京 都 大 学 防 災 研 究 所 年 報 第47号 C 平成16年3月
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 47 C, 2004

Active Response of the Structure beneath Japan due to Static and Dynamic Stress
Changes from Large Earthquakes in 2003

Masatoshi MIYAZAWA* and James J. MORI

* COE Researcher, DPRI, Kyoto University

Synopsis
In order to monitor the crust and upper mantle beneath Japan, we investigate theactive response to large earthquakes in 2003. The earthquake off the coast of Miyagi
prefecture on 26 May (Mw 7.0) triggered local earthquakes in the Tohoku region due tostatic stress perturbations. The 2003 Tokachi-oki earthquake (Mw 8.3) on 26 September
statically triggered the shallow earthquakes in the central Hokkaido and dynamicallydeep low frequency tremor at distances of more than 1000 km. We suggest seismic
approaches using the z-value and statistic E value to illustrate the active response forthe observed waveforms at Hi-net stations.
Keywords: active response, static triggering, dynamic triggering,
2003 Tokachi-oki earthquake

1. Introduction
After the occurrence of large earthquakes,
seismic and volcanic events are often excited bystress changes and/or perturbations in the crust
associated with the events (Harris, 1998). The
excitation includes static and dynamic triggering ofearthquakes, where the threshold for the stresscondition to rupture a fault is exceeded. Permanentstress changes caused by a large earthquake cantrigger nearby earthquakes, which is called “static
triggering”. Transient stress changes from the source
propagate through the crust as seismic waves and areable to remotely trigger events, which is called“dynamic triggering”.
Static triggering is usually observed within a
few fault lengths from the main event, and is
explained by Coulomb failure stress changes ('CFF).
For example, Hashimoto (1995) reported theincreases of seismic activity on the Tanba Plateau
after the 1995 Kobe earthquake and Toda et al.
(2001) argued that the western Tottori-ken

earthquake in 2000 triggered the group of offset
aftershocks, located about 25 km WSW from themain fault.
Investigation of active responses to static stresschanges is restricted to a local area within a few fault
lengths. However, we can study the active responseover all of Japan, with the use of seismic waves fromlarge earthquakes, which cause transient stressperturbations and possible dynamic triggering.
Miyazawa (2002) showed that the dynamic triggeringof volcanic tremor at Aso had a relation to theactivity of the volcano, and the dynamic triggeringreflects the current state of the volcano. However, themeaning of the active response in non-volcanic areas
is not yet clear.
The deformation of the crust has been wellstudied by geodetic methods (e.g., GPS and strain
meters), but the time-scale is generally not shorter
than a few months or a few weeks, with the
exception of large coseismic changes associated withthe large earthquakes. The use of seismicity changes
is another possible method to detect the changes inthe crust (e.g., Habermann, 1983; Reasenberg andSimpson, 1992), however, recognizing changes inseismicity patterns also usually take long observation
periods. This long time-scale makes it difficult to
detect precursory events immediately before large
earthquakes on the time scale of a few days to a few
hours.
If the relationship between the activity in thecrust or upper mantle and the dynamic triggering isclear, as for the case of volcanoes, the monitoring of
the dynamic triggering would provide information
about the present state of the structure, because thedynamic triggering could be detected within a few
hours. We suggest a new seismic method to detect
dynamic triggering which would occur on short timescales by use of continuous seismic waveforms when(a)some large earthquakes occur.
In the present study, we illustrate the triggered
events in the crust and upper mantle beneath Japan
from the Mw 7.0 earthquake off the coast of Miyagi
prefecture on 26 May 2003 (hereafter, called the
Miyagi-oki earthquake) and the 2003 Tokachi-oki
earthquake (Mw 8.3) to compare the static stresschange (the spatial pattern of 'CFF) with thechanges of seismic activity, to evaluate the active
response of the crust. For the 2003 Tokachi-oki
earthquake, the detection of dynamic triggering
makes it possible to monitor the active response overmost of the main part of Japan.

(b)Fig. 1 Z-values estimated from shallow earthquakes (depth d 20 km) of magnitude 0 to5. Rectangular is the fault
plane of the Miyagi-oki event on May 26, 2003. The two time windows used for calculating z-values are March
26 to May 25 and May 26 to July 25, which correspond to the pre- and post-seismic periods, respectively.
Negative values in the bright areas indicate increases of activity and positive values in the dark areas decreases.
Triangles are active volcanoes and dots are the hypocenters of the shallow earthquakes during the post-seismic
period. Contour lines in (a) and (b) show the 'CFF values in MPa calculated for a depth of 10 km, for two
possible types of faults of the triggered seismicity. Both orientations have NS strikes and (a) has eastward
dipping faults and (b) westward dipping, as illustrated by the stereoplots in the upper right of each figure. Stars
are hypocenters of a sequence of moderate earthquakes (MJMA 5.6, 6.2, 5.4) on July 26, 2003.

