.
Numerical
simulations based on the recent friction laws predict that an isolated
coupled area surrounded by a creep region will slip with a constant
recurrence interval when the creep region slips steadily (e.g. Kato and
Hirasawa [1]). For real earthquakes, however, aperiodicities of
earthquake recurrence have been observed.
For example, Shimazaki
and Nakata [2] concluded that the recurrence of large thrust earthquakes
around Japan followed a time-predictable model rather than a
slip-predictable model or a constant recurrence model.
Characteristic or repeating earthquake sequences along the San
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Andreas Fault have been investigated in detail.
Bakun and McEvilly [3]
compared source parameters of the three Parkfield earthquakes (1922,
1934 and 1966) and attempted to explain the aperiodicity of these
earthquakes.
Small-size repeating earthquake sequences have been
found in Stone Canyon [4], Calaveras fault [5], Parkfield [6], and
northern Hayward fault [7].
These small-size earthquake sequences
were interpreted as repeating ruptures of small isolated patches and,
recently, some studies have attempted to construct physical models of
repeating microearthquake (e.g. [8], [9], and [10]). Vidale et al. [5]
analyzed spectral differences between 18 small repeating earthquakes in
Calaveras fault. They showed that events belonging to a cluster with
longer recurrence interval had smaller source duration.
Nadeau and
McEvilly [11] compared the recurrence intervals of repeating
earthquakes with surface laser ranging data and concluded that episodic
creeps control the recurrence intervals of the small repeating earthquakes.
Many large earthquakes have occurred along the plate boundary east
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off northeastern (NE) Japan, but no large earthquakes have occurred in
the area of N39-40, E142-143 (Fig. 1 (a)). Smaller-size earthquakes,
however, occur very actively in this area as in the other areas (Fig. 1 (b)).
One possible interpretation for this is that only smaller-size coupled areas
that slip seismically during earthquakes sparsely distributed on the plate
boundary there.
We [12] have detected in this area, M4.8 +- 0.1
earthquakes which have occurred regularly since 1957 with a recurrence
interval of 5.52 +- 0.68 yrs at the same location off Kamaishi, Iwate
Prefecture (Fig. 2). We interpreted that this characteristic earthquake
sequence was caused by repeating ruptures of a coupled area with a
dimension of ~1km.
Its very regular occurrence is perhaps due to the
repeating slips of the isolated coupled area surrounded by the stable
sliding area slipping at nearly a constant rate.
Based on this
interpretation we [14] reported in 1999 that the next event was expected
to occur by the end of November 2001 with 99 % probability. An
earthquake with M4.8 actually occurred on November 13, 2001 as
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expected [12].
The coupled area is reflected as a large slip area i.e.,
asperity at the time of each rupture. Therefore, the asperities by the
repeating earthquakes should be consistent with each other, if our
interpretation described above is correct.
In the present paper, we tested
the hypothesis of persistent asperities by comparing slip distributions of
the recent two events based on wave form inversions.
2. Hypocenter relocation
First, we precisely relocated hypocenters of the 1995 and 2000 events,
the initial points of the ruptures．We adopted the homogeneous station
method [15] to avoid the effect of inhomogeneous structure of the crust
and upper mantle caused by the use of different station set.
We used
twelve stations for picking P-wave arrival times and three stations for
S-wave arrival times; the station sets were the same for both of the two
events. We carefully compared P- and S- wave waveforms of the 2001
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event with those of the 1995 event and tried to pick the same phases of
the two events. The seismic velocity model routinely used in the Tohoku
University seismic network [16] was adopted in the calculation of travel
times.
Hypocenter of the 2001 event is located about 200m to the west of the
1995 event. Relative location errors are about 50m in north-south
direction, 70m in east-west direction, and 100m in depth, respectively.
3. Moment tensors
We determined moment tensors of the two events using regional
broad-band seismograms. The method is based on Dreger and
Helmberger [17] and we used the program TDMT-INVC developed by
Dreger in the inversion (see also [18]). Green’s functions are calculated
by using the program FKRPROG by Saikia [19]. The seismic velocity
structure used in the calculation of Green’s function is shown in Table 1.
