13th World Conference on Earthquake Engineering
Vancouver, B.C., Canada
August 1-6, 2004
Paper No. 835

CHARACTERIZATION OF SOURCE MODELS OF SHALLOWINTRASLAB EARTHQUAKES USING STRONG MOTION DATA
Kimiyuki ASANO1, Tomotaka IWATA2, and Kojiro IRIKURA3SUMMARY
Several differences of observed ground motion characteristics between crustal and intraslab earthquakes
seem to be caused by not only propagation-path but also source characteristics. We studied thecharacterization of source models for shallow intraslab earthquakes based on strong motion simulations.
Appropriate source models of several shallow intraslab earthquakes (MW 5.2 - 7.0) that occurred in Japan
were constructed to explain the broadband ground motion based on strong motion simulations using theempirical Green's function method. Our source model is expressed by one or several rectangular regions
called strong motion generation area on an assumed fault plane, which can be thought to be equivalent tothe asperities of heterogeneous source model by previous studies. We summarized the results and foundthat the strong motion generation area of the source model for each shallow intraslab earthquake issmaller than the prediction by the empirical relationship for inland crustal earthquakes proposed bySomerville et al. (1999). This means that the stress drop on the asperity or the strong motion generation
area of a shallow intraslab earthquake is higher than that of an inland crustal earthquake. Among shallow
intraslab earthquakes, relatively deeper event with the source depth of about 70 – 100 km has very highstress drop, that is more than 100 MPa, though shallower events with the source depth of about 30 km has
relatively low stress drop, that is almost same or slightly higher than inland crustal events. Our result
suggests that the stress drop on the asperity of shallow intraslab earthquakes depends on its depth. When
we assume the value of the stress drop in case of the strong motion prediction for an intraslab earthquake,
we have to consider include the focal-depth dependence of the stress drop.
INTRODUCTION
Shallow intraslab earthquakes occurring in subducting slab bring strong ground motion on surface despite
its relatively deeper focal depths compared with inland crustal earthquakes. For engineering purpose,
empirical attenuation relationships for peak ground acceleration and velocity have been developed
considering the difference in the type of earthquake source and the focal depth by Si and Midorikawa [1].
According to their attenuation relationships, the peak ground motion from an intraslab event is larger thanthat from an interplate event. Such a large ground motion will be caused by not only the low-attenuation(high Q-value) along the propagation path but also the high stress drop on the source. Compared to
1Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan.
Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan.
3
Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan.
2