2. Triggering
earthquake

from

the

2003

Miyagi-oki

We investigate the relationship between static
stress changes due to the 2003 Miyagi-oki
earthquake (Mw 7.0) and changes of seismicity
around the Tohoku region, by evaluating the
Coulomb failure function change ('CFF) and the zvalue.
'CFF is expressed as'W  P ' 'V n

'CFF

(1)

where 'W is the shear stress change across a fault in
the area of interest and takes positive value if theincrease is in the same direction as the slip, P’ is theapparent coefficient of friction including porepressure effects, and 'Vn is the normal stress change
and positive for traction across the fault.
Some fault models of this earthquake have been
determined by the National Research Institute for
Earth Science and Disaster Prevention (NIED), theJapan Meteorological Agency (JMA) and theGeographical
SurveyInstitute
(GSI)
(the
coordinating committee for earthquake prediction
Japan (CCEP), 2004). The models of NIED and JMA
were determined using seismic waveform data andthe GSI model used GPS data. To calculate static
stress changes, it is probably better to use a modelobtained from geodetic data, so we adopt parametersmainly from the GSI model. We assume a simple
rectangular fault plane (19 km u 17 km), where theslip is uniform, and choose the other parameters as
follows. The northeastern corner of the fault is
located at 38.94N, 141.81E, the depth of the top of
the fault is 52 km, strike = 192 deg, dip = 68 deg,rake = 73 deg, and slip = 2.1 m. We also assume P’ =0.4 and the rigidity = 40 GPa. Spatial stress changes
due to the earthquake are calculated by the method of
Okada (1992). The orientation of fault planes on
which the static stress changes are evaluated isassumed to have a NS strike and dip of 45 deg (i.e.
NS-reverse fault) because the Tohoku region is under
EW compression from the subducting Pacific plateand most earthquakes in this region have this type of
focal mechanism. We also tried other fault models,
however the pattern of 'CFF is similar and thedifferences are not significant at far distances.
Changes of seismicity are statistically evaluatedby the z-value (Habermann, 1983),z

m1  m2
2
1

deviation, and suffixes 1 and 2 stand for “preseismic” and “post-seismic”, respectively. Whenseismicity increases and decreases after theoccurrence of the main earthquake, the z-value has
negative and positive values, respectively. In thisstudy we use time-windows of two months before
and after May 26, 2003 when the Miyagi-oki
earthquake occurred. In the pre- and post-seismic
periods, the analyzed earthquakes from JMA data
have magnitudes of M0 to M5 and depths shallower
than 20 km.

(2)

2
2

V / n1  V / n2

where, in an arbitrary time-window, n is the total
number of earthquakes, m is the mean number of
events in the time unit (one day), V is the standard

Fig. 2 Cumulative numbers of shallow earthquakes(depth d 20 km) in AREA1 to 4, as indicated in Fig.
1. The numbers of the earthquakes of M0 to 5 isshown in solid lines and M1-5 in broken lines. May
26 and July 26, when large earthquakes occurred, areindicated by vertical lines.
Figure 1 shows the z-value and the 'CFF for
two possible orientations of the faults of the triggered
seismicity. Both orientations have NS strikes with thefirst type having eastward dipping faults and thesecond type having westward dipping faults. Brightregions indicate areas where post-seismic seismicity
becomes higher than pre-seismic seismicity. We cansee the increases of seismicity in AREA1 and
AREA3, and the cumulative number of earthquakes(Fig. 2) also reflects the increases. We do not use any
de-clustering algorithms in analyzing the backgroundseismicity, because this method could cause loss of
information of the events we are studying. In AREA1,
the values for 'CFF are positive (0.001 to 0.007
MPa) both in Figs. 1a and 1b. In AREA2, the zvalue is almost zero indicating that the seismicity
barely changed in the two months after theearthquake. This region includes hypocenters of asequence of moderate earthquakes (MJMA 5.6, 6.2,
5.4) on July 26, 2003, of which mechanisms are close(a)to supposed one (Fig. 1b). The 'CFF values near the
hypocenters have positive value of about 0.003 to
0.005 MPa in Fig. 1b. However, the high gradient for
the 'CFF values means that small changes of fault
parameters could result in different 'CFF values.
AREA3 includes the locally highest 'CFF value ofabout 0.01 MPa. Around the region with much lower
z-values in the center of AREA3, the 'CFF values
are 0.005 to 0.008 MPa in Fig. 1a and 0.005 to 0.01
MPa in Fig. 1b. AREA4 has positive z-values,
however Fig. 2 indicates that this could be caused by
the events that occurred in early May.
(b)

Fig. 3 Z-value and 'CFF value. Z-values are estimated from shallow earthquakes (depth d 20 km) of magnitude
0 to 5. Rectangular is the fault plane of the 2003 Tokachi-oki earthquake. The pre- and post-seismic periods are
Jul 27 to Sep 25 and Sep 26 to Nov 25. Triangles are active volcanoes and dots are the hypocenters of theshallow earthquakes during the post-seismic period. Contour lines in (a) and (b) show the 'CFF values in MPa
calculated for a depth of 10 km, for two types of faults of the triggered seismicity. The types of faults are (a) EW
right lateral and (b) NS strike with eastward dipping (45 deg), as illustrated by the stereoplots in the upper rightof each figure. Absolute 'CFF values larger than 0.3 MPa are clipped.