This is the same as the structure used in Fukuyama et al. [20].
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Waveform data are from the Tohoku University’s broad-band seismic
network. STS-1 or STS-2 type seismograph is deployed at each station.
Waveform data from each station are digitized at a sampling frequency of
100Hz with a wide dynamic range of 20 or 22 bits, and they are
transmitted to the observation center by telephone or satellite telemetry
system. In the present analysis, waveform data thus recorded were
low-pass filtered with a pass-band between 0.02 and 0.05Hz and
resampled at a sampling rate of 1Hz.
Waveform data were from
common three stations (SWU; dist. 68km, baz. 279degrees, HSK; dist.
130km, baz. 338 degrees, HMK; dist. 96 km, baz. 309 degrees) of the
Tohoku university's broad-band seismograph network.
Obtained moment tensors of the two events are almost the same (Fig.
3). They are thrust-type solutions with low-angle westward dipping
planes, which are consistent with the local geometry of the plate
boundary between the subducting Pacific plate and the overlying plate.
Non-double-couple component is quite small (3 to 12 %) compared to
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double-couple component.
Scalar moments of the 1995 and 2001
events are 1.05 and 1.12 x 10^23 dyne.cm, respectively.
4. Source processes
We determined moment release distributions on the fault planes of the
two events by a waveform inversion method [21] based on the concept of
the empirical Green’s function method (e.g., [22], [23], and [24]).
Observed waveforms are inverted to determine the source time function
of each grid distributed on the fault plane.
In the inversion, we used the
multiple time window method [25]. We corrected the difference in rise
time between the target earthquake and a smaller event used as empirical
Green's function and the perturbation of rupture velocity is represented
by the multiple time window method.
Slip direction of the target
earthquake was assumed to be the same as that of the empirical Green's
function event over the whole fault plane.
Relative source time function
for each grid is assumed to be expressed as a sum of isoscales triangles
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with a base of trise. The interval between the triangles having a half of
duration trise. Thus, the unknown parameters are these heights of the
triangles. To stabilize the solution, we add smoothing constraints in the
inversion. The equation to be solved by the inversion is
 A 
b

 x    ,
s  D
 0
where A is the matrix of the Green’s function, b is the observed
seismograms (data), x is the amplitude of relative moment release (i.e.
the heights of the triangles) for each time window of each grid, D is a
first-degree differential operator for the time and space domains, and s is
a constant to weight the smoothing.
Rupture front is assumed to propagate circularly with a velocity of 3.8
km/s (80% of S-wave velocity at the source depth).
resolve the rupture velocity using the variance [23].
We attempted to
The adopted range
of rupture velocity was 2.6 to 4.4 km/s and the rupture velocity of 3.8
km/s gives a comparatively small variance. Amount of relative moment
release with duration of 0.1 sec is determined at each grid point and at
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four time intervals of 0.05s by waveform inversion. Their fault planes
were assumed as a low-angle westward dipping plane of the moment
tensor solutions. We used a grid net of 3km x 3km with 81 points spaced
300m on the fault plane. An event (8/17/2001 12:14 M3.1) located near
the 1995 and 2001 events was selected as the empirical Green's function.
We used the seismic wave velocity model routinely used in the Tohoku
University seismic network [16] for calculating the ray paths.
Waveform data were from the Tohoku University's seismic network.
Figure 4 shows the locations of stations used in the present analysis.
Data used are of short-period seismographs (1Hz) or broad-band (STS-1
or STS-2) seismographs. For the 2001 event, we also used waveform data
from JOF station of JMA and those from OB3 station, which is one of
cabled ocean bottom seismometers (accelerometers) off Kamaishi, of
ERI, Univ. of Tokyo. Waveform data recorded at these stations were
low-pass filtered with a cut off frequency of 5Hz and resampled at a
sampling rate of 20Hz. We used 5 second, three-component (east-west,
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north-south and vertical) seismograms starting 2 second before S wave
arrival.