shallow crustal earthquakes occurring at inland regions and interplate earthquakes occurring at subducting
plate boundary regions, the study on source characteristics of shallow intraslab earthquakes in the view ofstrong motion seismology are not sufficient because the number of observed events seems to be smaller
than other types of earthquakes and the place of large intraslab earthquake occurrences is believed to belimited. These intraslab earthquakes are thought to be related to the dehydration of hydrated oceanic crustand serpentinized mantle [2].
In recent years, Japan has been suffered by two large shallow intraslab earthquakes, which were the 2001
Geiyo earthquake and the 2003 off Miyagi earthquake. The strong motion waveforms from theseearthquakes have been obtained through the dense Japanese strong motion networks, the K-NET [3] andthe KiK-net [4], operated by the National Research Institute for Earth Science and Disaster Prevention(NIED). In this paper, we call the intraslab earthquake occurring at the depth of about 30-100 km theshallow intraslab earthquake. The detailed source processes of large earthquakes mentioned above havebeen obtained by several authors through the kinematic waveform inversion using the strong motion data
[5-8]. However, the frequency range for these analyses is restricted to lower than about 1Hz because of
the accuracy in the underground structure information to calculate Green’s functions. To explain therealistic strong ground motions from shallow intraslab earthquakes quantitatively, it is needed to takemore wide-range including high-frequency components of ground motions in consideration. One of theeffective methods to simulate broadband ground motions is employment of observed aftershock records
as the empirical Green’s functions. Irikura [9] has developed the methodology to simulate strong groundmotions using empirical Green’s functions based on a scaling relationship of the ω−2 source spectrum [10].
The waveform for a large event is synthesized by summing the records of small events with corrections
for the difference in the slip velocity time function between the large and small events following thescaling laws. Successive source modeling by the empirical Green’s function method [9] has been done by
Kamae and Irikura [11, 12], Ikeda et al. [13], Miyake et al. [14], Morikawa et al. [15], and others. In thisstudy, we will construct source models for seven large and moderate-size shallow intraslab earthquakesby the forward waveform modeling using the empirical Green’s function method [14], and will discuss onsome remarkable features of obtained source models.
METHODOLOGY AND DATA
Methodology
In this study, a characterized source model consisting one or several “strong motion generation areas” inthe fault plane is assumed. The strong motion generation area is defined as the extended area having
relatively large slip velocity within the total rupture area [14]. We assume that the area has uniform slip
and rise time. For broadband strong motion simulation up to about 10 Hz using the empirical Green’s
function method [9], such a simple source model is able to reproduce observed ground motions and isconfirmed that the strong motion generation area is equivalent to the asperity area characterized from theheterogeneous slip distribution estimated by the kinematic waveform inversion using low-frequency
waveforms [11-16].
We determined the number and the location of strong motion generation areas, the size and the rise timeof each strong motion generation area, and the rupture propagation velocity by the forward modelingusing the empirical Green’s function method. Radial rupture propagation inside each strong motion
generation area is also assumed. The best source model is chosen to minimize the residual of
displacement waveforms and envelopes of acceleration waveforms using the criterion proposed by
Miyake et al. [14]. The detailed description about the forward modeling and the summation technique ofthe empirical Green’s function method are written by Irikura [9] and Miyake et al. [14].

Analyzed events and data set
We have analyzed seven shallow intraslab earthquakes listed in Table 1. The locations of epicenters arealso indicated in Fig.1. The source mechanism determined from the moment tensor inversion using
broadband seismograph data by the F-net and the Harvard University CMT project is also shown in Fig.1
using lower hemisphere projection. The focal depths of these events range from 30 to 72 km. Magnitudes
determined by the Japan Meteorological Agency (JMA) are from 5.3 to 7.1. Horizontal components of the
S-wave part at four strong motion stations are used to construct the source model by the forward
waveform modeling.
Table 1: Information of target events and small events used as empirical Green’s functions
Event No. Origin time (JST)Lat. (deg)Long. (deg) Depth (km) MJMA
M0 (Nm) / MW
171m
1997/03/16 14:51
34.925°N
137.528°E
39.12
5.9
2.97×10 / 5.6
1s
1997/03/16 14:53
34.901°N
137.515°E
36.21
4.0
N.A.
17
2m
1999/08/21 05:33
34.028°N
135.473°E
65.83
5.6
2.79×10 / 5.6
152s
2000/06/02 15:05
34.002°N
135.407°E
59.85
4.1
2.10×10 / 4.2
193m
2000/01/28 23:21
43.006°N
146.749°E
58.51
7.0
1.21×10 / 6.7
17
3s
2000/09/03 20:01
42.984°N
146.846°E
49.40
5.7
1.39×10 / 5.4
194m
2001/03/24 15:27
34.129°N
132.696°E
46.46
6.7
1.51×10 / 6.8
164s
2001/03/26 05:40
34.114°N
132.712°E
45.86
5.2
4.76×10 / 5.1
165m
2001/04/03 23:57
35.021°N
138.097°E
30.34
5.3
8.17×10 / 5.2
14
5s
2001/04/04 00:04
35.011°N
138.089°E
31.31
4.1
8.73×10 / 3.9
176m
2001/04/25 23:40
32.796°N
132.342°E
39.30
5.8
4.00×10 / 5.7
15
6s
1999/01/25 05:03
32.694°N
132.286°E
39.23
3.9
1.33×10 / 4.0
197m
2003/05/26 18:24
38.818°N
141.654°E
72.03
7.1
3.49×10 / 7.0
167s
2003/05/27 00:44
38.948°N
141.665°E
68.52
4.9
1.43×10 / 4.7
Event No. with ‘m’ indicates the mainshock that is studied as the target event, and that with ‘s’ indicatesthe small event that is used as the empirical Green’s function.
Hypocentral information is provided from the Japan Meteorological Agency (JMA).
Seismic moment (M0) was determined by the F-net (Full Range Seismograph Network of Japan), NIED[17]. These data is available via the World Wide Web (http://www.fnet.bosai.go.jp/).
130˚E