3 Detection and illustration of active response toteleseismic waves
When we study the active response to teleseisms,
we can discount the possibility of the static
influences. An effect of 'CFF values is usually
negligible because of small values much less than
0.01 MPa, which is thought to be required for thestatic triggering, by a few orders. Hence, we consider
the possibilities of the dynamic triggering due to theteleseismic waves if the large earthquakes are far
from Japan.
3.1 The 2003 Tokachi-oki earthquake
For this large earthquake of Mw 8.3 in Japan, we
consider both static triggering within a few fault
lengths from the main fault and dynamic triggering in
main part of Japan.
(1) Seismicity changes
Figure 3 shows z-value for shallow earthquakes(depth d 20 km) and 'CFF for a depth of 10 km. To
calculate z-values we use time-windows of 60 days
before and after 26 September when the 2003
Tokachi-oki earthquake occurred. In the pre- andpost-seismic periods, the analyzed earthquakes from
JMA data have magnitudes of M0 to 5 and depths
shallower than 20 km. We adopt the GSI model(CCEP, 2004) to calculate 'CFF values as theprevious section. We assume a simple rectangularfault plane (90 km u 92 km), where the slip is
uniform, the eastern corner of the fault is located at42.05N, 144.64E, the depth of the top of the fault is
15.7 km, strike = 241 deg, dip = 23 deg, rake = 124
deg, and slip = 4.84 m. The other parameters are thesame as in the previous section. 'CFF values arecalculated for the faults with compressional
mechanism due to the subduction of Pacific plate.
Negative z-values (increasing activities) are seen
about 100 km NNW from the main fault, where'CFF values for EW right lateral fault have about 0.1
to 0.3 MPa (Fig. 3a) and those for NS reverse fault
with eastern dipping about 0 to 0.02 MPa (Fig. 3b).
The region includes active volcanoes; Tokachidake
erupted in 1989 and Maruyama in 1898.
At the northern Miyagi regions where thesequence of moderate earthquakes (MJMA 5.6, 6.2,
5.4) on July 26, 2003 occurred, 'CFF values are
scarcely changed while z-values there have large
values because activities of the aftershocks are
decreasing.
(2) Response to dynamic stress perturbations
To detect the active response to seismic waves
from the 2003 Tokachi-oki earthquake, we use
continuous velocity waveform data of Hi-net
provided by NIED to look for high-frequency signals
from possibly triggered small earthquakes. Observedhigh-frequency signals can be associated withsources in the crust or upper mantle in the vicinity ofthe observation station, however, these signals haveto be distinguished from the seismic waves of thelarge mainshock. Generally, this can be done because
at far distances the frequency content of seismic
waves from the mainshock is different due to the lossof the high-frequencies from attenuation. We detect
high frequency signals by the following process[1] Filter the chosen velocity waveforms with afrequency pass-band of 5-20 Hz for each of
three components (UD, NS, EW).
[2] Construct root-mean-square (RMS) envelopes of[1] with one second smoothing.
[3] Calculate the z-value and statistic E value for
amplitudes of the RMS envelopes.

Fig. 4 Hi-net observation stations. 642 stations
analyzed in this study are indicated by crosses. In Fig.
6, RMS envelopes for the 7 labeled stations areshown.
Figure 4 shows the 642 observation stations
used in this study. The RMS envelope constructedfrom three components of velocity waveforms,
filtered with a pass-band of 5-20 Hz at station OOZ,
is shown in Figs. 5 and 6. The station OOZ is locatedin western Shikoku, and the epicentral distance isabout 1400 km. To illustrate that this envelope
includes local events near the station and mainly
excludes teleseismic waves, Fig. 5 compares theRMS envelope waveforms filtered with different
pass-bands of 0.2-2, 2-5 and 5-20 Hz. The RMS
envelope of 0.2-2 Hz shows both body waves
followed by surface waves from the Tokachi-oki
earthquake, while the envelope for 2-5 Hz shows
mostly body waves (200-500 sec). Higher frequency
components should include only direct body waves,because surface waves have longer period

components and scattered waves of the body waves
in the coda are largely attenuated. However, the 5-20
Hz envelope includes energy around 400 sec, whichis thought to be associated with local triggered events.
These are also seen in the waveform filtered between
2-5 Hz.

Fig. 5 RMS envelope waveforms filtered with passbands of 0.2-2, 2-5 and 5-20 Hz, at OOZ. Zero is theorigin time of the 2003 Tokachi-oki earthquake(4:50’06 Sep 26, 2003 (JST)).

To statistically see changes of the envelope
waveforms (5-20 Hz) we apply the z-value obtainedby Eq. (2) for the amplitude of envelope waveforms
instead of the number of events. The pre-seismic
window is the 1000 sec up to the time ~200 sec
before the arrival of the P wave. The post-seismic
window is 1000 sec from ~500 sec after the arrival of
the P wave (Fig. 6). In the previous section we
calculated the z-values with a time unit of one day
and here we use one second. Figure 7a illustrates theresults of the z-value analysis for all of Japan. Low(negative) z-values are found around the Hokkaido
and Tohoku regions, and are especially low on the
Pacific side, indicating that aftershocks are includedin the envelopes. The differences between the eastern
and western regions of northern Honshu are due to awell-known difference in attenuation properties. Also
the envelope from KAK may include continuing
aftershocks of the events on 26 July. The western
Shikoku, Kii peninsula, and the Tokai regions also
have low z-values and observe swam-like tremor (Fig.
6). The tremor seems to have begun after the arrivalsof the direct body waves and gradually disappeared
in a few thousands of seconds. HIN is located in thevicinity of the fault of the western Tottori-ken
earthquake in 2000, and observes one event in the
post-seismic window.
Some positive and negative changes observed atonly one station are sometimes local earthquakes or
effects of the station due to noise. For example, astation KAM near the Yamasaki-fault shows a large
negative z-value, while the surrounding stations
show values of close to zero. Careful examination of
the waveforms indicates that this was due to somenoises after 5:00. We could not detect what are thesources of the noise, however this would not be
related with the site effects because of no similar
observations even in cases of the arrivals of the otherteleseismic waves.
To reduce the influence of the aftershocks of the2003 Tokachi-oki earthquake, we apply the statistic E
value defined by Matthews and Reasenberg (1988) asE