Figure 5 shows the obtained moment release distribution of the 1995
and 2001 events.
For the 2001 event, spatial extent of the rupture area
is about 1.5 x 1.5 km^2. Moment release distribution has a simple shape
with a single peak. The peak of moment release is located near the
hypocenter, the initial point of the rupture. For the 1995 event, spatial
extent of rupture area is about 1.5 x 1.5 km^2, which is the same as that
of the 2001 event. Moment release distribution has a slightly complicated
shape compared with the 2001 event. The peak of moment release is
located about 300m to the west of the hypocenter. Scalar moments of the
1995 and 2001 events are 0.82 and 1.12 x 10^23 dyne.cm, respectively.
Ratio of seismic moment of the 2001 event to the 1995 event is 1.35.
Observed and synthesized seismograms are shown in Fig. 6 for the 1995
event and in Fig. 7 for the 2001 event.
They are very consistent with
each other.
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Figure 5 (c) shows a comparison of the rupture area of the 2001 event
with that of the 1995 event.
They are almost overlapped with each other,
although their hypocenters are different from each other.
Especially,
spatial extents of the asperities (the areas with larger moment release) are
also same and locations of the peaks of the two moment release
distributions are closely located to each other.
The present observations
clearly show that the 1995 and 2001 events are caused by the repeating
ruptures of the same asperity patch, and that the asperities are persistent
features.
5. Discussion
Source inversions using an EGF are strongly affected by the selection
of the EGF event.
To test the stability of the present source process
inversion, we used a different small event as empirical Green’s function
and did the same procedure.
We selected a small event with a
magnitude of M3.1 that occurred on Oct. 13, 2001. Figure 8 (b) shows
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the result obtained by using this small event as EGF event.
In this case,
rupture areas of the 2001 event and of the 1995 event extend in areas
with about 1 x 1.5 km^2 and they are almost overlapped with each other
as in the case shown in Figure 5 and Fig. 8 (a).
Selection of waveform
data also affects the results of inversions. We also inverted different
sets of data to check the stability of the present source inversion.
We
showed the moment release distributions obtained by using only vertical
component seismograms, and those obtained by using waveforms from
common 5 stations (see Fig. 4) in Figs. 8 (c) and (d), respectively.
Again, these cases, rupture areas of the 2001 event and of the 1995 event
are almost overlapped with each other as in the case shown in Figure 5
and Fig. 8 (a).
The present 'characteristic' earthquake sequence off Kamaishi has
periodically occurred in an average period of 5.6 yrs with a standard
deviation of about 10% of the mean [12]. No earthquakes which size is
equal to or larger than that of the M4.8 event off Kamaishi have occurred
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nearby the M4.8 event and the asperity of the M4.8 event is isolated from
other asperities. The interaction between the asperities sparsely
distributed would be neglible and the recurence interval will be constant.
For example, the 1990 event have occurred at 5.3 yrs after the previous
event (the 1985 event). But the recurrence intervals of the two recent
events fluctuated from the average interval.
The 1995 event occurred at
4.7 yrs after the 1990 event, but, in contrast, the 2001 event occurred at
6.7 yrs after the 1995 event.
We re-examined seismograms of the 1990,
1995, 2001 events to determine their seismic moments, and compared the
seismic moments with their recurrence intervals.
Seismic moments of the 1990, 1995 events relative to that of the 2001
event. Matsuzawa et al. [12] described that the waveform similarities of
these events are quite good up to about 3 Hz. As a simple procedure
made in Bakun and McEvilly [3], we measured maximum peak-to-peak
amplitudes of these low-pass (to 2Hz) filtered seismograms and
calculated the ratios of these amplitudes.
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In the first case, we measured maximum peak-to-peak amplitudes of
vertical seismograms in a length of 30 sec from P-wave arrival.
Each
waveforms are low-pass filtered with a passband of 2 Hz.