135˚E

140˚E

150˚E

145˚E

45˚N

45˚N
1
2
3

40˚N

40˚N
4
7

35˚N

35˚N
5
6

30˚N

km
130˚E

135˚E

30˚N

0

500

140˚E

145˚E

150˚E

Figure 1: Locations and mechanisms of events studied

RESULTS
The 2001 Geiyo earthquake
The 2001 Geiyo earthquake (MJMA=6.7, MW=6.8) occurred at the depth of 46 km within the Philippine
Sea slab on March 24, 2002 (JST). At this region, a large historical intraslab earthquake called the 1905
Geiyo earthquake (M=7.2) has occurred. The epicenters of the target and the small events are shown by
the large and the small stars in Fig.2. Strong motion stations used for the source modeling are indicatedby solid triangles in Fig.2.
131˚ 30'35˚ 00'132˚ 00'

132˚ 30'133˚ 00'

133˚ 30'35˚ 00'50 km
34˚ 30'34˚ 30'

HRS01334˚ 00'

HRS02034˚ 00'

YMGH04
EHMH0433˚ 30'33˚ 30'131˚ 30' 132˚ 00' 132˚ 30' 133˚ 00' 133˚ 30'
Figure 2: Map of the 2001 Geiyo earthquake

Our forward modeling result suggests that the 2001 Geiyo earthquake has two strong motion generation
areas. The obtained source model is shown in Fig.3. The large star in Fig.3 is the starting point of thewhole rupture fixed at the hypocenter determined by the JMA, and the small star is the rupture starting
point for the strong motion generation area. The rupture of the second (southern) strong motion
generation area started at 4.81 s after the rupture of the first (northern) strong motion generation area.

along dip (km)10N181˚EDip 57˚
5
0
-5
-10
0

5

10

15

20

25

along strike (km)Figure 3: Obtained source model of the 2001 Geiyo earthquake

Observed and synthetic acceleration, velocity, and displacement waveforms of each component are shownin Fig. 4. These records are band-pass filtered between 0.2 and 10 Hz. The number between observed andsynthetic waveforms of each component is the maximum amplitude of the observed records. Observed
waveforms are well reproduced by synthetic waveforms.

Acc. (cm/s**2)

Vel. (cm/s)

Disp. (cm)

8.8

2.75

12.7

Acc. (cm/s**2)

Vel. (cm/s)

Disp. (cm)

82

6.2

1.82

1.45

310

17.9

1.64

17.5

2.20

248

16.5

2.09

5.5

1.33

47

4.5

1.33

obs.

EHMH04 130
syn.

obs.

HRS020 306
syn.

obs.

HRS013 239
syn.

obs.