Fig. 6 RMS envelopes constructed from threecomponent waveforms filtered with a pass-band of 520 Hz at HNK, KAK ASU, TKW, OOZ, HIN and
KAM. The locations of the stations are shown in Fig.
4. Horizontal bars show the 1000 sec time windows
for the pre- and post-seismic periods used for
calculating z-values and statistic E values. ' ishypocentral distance.

n2  E (n2 ) / V 2(3)

where n2 is the total number of earthquakes in the
post-seismic term, E(n2) is the expected value
estimated from the activity in the pre-seismic period,V is the standard deviation of the post-seismicity. A
positive value indicates an increase of activity, and anegative value indicates a decrease. We use theamplitude of the envelope waveform as the numberof events and obtain the statistic E value for theobserved waves.
The statistic E value is based on a zero
hypothesis that post-seismic activity is equal to thebackground with Poisson process, providing low
absolute values in the regions influenced by theaftershocks because of large standard deviations.

Then the activity changes with small deviation aremuch cleared. We note that the statistic E value isvery sensitive to noise perturbations. Hence, eventhough increasing activity (positive statistic E values)
are dominantly found in Shikoku, Kii and Tokai (Fig.
7b), which is consistent with the negative z-values inthese areas, we can not conclude whether the effect isdue to natural events or noises.
For the three regions that show the high statistic
E values and negative z-values, we calculate thecorrelation coefficients of envelope waveforms
between the observation stations and a reference
station in each region. The reference stations are
OOZ, TKW and ASU, where the statistic E values
are high and the z-values low compared withsurrounding stations’. Contours in Figs. 7a and b
show that similar envelope waveforms are observed
over a wide area, therefore the increases of activity inthose areas are not caused by local noises. Supposing
a spherical and/or a cylindrical propagation of thesignals (i.e. geometrical spreading), we can estimate
the souses are approximately within a few tens ofkilometers to 100 km from each reference stations. If
the sources are in shallow region, much large
increases of activity should be observed in contoured
regions, which, however, are not found. Then they
are caused by deep sources but not in shallow region.
(a)

Excitation of the tremor is especially strong in
Shikoku among the three regions. The epicentral
distance is more than 1400 km and the direct bodywaves are large attenuated and cannot be dominantly
observed in very high frequency range. The
excitation of the tremor appears to start from about
400 sec after the origin time of the earthquake, whichis consistent with travel-times of S-waves and/or
surface waves, although some similar tremor is alsointermittently observed before that time (Figs. 5 and
6). It is difficult to determine the exact beginning of
the excitation and whether it coincides with arrivalsof the direct waves or later.
Figure 8 shows the hypocenters of earthquakes
and deep low frequency (DLF) events in westernJapan. The DLF events (M0 to 2) occur at depths ofabout 30 to 60 km on the subducting plate and havedominant frequencies of 1-10 Hz, while the ordinary
earthquakes with the similar magnitudes havedominant frequencies of 10-20 Hz (Obara, 2003).
Obara proposed that the DLF events are caused by
fluid flow. The areas of small z-value and the large
statistic E value seem to be coincident with the areas
where DLF events are observed, suggesting thepossibility that the swam-like tremor is excitation of
DLF events.

(b)

Fig. 7 Results showing the seismic amplitudes (5-20Hz) following the 2003 Tokachi-oki earthquake in Japanestimated by (a) z-value and (b) statistic E value. The white areas show where the mean amplitude of the RMS
envelope after the arrivals of teleseismic waves is larger than before. Clipped values less than -0.01 in (a) are
shown in broken lines. We contour with solid lines the regions where the RMS envelope waveforms havecorrelation coefficients larger than 0.5, with respect to reference stations. The reference stations are OOZ, TKW
and ASU (see Fig. 4).

Fig. 8 Hypocenters of earthquakes and DLF events
during 2003 (Jan to Dec) in western Japan. Dots areearthquakes and gray circles DLF events (M0 to 2).

3.2 Other teleseismic events
We investigate the active response to teleseismic
waves from other earthquakes in 2003. We choose
teleseisms, which occurred at night or in the earlymorning in Japan, in order to reduce cultural noise.
Also, Hi-net records bore-hole data with generally
low noise levels. We chose events with epicentral

distances less than 8000 km and moment magnitudes(Mw) larger than about 7.0. We select nine
earthquakes from the National Earthquake
Information Center, U.S. Geological Survey, which
are listed in Table 1, including the 2003 Miyagi-oki
earthquake and the 2003 Tokachi-oki earthquake.
The RMS envelopes at OOZ are shown in Fig. 9.
The swam-like tremor is not always observed in
the Shikoku, Kii, and Tokai regions following theseearthquakes, confirming that the inferred activity
following the 2003 Tokachi-oki event cannot be
attributed to stable site effects.
In Fig. 10 the DLF events in 2003 near Shikoku
increase at the end of August around Bungo Straitand early in September around western Shikoku.
Excitations of the tremor from the distantearthquakes were observed on September 26 and 27
in the Shikoku, Kii and Tokai regions, however,there was little change of earthquake activity. The
excitation of tremor on September 27 starts from1000 sec after the origin time and not at arrivals of
the faster body waves.

Fig. 9 RMS envelopes
constructed from threecomponent
waveforms
filtered with a pass-bandof 5-20 Hz at OOZ. Zeroindicates the origin timeof earthquakes listed inTable 1. Horizontal bars
show the 1000 sec timewindows
usedforcalculating the statistic E
value and z-value. The
gray arrows indicate
arrival times of the direct
P-wavesfromthecorresponding
distant
earthquake. Vertical scale,velocity, is common. For
each event the epicentral
distance ', statistic E
value and z-value areindicated on the right.