These
waveforms of the 1995 and the 2001 events are very similar to each other
as discussed in Matsuzawa et al. [12]. We can measure the maximum
amplitudes of the 1995 and the 2001 events at 14 stations with epicentral
distances ranging from about 80km to 320km. Figure 9 shows relative
amplitudes of the 2001 event to the 1995 event as a function of epicentral
distance. Relative amplitudes are greater than 1.0 at most of stations,
although the data is some what scattered.
The mean and standard
deviation is 1.3 ± 0.2.
For the events before the 1995 event, we cannot measure the
maximum amplitudes due to saturations of the seismograms. This is
because the dynamic range of our telemetry system was low (12 or 16
bits) before the 1995 event occurred. However, fortunately, P-wave first
motions were not saturated with a good S/N at a few stations. Then, in
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the second case, we measured the maximum peak-to-peak amplitudes of
vertical components of P-wave first motions for the 1985, 1990, 1995,
and 2001 events.
Figure 10 (a) and (b) shows examples of waveforms
observed at AOB station. These waveforms were not saturated from
P-wave arrivals (0 sec) to 5 sec. They are very similar to each other.
Figure 10 (c) shows relative amplitudes measured from vertical
components of P-wave arrivals of the 1985, 1990 and 2001 events to that
of the 1995 event.
We could measure peak-to-peak amplitudes of
P-wave first motions at four stations.
The 2001 event has larger
amplitudes, which is same as in the first case.
The mean and standard
deviation is 1.1±0.1．In contrast, the amplitude of the 1990 event at each
station was nearly identical with that of the 1995 event.
The mean and
standard deviation is 1.0±0.1．The amplitude of the 1985 event, which
could be obtained at only one station, was also nearly identical with those
of the 1995 event.
Seismic moments obtained in this study are summarized in Table 2.
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In all cases, the estimated seismic moment of the 2001 event is slightly
larger than those of the 1995 and 1990 events.
The ratio of the moment
of the 2001 event to that of the 1990 event (about 1.2 times) is consistent
with the ratio of the time interval from the previous event of the 2001
event to that of the 1990 event.
This positive correlation between
seismic moment and recurrence interval leads to the relative change in
static stress drop of about 0.2MPa/year (2MPa/decade) by assuming the
spatial extent of the rupture area of the 1990 event is the same as that of
the 2001 event.
This supports the typical values previously reported for
large earthquakes (4MPa/decade; [26]) and for microearthquakes
(2MPa/decade; [27]).
If the slip rate of the aseismic slip in the surrounding creeping area is
constant and the spatial extent of the 1990 event is the same as that of the
2001 event, the slip-predictable model would be suitable for the
interpretation of the aperiodicity of the recurrence of this ‘characteristic’
earthquake sequence off Kamaishi.
In contrast, the ratio of the moment
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of the 1995 event to that of the 1990 event (almost identical) is larger
than the ratio of the time interval from the previous event of the 1995
event to that of the 1990 event (about 0.8 times).
In the interval
between the 1995 and the 1990 events, the 1994 M7.5 Far off Sanriku
earthquake occurred at Dec. 28, 1994, about 150km north from the focal
area of this ‘characteristic’ earthquake sequence.
Based on GPS data
surrounding the focal area of this M7.5 event, Nishimura et al. [28]
revealed the spatio-temporal distribution of the after-slip.
The
estimated after-slip area was extended to the focal area of this
‘characteristic’ earthquake sequence off Kamaishi.
We infer that the
after-slip of the 1994 M7.4 Far off Sanriku earthquake accelerated the
occurrence of the 1995 event and the rate of the aseismic slip in the
surrounding creeping area became larger before the 1995 event.
6. Conclusions
M4.8 +- 0.1 earthquakes have occurred regularly since 1957 with a
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recurrence interval of 5.52 +- 0.68 yrs at the same location off Kamaishi,
Iwate Prefecture, NE Japan.
We interpreted that this peculiar
characteristic earthquake sequence is caused by repeating slips of an
isolated small asperity surrounded by stable sliding area on the plate
boundary.