YMGH04 46
syn.
0

4

8

s

12

16

20

0

4

8

s

12

16

20

0

4

8

s

12

16

20

0

4

8

s

12

16

20

0

4

8

s

12

16

20

0

4

8

s

12

16

20

Figure 4: Comparison of observed and synthetic waveforms for the 2001 Geiyo earthquake

The 2003 Off-Miyagi earthquake
The 2003 Off-Miyagi earthquake (MJMA=7.1, MW=7.0) occurred at the depth of 72 km near the coast of
Northeastern Honshu, Japan, within the Pacific slab on May 26, 2002 (JST). The epicenters of the target
and the small events are shown by the large and the small stars in Fig.5. Strong motion stations used forthe source modeling are indicated by solid triangles in Fig.5.
140˚ 30'39˚ 30'141˚ 00'

141˚ 30'

IWTH24142˚ 00'

142˚ 30'39˚ 30'

IWTH2339˚ 00'39˚ 00'

MYGH0438˚ 30'38˚ 30'

MYGH1150 km

38˚ 00'38˚ 00'140˚ 30' 141˚ 00' 141˚ 30' 142˚ 00' 142˚ 30'
Figure 5: Map of the 2003 Off-Miyagi earthquake

Our forward modeling result suggests that the 2003 Off-Miyagi earthquake has three strong motion
generation areas within a total rupture area. The obtained source model is shown in Fig. 6. This spatial

distribution of strong motion generation area is close to the large slip area from the kinematic waveform
inversion [7]. The large star in Fig.3 is the starting point of the whole rupture fixed at the hypocenter
determined by the JMA, and the small star is the rupture starting point for the strong motion generation
area. The ruptures of the strong motion generation area B and C started at 0.36 s and 3.28 s after the
rupture of the strong motion generation area A, respectively.

N190˚EDip Direction (km)10

Dip 90˚

5

A

0

Dip 69˚
B

-5
-10

C

-15
-20
-20

-15

-10

-5

0

5

Strike Direction (km)Figure 6: Obtained source model of the 2003 Off-Miyagi earthquake

Observed and synthetic acceleration, velocity, and displacement waveforms of each component are shownin Fig. 7. These records are band-pass filtered between 0.3 and 10 Hz. The number between observed andsynthetic waveforms of each component is the maximum amplitude of the observed records. Observed
waveforms are well reproduced by synthetic waveforms.
Acc. (cm/s**2)

EW component
Vel. (cm/s)

Disp. (cm)

11.7

2.87

15.2

Acc. (cm/s**2)

NS component
Vel. (cm/s)

Disp. (cm)

102

4.4

0.81

6.65

77

8.2

2.85

8.4

1.14

102

6.3

0.90

4.5

0.76

42

3.4

0.86

obs.

IWTH23 105
syn.

obs.

IWTH24 85
syn.

obs.

MYGH04 109
syn.

obs.

MYGH11 55
syn.
0

3

6

s

9

12 15 0

3

6

s

9

12 15 0

3

6

s

9

12 15 0

3

6

s

9

12 15 0

3

6

s

9

12 15 0

3

6

s

9

12 15

Figure 7: Comparison of observed and synthetic waveforms for the 2003 Off-Miyagi earthquake

Summary of all events
Source parameters of all events analyzed here are listed in Table 2.
Table 2: Source parameters of Strong Motion Generation Areas obtained in this study
2
Rise time (sec)Vr (km/s)
No. Length (km)Width (km)Area (km )∆σ (MPa)
1m
1.5
1.8
2.7
0.09
3.05
N.A.
2m
1.2
1.2
1.44
0.04
4.2
314
3m
4.4
5.6
24.64
0.12
3.7
261
4m
7.2 and 5.4
4.6 and 4.6
57.96
0.28 and 0.21
2.7
47 and 41
5m
1.8
2.2
3.96
0.04
3.1
23
6m
2.2
3.4
7.48
0.12
3.4
19
7m
3.0, 4.0,
3.0, 4.0,
61.0
0.18, 0.24,
2.75
105, 105,
and 6.0
and 6.0
and 0.36
and 105
Event 4m and 7m has two and three strong motion generation areas, respectively.
Vr is the rupture propagation velocity inside the strong motion generation area.
∆σ is the stress drop on the strong motion generation area calculated assuming a circular crack [18, 19].