Origin time anddate (JST)Mar 18 01:36’17
May 26 18:24’33
May 27 04:23’28
Jun 23 21:12’34
Jul 16 05:27’51
Sep 26 04:50’06
Sep 27 20:33’25
Dec 06 06:26’09
Dec 28 01:00’59

Table 1 Teleseisms and large earthquakes in 2003
LatitudeLongitude
Depth
Mw
Regional Location(km)
51.272 N
177.978 E
33 7.0 Rat Islands, Aleutian Islands, Alaska.
38.849 N
141.568 E
68 7.0 Off coast of Miyagi prefecture, Japan.
2.354 N
128.855 E
31 7.0 Halmahera, Indonesia.
51.439 N
176.783 E
20 6.9 Rat Islands, Aleutian Islands, Alaska.
2.598 S
68.382 E
10 7.6 Carlsberg ridge.
41.815 N
143.910 E
27 8.3 Tokachi-oki, Japan.
50.038 N
87.813 E
16 7.3 Southwestern Siberia, Russia.
55.538 N
165.780 E
10 6.7 Komandorskiye Ostrova, Russia region.
22.015 S
169.766 E
10 7.3 Southeast of the Loyalty Islands.

Fig. 10 Cumulative numbers of DLF events (M0-2)
and earthquakes (M0 to 5) in the three areas of
Shikoku, Kii and Tokai indicated in Fig. 8. The datesof occurrence of teleseisms in Table 1 are shown by
the arrows.
4 Discussion and Conclusions
4.1 Triggered earthquakes in the Tohoku region
due to the 2003 Miyagi-oki earthquake
In the region west of the earthquake where there
was considerable seismic activity following themainshock (AREA1 and AREA3 of Fig. 1) there
were positive but small 'CFF values (< 0.01 MPa),
and it may be difficult to say that the events in thisregion were statically triggered. It is thought thatstatic triggering requires changes of 0.01-0.1 MPa or
greater. For these two regions, there is the possibilityof dynamic triggering by the waves from the Miyagioki earthquake. The dynamic stress perturbations in
AREA1 are order of 10-1 MPa, which would bethought to be much plausible reason than the static
stress changes. The dynamic triggering does not

involve a time-delay to excitation (Belardinelli et al.,2003) and the seismicity could increase immediately
after the Miyagi-oki earthquake. Also, the areas
include active volcanoes, which are susceptible todynamic triggering, as reported by Hill et al. (1993)
and Miyazawa (2002).
In and near the volcanic regions the mechanismsof earthquakes, for which 'CFF is evaluated, are notalways consistent with the supposed types. However
it was difficult for 'CFF to exceed 0.01 MPa in theseregions, even if various mechanisms are chosen.
There was stable seismicity (no increases) in
AREA2 to the southwest for two months following
the mainshock, in an area where there were increases
in the 'CFF values, and then a sequence of moderate
earthquakes occurred on July 26. If the values had
major influence on the occurrences, this might havebeen an example of a delayed triggering due to static
stress changes (Roy and Marone, 1996; Belardinelli
et al., 2003). We actually provided information about
this one week before the northern Miyagi
earthquakes in July. Even if we could confirm thatthe triggering after May 26 is due to static stresschanges and that time-delayed earthquakes mightoccur in areas of large 'CFF values but low triggered
seismicity, it is still difficult to predict the magnitudeof the events (Dieterich, 1994).
In AREA1-3, 'CFF values were small for thestatic triggering, however the increases of seismicity
were observed in the western part of mainshock,where the 'CFF values were positive. Or our results
may show the possibility of the static triggering evenwith the order of 10-3 MPa one order smaller than it
has ever been thought, which is still larger than theearth tide by 1-2 orders.
4.2 Seismicity after the 2003 Tokachi-oki
earthquake
Seismic activity increased around volcanic
regions in the center area of Hokkaido after the 2003
Tokachi-oki earthquake. These events would betriggered, however there are some possible reasons.
One is the static triggering, because 'CFF values
were larger than 0.1 MPa, if the triggered
earthquakes had mechanisms being close to supposed

NW-SE compression. We only consider the staticchanges due to the main event, however we cannot
discount the coseismic slip with a large magnitude (>Mw 7.5) in one month after the mainshock. Also
there is the possibility of dynamic triggering due tothe seismic waves.
4.3 Dynamic triggering by the 2003 Tokachi-oki
earthquake
Some observed active responses to the seismic
waves of the Tokachi-oki mainshock could be
attributed to dynamic triggering of small earthquakes.
Stations KAK and HIN showed large seismic
amplitudes following the mainshock, however thismay be due to coincidence and not a result ofdynamic triggering. Both stations are close to theaftershock areas of recent earthquakes. KAK is closethe 2003 Northern Miyagi sequence and HIN is close
to the 2000 western Tottori earthquake (Mw 6.6). In
areas of recent aftershocks, where the seismicity rate
is relatively high, it is likely that small earthquakes
occurred by chance during the 1000 seconds
following the Tokachi-oki mainshock.
However, the excitation of the tremor observedon September 26 and 27 around the Shikoku, Kii,
and Tokai regions is thought to be dynamic
triggering due to seismic waves, probably S-waves
and/or surface waves. The evidences from ouranalyses are as follows;[1] The sources of the tremor are not in theimmediate vicinity of one station but occur insome deeper regions around the slab, as inferredfrom the similar shape of RMS envelopes atdifferent stations over wide area.
[2] Observed waves of this frequency range (5-20
Hz) are generally attributed to DLF events.
[3] The excitation appears to begin after arrivals ofseismic waves with apparent velocities of 3-4
km/s, which are S-waves and surface waves.
Ukawa et al. (2002) argued that surface waves
from shallow earthquakes tended to trigger local
earthquakes in volcanic regions. Voisin (2002)
numerically showed that, not only large amplitudes,
but also low frequencies, such as surface waves, werelikely to exceed the threshold for starting a rupture.
Another possibility is that the triggering is caused byyield stress drops to the initial stress (Kilb et al.,2000), by cyclic fatigue on the fault (Gomberg et al.,2001). The stress perturbation at OOZ from the 2003
Tokachi-oki earthquake was on the order of 10-3 MPa,
which would be too small to directly exceed thethreshold for triggering events for each pulse of theseismic waves. DLF events are caused by fluid flows(Obara, 2002) and the triggering mechanism may bedifferent from ordinary earthquakes, however they
may be strongly influenced by the long period Sand/or surface waves. The faster body waves, Pwaves, seem not to trigger the tremor, because thedynamic triggering involves instantaneous failures