In order to test whether this interpretation is correct or not,
we compared the rupture area of the latest (2001) event with that of the
previous (1995) event by a waveform inversion method. The result
shows that the asperities by them are almost overlapped with each other,
although their hypocenters are separated by about 200m from each other.
The ratio of the moment of the 2001 event to that of the 1990 event is
consistent with the ratio of the time interval from the previous event of
the 2001 event to that of the 1990 event. In contrast, the ratio of the
moment of the 1995 event to that of the 1990 event is greater than the
ratio of the time intervals from the previous event of the 1995 event to
that of the 1990 event. This might be caused by the after-slip of the 1994
M7.4 Far off Sanriku earthquake, which was expanded significantly to
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the surrounding area. The southern edge of the estimated after-slip area is
located approximately on the focal area of this ‘characteristic’ earthquake
sequence off Kamaishi, which perhaps accelerated the occurrence of the
1995 event.
Acknowledgment:
We used data from JMA, and from the Off-Kamaishi cabled
observation system of ERI, Univ. of Tokyo. We are very grateful to the
staffs for their effort of observations. Moment tensors were computed
using the tdmt-invc package developed by Douglas Dreger of the
Berkeley Seismological Laboratory, and Green's functions were
computed using the FKRPROG software developed by Chandan Saikia
with URS Granger, Woodward Clyde Federal Services. The authors are
grateful to Prof. Y. Tajima for valuable suggestions regarding the moment
tensor inversion, and Prof. K. Farley and two anonymous reviewers for
constructive comments.
We thank Dr. N. Umino, Dr. T. Igarashi and Dr.
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N. Uchida for many valuable discussions. We also thank Mr. T.
Nakayama for his assistance in moment tensor inversion.
References:
[1] N. Kato and T. Hirasawa, A numerical study on seismic coupling
along subduction zones using a laboratory-derived friction law, Phys.
Earth. Planet. Inter., 102 (1997), 51-68.
[2] K. Shimazaki, and T. Nakata, Time-predictable recurrence model
for large earthquakes, Geophys. Res. Lett., 7 (1980), 279-282.
[3] W. Bakun, and T. McEvilly, Recurrence models and Parkfield,
California, earthquakes, J. Geophys. Res., 89 (1984), 3051-3058.
[4] W. L. Ellsworth, Characteristic earthquakes and long-term
earthquake forecasts: implications of central California seismicity, in F. Y.
Cheng and M. S. Sheu (Ed.), Urban Disaster Mitigation: The Role of
Science and Technology, Elsevier, Oxford, 1995.
[5] J. E. Vidale, W. L. Ellsworth, A. Cole, and C. Marone, Variations in
22
rupture process with recurrence interval in a repeated small earthquake,
Nature, 368 (1994), 624-626.
[6] R. M. Nadeau and L. R. Johnson, Seismological studies at
Parkfield V: moment release rates and estimates of source parameters for
small repeating earthquakes, Bull. Seism. Soc. Am., 88 (1998), 790-814.
[7] F. Waldhauser, and W. L. Ellsworth, Fault structure and mechanics
of the Hayward Fault, California, from double-difference earthquake
locations, J. Geophys. Res., 107 (2002), B3, 10.1029/2000JB000084.
[8] C. G.. Sammis and J. R. Rice, Repeating earthquakes as
Low-stress-drop events at a border between locked and creeping fault
patches, Bull. Seism. Soc. Am., 91 (2001), 532-537.
[9] N. M. Beeler, D. L. Lockner, and S. H. Hickman, A simple
stick-slip and creep slip model for repeating earthquakes and its
implication for microearthquakes at Parkfield, Bull. Seism. Soc. Am., 91
(2001), 1797-1804.
[10] L. R. Johnson and R. M. Nadeau, Asperity model of an
23
earthquake: Static problem, Bull. Seism. Soc. Am., 92 (2002), 672-686.