DISCUSSION

Strong Motion Generation Area (km2)

Scaling relationship between strong motion generation area and seismic moment
Strong motion generation area obtained here is plotted against the seismic moment in Fig.8. Strong
motion generation areas for two great intraslab earthquakes in Hokkaido derived by Morikawa et al. [15]are also plotted in the same figure. For comparison, strong motion generation areas of inland crustal
earthquakes estimated by the forward waveform modeling using the empirical Green’s function method[11-14] are plotted by squares in Fig.8. The solid line in Fig.8 is the empirical relationship between
combined area of asperities and seismic moment proposed by Somerville et al. [20]. Miyake et al. [14]has concluded that strong motion generation area of inland crustal earthquakes follow the empirical
relationship by Somerville et al. [20]. However, shallow intraslab earthquakes studied in this study show
smaller strong motion generation area than that predicted from the empirical relationship by Somerville etal. [20]. Since their empirical relationship is a self-similar relationship with constant stress drop on
asperities, it means the stress drop on strong motion generation area or asperities of shallow intraslab
earthquakes are larger than that of inland crustal earthquake. That is a quite important characteristic forthe source modeling of shallow intraslab earthquakes.
10000

Inrand Crustal Earthquakes
by Kamae and Irikura [11,12], Ikeda et al. [13],and Miyake et al. [14]Intraslab Earthquakes by Morikawa et al. [15]This study

1000

Somerville et al. (1999)

100

10

1
1016

1017

1018

1019

1020

1021

1022

Seismic Moment (Nm)Figure 8: Relationship between strong motion generation area and seismic moment

Stress Drop on SMGA (MPa)

Depth dependence of stress drop on strong motion generation area
Fig.9 shows the relationship between the stress drop on strong motion generation area and the depth ofthe center of strong motion generation area. Solid circles indicate normal-fault intraslab events in thePhilippine Sea slab, respectively. Open squares indicate reverse-fault events in Pacific slab. Horizontal
bars accompanying with these symbols correspond to the spatial extent of strong motion generation areain the depth direction. The relation shows the depth dependence of stress drops, in which the deeper event
has larger stress drop. In addition, the reverse fault intraslab events seem to have relatively larger stressdrop than the normal fault events. Another possible controlling factor for the difference in stress drop is
the regional difference between the Pacific slab and Philippine Sea slab.
Normal fault in Philippine Sea slab
Reverse fault in Pacific slab

500
200
100
50
20
10
20 30 40 50 60 70 80 90 100 110

Depth (km)Figure 9: Relationship between the stress drop on strong motion generation area and the depth

CONCLUSIONS
Source modeling of seven shallow intraslab earthquakes occurred in Japan were studied via the forward
modeling using the empirical Green’s function method. Obtained result indicate that strong motion
generation area of shallow intraslab earthquakes are smaller then the predicted values from the empirical
relationship between seismic moment and combined area of asperities for inland crustal earthquakes.
And, the stress drop on the strong motion generation area seems to depend on its depth, which should be
one of important source characteristics for shallow intraslab earthquakes. These results suggests that weneed to consider the depth dependence of the stress drop on the strong motion generation area or theasperity in case of the source modeling for the strong motion prediction for shallow intraslab earthquakes.
ACKNOWLEDGMENTS
We would like to thank the National Research Institute for Earth Science and Disaster Prevention (NIED)
for strong motion data used in this study, the Japan Meteorological Agency (JMA) for providing
hypocentral information, and the F-net/NIED and the Harvard University CMT Project for moment tensor
solutions catalog. This study is partially supported by the study on characterization of earthquake faults
and crustal structure of special project for earthquake disaster mitigation in urban areas by the Ministry of
Education, Culture, Sports, Science, and Technology (MEXT), Japan. We used the Generic Mapping
Tools (Wessel and Smith, 1998) to draw some of figures in this paper.

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