without a significant time delay (Belardinelli et al.,2003) and the excitation did not start immediatelyfrom the arrivals of those waves. Once triggered thetremor it seems to continue even without enoughexternal dynamic perturbations of stress.
The high activity of DLF events for the active
response under Shikoku (Fig. 10) is plausibly
explained if they can be easily excited by the 2003
Tokachi-oki earthquake and later teleseisms. Also theactivity seems to be related to the slow slip event(Mw 6.9) beneath Bungo Strait, west of Shikoku,
which occurred from June to November in 2003.
This suggests that dynamic triggering is related to not
only high seismicity but also activity in the
subduction zone. However, the excitation is also
observed in other regions, Kii and Tokai, so therelationship between the dynamic triggering and theactivity of the subduction zone is not well understood.
We, however, note the excitation at Shikoku issignificant.
The observation of the excited tremor only in
September may be due to the large size of the waves
from the 2003 Tokachi-oki earthquake. The seismic
waves have the largest amplitudes and long period
components that might be enough to trigger the
events. The tremor sources may then become
unstable and very sensitive to external perturbations
for a certain time because of the fluid flow. Hencethe next teleseism could trigger the same events evenwith small perturbations.
The mapping of z-values and statistic E valuesof observed envelopes following large earthquakes
may provide a way to monitor the activity of the crustand upper mantle including the subduction zone, inregions where large earthquakes may occur in the
future, although application of this monitoring toactive faults is not presently possible, because ofpoor coverage of observation stations in local scale.
Challenges for the future include the investigation ofthemechanismsofdynamic
triggering,understanding the relationship between dynamic
triggering and the activity of the crust and thesubduction zone, resolution of the source depth of theactive responses, and the separation of the active
response from other events, such as random
coincidences and noise.
Acknowledgments
This research was supported by the Japanese
Ministry of Education, Culture, Sports, Science andTechnology (MEXT) 21st Century COE Program for
DPRI, Kyoto University (No.14219301, Program
Leader: Prof. Yoshiaki Kawata). We used hypocenter
data processed by the Japan Meteorological Agency(JMA) and the networks supported by MEXT. We
used Hi-net data from the National Research Institutefor Earth Science and Disaster Prevention (NIED).