[11] R. M. Nadeau and T. V. McEvilly, Fault slip rated a depth form
recurrence intervals of repeating microearthquakes, Science, 285 (1999),
718-721.
[12] T. Matsuzawa, T. Igarashi, and A. Hasegawa, Characteristic
small-earthquake sequence off Sanriku, northeastern Honshu, Japan,
Geophys. Res. Lett., 29(2002), 1543, doi:10.1029/2001GL014632.
[13] K. Aki, Scaling law of earthquake source time function, Geophys.
J. R. astr. Soc., 31 (1972) 3-25
[14] T. Matsuzawa, T. Igarashi, and A. Hasegawa, Characteristic
small-earthquake sequence off Sanriku, Japan, EOS Trans. AGU,
80(1999), no. 46, F724.
[15] J. H. Ansel and E. G. C. Smith, Detailed structure of a mantle
seismic zone using the Homogeneous Station Method, Nature, 253,
(1975) 518-520.
[16] A. Hasegawa, N. Umino, and A. Takagi, Double-planed structure
24
of the deep seismic zone in the northeastern Japan arc, Tectonophysics,
47 (1978), 43-58.
[17] D. S. Dreger, and D. V. Helmberger, Determination of source
parameters at regional distances with single station or sparse network
data, J. Geophys. Res., 98 (1993), 8107-8125.
[18] T. Okada, T. Yamashita, T. Nakayama, T. Matsuzawa, A.
Hasegawa, and F. Tajima, Determination of moment tensors using
Tohoku University’s broad-band seismic network (in Japanese), Prog.
Abst. 2001 Fall meet. Seism. Soc. Japan (2001a), P105
[19] C. K. Saikia, Modified frequency-wavenumber algorithm for
regional seismograms using Filon’s quadrature:
modeling of Lg waves
in eastern North America, Geophys. J. Int., 118 (1994), 142-158.
[20] E. Fukuyama, M. Ishida, D. S. Dreger, and H. Kawai, Automated
seismic moment tensor determination by using on-line broad band
seismic waveforms (in Japanese with English abstract), Jour. Seism. Soc.
Japan, 2, 51 (1998), 149-156.
25
[21] T. Okada, N. Umino, Y. Ito, T. Matsuzawa, A. Hasegawa and M.
Kamiyama, Source processes of 15 September 1998 M5.0 Sendai, NE
Japan, earthquake and its M3.8 foreshock by waveform inversion, Bull.
Seism. Soc. Am., 91 (2001), 1607-1618.
[22] S. Hartzell, Earthquake aftershocks as Green's functions,
Geophys. Res. Lett., 5 (1978), 1-4.
[23] J. Mori and S. Hartzell, Source inversion of the 1988 Upland,
California, earthquake: Determination of a fault plane for a small event,
Bull. Seism. Soc. Am., 80 (1990), 507-518.
[24] J. B. Flecther, and P. Spudich, Rupture characteristics of the thee
M-4.7 (1992-1994) Parkfield earthquakes, J. Geophys. Res., 103 (1998),
835-854.
[25] S. Hartzell, and T. Heaton,
Inversion of strong ground motion and
teleseismic waveform data for the fault rupture history of the 1979
Imperial Valley, California, earthquake. Bull. Seism. Soc. Am., 73 (1983),
1553-1583.
26
[26] C. Scholz, The mechanics of earthquakes and faulting (1990), 439
p.p., Cambridge Univ. Press, New York.
[27] C. Marone, J. E. Vidale, and W. L. Ellsworth, Fault healing inferred
from time dependent variations in source properties of repeating
earthquakes, Geophys. Res. Lett., 22 (1995), 3095-3098.
[28] T. Nishimura, S. Miura, K. Tachibana, K. Hashimoto, T. Sato, S.
Hori, E. Murakami, T. Kono, K. Nida, M. Mishina, T. Hirasawa, and S.
Miyazaki, Distribution of seismic coupling on the subducting plate
boundary in northeastern Japan inferred from GPS observations,
Tectonophysics, 323 (2000), 217-238
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