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ⷐ ᣦ
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COE-10ᣣᧄ೉ፉਅߦ߅ߌࠆᵴേ⊛࿾Ზߩ㆙࿾࿾㔡ᵄࠍ↪޿ߚ࿾㔡ቇ⊛ࡕ࠾࠲࡝ࡦࠣ                 ‫ޚ‬ችỈ ℂ⒤࡮James J. Mori㧝㧚ߪߓ߼ߦ
ㄭᐕᓸዊ࿾㔡᷹ⷰ✂߇ో࿖⊛ߦᢛ஻ߐࠇ‫ޔ‬ࠗࡦ࠲࡯ࡀ࠶࠻ࠍ⚻↱ߒߡ࿾㔡ᵄᒻ⸥㍳߇හᤨߦᓧࠄࠇࠆࠃ߁ߦߥߞߡ߈ߚ‫⸥ߩࠄࠇߎޕ‬㍳ࠍ↪޿ߡ‫ޔ‬࿾㔡ᵄߦኻߔࠆ᷹ⷰὐㄭறߩ᭴
ㅧߩ⢻േ⊛ᔕ╵ࠍ⺞ߴࠆߎߣߢ‫ޔ‬ᣣᧄ೉ፉ⋥ਅߩ࿾Ზߣ਄ㇱࡑࡦ࠻࡞ߩᵴേࠍḰ࡝ࠕ࡞࠲ࠗࡓߦࡕ࠾࠲࡝ࡦࠣߔࠆߎߣ߇ߢ߈ࠆ‫ޕ‬㨪
10Hz ߦ⷗ࠄࠇࠆᵴേ߇᷹ⷰน⢻ߢ޽ࠅ‫ޔ‬GPS ╬ߩ᷹࿾ቇ⊛ᚻᴺߦᲧߴචಽ⍴޿ᤨ㑆ࠬࠤ࡯࡞ߢ޽ࠆ‫ߥ߁ࠃߩߎޕ‬ᚻᴺߪᚗ޿ߪ࿾㔡ᵄߦኻߔࠆ࿾㔡ߩ࠳ࠗ࠽ࡒ࠶ࠢ࠻࡝ࠟ࡝ࡦࠣ
㧔േ⊛⺃⊒㧕ࠍ⺞ߴࠆߎߣߢ߽޽ࠆ‫࠽ࠗ࠳ޕ‬ࡒ࠶ࠢ࠻࡝ࠟ࡝ࡦࠣߣߪኻᭂ⊛ߦ‫ޔ‬๟ᵄᢙߩ࠯ࡠẋㄭߦࠃࠆ⢻േ⊛ᔕ╵ߣߒߡ‫࠶ࠖ࠹ࠬޔ‬ࠢ࠻࡝ࠟ࡝ࡦࠣ㧔㕒⊛⺃⊒㧕߇޽ࠆ‫ࠍࠇߎޕ‬⺞ߴࠆ㓙ߪㆃᑧᤨೞࠍ⠨ᘦߔࠆᔅⷐ߇޽ࠆߩ
ߢ‫ޔ‬᳇⽎ᐡ৻రൻ࿾㔡ࠞ࠲ࡠࠣࠍ↪޿ߡ࿾㔡ᵴേࠍਛᔃߦ⸃ᨆߔࠆ‫⽎⃻⊒⺃ߥ߁ࠃߩߎޕ‬ߪ࿾Ზߩᵴേ⁁ᘒࠍ෻ᤋߒߡ޿ࠆߣ⠨߃ࠄࠇࠆߩߢ‫ߦ߼ߚߩࠣࡦ࡝࠲࠾ࡕޔ‬㊀ⷐߢ޽ࠆ‫ޕ‬ᧄ⎇ⓥߢߪ 2003 ᐕߩ⸃ᨆ⚿ᨐࠍႎ๔ߔࠆ‫ޕ‬㧞㧚㆙࿾࿾㔡ᵄߦኻߔࠆേ⊛ᔕ╵ߩᬌ಴࿾Ზߣ਄ㇱࡑࡦ࠻࡞ߩ⢻േ⊛ᔕ╵ࠍ⺞ߴࠆߚ߼ߦ‫ޔ‬㆙࿾࿾㔡ᵄߩ౉኿ࠍ઒ቯߔࠆ‫ߎߎޕ‬ߦ฽߹ࠇࠆ⍴๟ᦼߩᵄߪવ᠞ㆊ⒟ߢᷫ⴮ߔࠆ
ߚ߼ߦ‫ޔ‬ਥߦ㐳๟ᦼߩᵄߩ౉኿߇ᦼᓙߢ߈ࠆ
ߩߢ‫ⷰޔ‬᷹ᵄᒻߦࡂࠗࡄࠬࡈࠖ࡞࠲࡯ࠍᣉߖ
߫‫ޔ‬㆙࿾࿾㔡ᵄߢߪߥ޿᷹ⷰὐㄭற߆ࠄユ኿ߐࠇߚᵄߩߺࠍ᛽಴ߔࠆߎߣ߇ߢ߈ࠆ‫ޕ‬㆙࿾࿾㔡ᵄ೔㆐೨ᓟߦ߅ߌࠆㅦᐲᵄᒻߩ RMS ࠛࡦࡌࡠ࡯ࡊߩᝄ᏷ࠍ⛔⸘⊛ߦᲧセߔࠆߎߣߢ‫ޔ‬⢻േ⊛ᔕ╵ࠍ⹏ଔߔࠆ‫ޕ‬㒐ἴ⑼ቇᛛⴚ⎇ⓥᚲ
ࠃࠅᢛ஻ឭଏߐࠇߡ޿ࠆ Hi-net ߩ‫ޔ‬700 ὐㄭߊߩ᷹ⷰὐߢߩਃᚑಽㅪ⛯ㅦᐲᵄᒻ⸥㍳ࠍ೑↪ߔࠆ‫⹏⊛⸘⛔ޕ‬ଔߩᜰᮡߣߒߡ‫ⷰޔ‬᷹ߐࠇ
ߚㅦᐲ⸥㍳ߦ 5-20Hz ߩࡃࡦ࠼ࡄࠬࡈࠖ࡞࠲
ࠍᣉߒߚ RMS ࠛࡦࡌࡠ࡯ࡊᵄᒻߩ‫ޔ‬࿾㔡ᵄ೔㆐ᤨ೨ᓟ 1000 ⑽ߩᝄ᏷ߦኻߒ‫ޔ‬z ୯෸߮
statistic Ǫࠍ⸘▚ߔࠆ‫ޕ‬z ୯߇ዊߐ޿(⽶୯)߶
ߤ‫ޔ‬ᚗ޿ߪ statistic Ǫ߇ᄢ߈޿(ᱜ୯)߶ߤ‫ޔ‬࿾㔡ᵄ೔㆐ᓟߩࠛࡦࡌࡠ࡯ࡊᝄ᏷߇ᄢ߈ߊߥ
ࠆ‫ޕ‬೨⠪ߪࡄ࡞ࠬ⊛ߥᵄᒻ‫ޔ‬ᓟ⠪ߪᢿ⛯⊛ߥᵄᒻߩᬌ಴߇น⢻ߢ޽ࠆ‫ޕ‬Hi-net ߩ⸥㍳ߩᓧࠄࠇࠆߔߴߡߩ᷹ⷰὐߦߟ޿ߡ‫ߩࠄࠇߎޔ‬୯ࠍ᳞߼ࠆߎߣߢ‫ޔ‬ർᶏ㆏߆ࠄ㣮ఽፉ߹ߢߩᣣ
ᧄ೉ፉߦ߅ߌࠆ⢻േ⊛ᚗ޿ߪฃേ⊛ᔕ╵ߩ᭽ሶࠍ␜ߔߎߣ߇ߢ߈ࠆ‫ޕ‬
 2003 ᐕචൎᴒ࿾㔡ߦ઻߁࿾㔡ᵄࠍ↪޿ߚ⸃ᨆࠍⴕߞߚߣߎࠈ‫ޔ‬ർᶏ㆏߆ࠄ᧲ർ࿾ᣇߩ
ᄥᐔᵗ஥ߦ z ୯ߩዊߐߥ㗔ၞ߇⹺߼ࠄࠇߚ‫ޕ‬ߎࠇߪ⇣Ᏹ㔡ၞߦᴪߞߚ㗔ၞߢ‫ޔ‬૛㔡߇ࠃߊ᷹ⷰߐࠇߚߎߣࠍᗧ๧ߔࠆ‫ޕ‬statistic Ǫߪ㔡ᄩ߆ࠄ 1000 km એ਄߽㔌ࠇߚ᧲ᶏ࿾ᣇ‫♿ޔ‬દඨፉ‫ޔ‬྾࿖ߦᄢ߈ߥ୯߇ᓧࠄࠇߚ‫ⷰޕ‬᷹ὐ㑆ߢ᷹ⷰᵄᒻߩ⋧㑐ࠍขࠆߎߣߢ‫ޔ‬ዪᚲ⊛ߥᵴേ߿ࡁࠗ࠭ߢߥ޿ߎߣ߇ಽ߆ߞߚ‫ࠄࠇߎޕ‬ߪᷓߐ 30-60 km ⒟ᐲߩࡊ࡟࡯࠻Ⴚ⇇ઃㄭߢ⊒↢ߔࠆᷓㇱૐ๟ᵄᓸേߢ޽ࠆߣ⠨߃ࠄࠇࠆ‫ޕ‬ߟ߹ࠅචൎᴒ࿾㔡ߦࠃߞߡᷓㇱૐ๟ᵄᓸേ߇
⺃⊒ߐࠇߚน⢻ᕈ߇޽ࠆ‫ޕ‬㧟㧚㕒⊛ᔕജ႐ᄌൻߦኻߔࠆ࿾㔡ᵴേ5 ᦬ 26 ᣣߩችၔ⋵ᴒߩ࿾㔡(MJMA7.1)೨ᓟ 2
ࡩ᦬㑆ߩ‫ޔ‬ห࿾ᣇߩ࿾㔡ᵴേᐲߩᄌൻࠍ z ୯ߣߒߡ⸘▚ߒߚ‫ޕ‬ౝ㒽ㇱߩᷓߐ 20 km એ਄ߩᵻㇱߢ᣿⍎ߥ࿾㔡ᵴേߩჇട߇⹺߼ࠄࠇߚ‫ޕ‬ߎߩ࿾ၞߪችၔ⋵ᴒߩ࿾㔡ߦኻߔࠆǍCFF(ૉߒੑᰴ⊛㔡Ḯࠍධർᣇะߦ⿛ⴕࠍᜬߟㅒᢿጀߣ઒ቯ)ߣᱜߩ⋧㑐߇޽ࠆ߇‫'߿߿ޔ‬CFF ୯߇ዊߐ޿ߩߢ࠳ࠗ࠽ࡒ࠶ࠢ࠻࡝ࠟ࡝ࡦࠣߩน⢻ᕈ߽޽ࠆ‫ޕ‬ǍCFF ߇ࠃࠅᄢ߈ߥ୯ࠍᜬߟߦ߽㑐ࠊࠄߕ‫ޔ‬࿾㔡ߩᵴേߩჇട߇ᱴߤ⹺߼ࠄࠇߥ߆ߞߚ࿾
ၞ߇޽ࠆ‫ ߪࠇߎޕ‬2 ࡩ᦬ᓟߩ 7 ᦬ 26 ᣣߦችၔ⋵ർㇱߢ‫৻ޔ‬ㅪߩ࿾㔡ᵴേ(M JMA 5.6, 6.2,
5.4)߇޽ߞߚ࿾ၞࠍ฽߻‫࡝࠻ࠢ࠶ࠖ࠹࠲ࠬޕ‬ࠟ࡝ࡦࠣߪ࠳ࠗ࠽ࡒ࠶ࠢ࠻࡝ࠟ࡝ࡦࠣߣߪ⇣ߥࠅ‫ޔ‬⎕უㆃᑧࠍ᦭ߔࠆߎߣ߇޽ࠆ‫ޕ‬ᓥߞߡ‫ޔ‬ߎࠇࠄߩ৻ㅪߩᵴേߪ 5 ᦬ 26 ᣣߩ࿾㔡ߦࠃߞߡ㕒⊛ߦ⺃⊒ߐࠇߚน⢻ᕈ߇޽ࠆ‫ޕ‬⻢ㄉ㧦㔡Ḯ࠺࡯࠲ߦߟ޿ߡ‫ޔ‬᳇⽎ᐡ৻రൻ࿾
㔡ࠞ࠲ࡠࠣࠍ૶↪ߒ߹ߒߚ‫)⁛ޕ‬㒐ἴ⑼ቇᛛⴚ
⎇ⓥᚲߩឭଏߔࠆ Hi-net ߩ⸥㍳ࠍ૶↪ߒ߹ߒߚ‫ޕ‬