source modelling and strong ground motion … modelling and strong ground motion simulation of the...
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Source Modelling and Strong Ground Motion
Simulation
of the 2011 Tohoku Earthquake
Hidenori Kawabe, Katsuhiro Kamae, Hirotoshi Uebayashi Research Reactor Institute, Kyoto University, Japan
SUMMARY:
The 2011 Tohoku earthquake of 11 March 2011 occurred in the subduction zone plate boundary between the
Pacific and North American plates along the Japan Trench. In this study, we try to estimate the source model and
simulate the strong ground motions of this earthquake. First of all, we try to estimate the source model by a
forward-modeling approach based on the characterized source model using the empirical Green's function
method. Based on the results, we proposed a source model composed of five strong motion generation areas on
the subducting plate boundary. Next, we performed a strong ground motion simulation using the 3D
finite-difference method. Our simulation target area was the region from Miyagi Prefecture to the Kanto basin.
The effective period of this simulation is from 3 to 10 sec. The resulting S wave amplitude and arrival time of
synthetic waveforms are in good agreement with the observed ones.
Keywords: Strong ground motion, Source modeling, Subduction zoon earthquake, Finite difference method
1. INTRODUCTION
On 11 March 2011, Japan was struck by a massive Mw 9.0 subduction zone earthquake whose
epicenter was off Miyagi Prefecture in the Tohoku region (the 2011 Tohoku earthquake). The source
area extends approximately 400 km along the Japan trench. The seismic ground motions of this earthquake caused severe damage and casualties over a wide area from Tohoku into the Kanto region.
Eastern Japan was also struck by a tsunami, wreaking catastrophic damage to coastal areas.
Many strong motion records of this earthquake were obtained by the strong motion observation
networks in Japan. The duration of observed strong ground motions was quite long. The long-period
strong ground motions associated with this earthquake had less effect on high rise buildings than one
would expect from the scale of the event. Nevertheless, skyscrapers suffered some ceiling collapses and damage to internal furnishings, elevators and other equipment. It is important to estimate the
amplification characteristics, attenuation characteristics and other propagation parameters of the
long-period strong ground motions associated with this earthquake to investigate measures that can be taken against such ground motions in future huge earthquakes.
The first of our objectives is to estimate the source model of the 2011 Tohoku earthquake using a forward-modeling procedure. Another of our objectives is to investigate how well the observed
long-period strong ground motions during the earthquake are reproduced using our source model and
the 3D subsurface structure model proposed by the Headquarters for Earthquake Research Promotion
(HERP).
2. STRONG MOTION RECORDS
The 2011 Tohoku earthquake was recorded by well over 1000 strong motion instruments of the
National Research Institute for Earth Science and Disaster Prevention (NIED) and other organizations.
Fig. 1 shows the ground acceleration records observed by KiK-net strong motion seismograph
networks belonging to NIED. These acceleration waveforms indicate two characteristic wave packets
in the region north of Miyagi Prefecture. In addition to these two wave packets, different characteristic
wave packets are observed in the waveforms observed in Fukushima Prefecture. In Ibaraki Prefecture,
a single characteristic wave packet is observed. These waveforms suggest that the source process of this earthquake was extremely complicated.
(a) Locations of epicenter and observation stations (b) Acceleration waveforms
Figure 1. Map of the Epicenter (a Red Star) of the 2011 Tohoku Earthquake,
KiK-net Strong-Motion Stations and Observed Waveforms at the Bottom of Boreholes (Band Pass Filter: 0.1–10 Hz). Dashed lines represent propagation of 5 wave packets.
3. SOURCE MODELING
In this section, we estimate the source model by a forward-modeling approach, based on the
characterized source model using the empirical Green's function method (Irikura, 1986). Using the
bandpass-filtered (0.1–10 Hz) subsurface records of KiK-net near the coast, strong motion generation areas (SMGAs) were designated. Considering the shape of the subducting Pacific Plate, the fault plane
for this earthquake was assumed to pass through the hypocenter determined by the Japan
Meteorological Agency (JMA) with a strike of 195° and dip of 13°. The records from the 4 events listed in Table 1 were used as the empirical Green’s functions. The records at the high S-wave velocity
layer (see Fig. 2) were used to minimize the influence of amplification of ground motions in the
surface layers on the observation records while estimating the source model. Here, the SMGA
locations, rupture start times and other parameters were estimated on the basis of the arrival times of the wave packets to each station, based on the wave propagation shown by the dashed line in Fig. 1.
Fig. 2, Fig. 3 and Table 2 show the SMGA locations and source parameters. This source model is composed of five SMGAs located on the sea off Miyagi, south Iwate, Fukushima and Ibaraki
Prefectures. Fig. 4 shows examples of comparison between synthetic waveforms calculated by the
empirical Green's function method with the observed waveforms. From this figure, it can be seen that the observed waveforms are well reproduced at each observation station.
Table 1. Source Parameters for Small Events
Event 1 Event 2 Event 3 Event 4
Origin Time (JST)* 2005/12/17 3:32 2011/3/10 3:16 2010/6/13 12:32 2005/10/19 20:44
Latitude (deg.)* 38.449 38.271 37.396 36.382
Longitude (deg.)* 142.181 142.879 141.796 141.043
Depth (km)* 39.9 28.9 40.3 48.3
Mj* 6.1 6.4 6.2 6.3
Mo (Nm)** 1.12×1018 1.10×1018 7.94×1017 3.18×1018
Strike/dip/rake** (deg.)20/72/91196/19/86
22/71/90201/19/89
247/47/7292/46/108
25/68/88209/22/94
*JMA, **F-net
Figure 2. Locations of Epicenters, Strong Motion Generation Areas and Seismic Stations.
Table 2. Source Parameters for SMGAs
SMGA1 SMGA2 SMGA3 SMGA4 SMGA5
Strike (°) 195 195 195 195 195
Dip (°) 13 13 13 13 13
Area (km2) 40×40 50×50 21×21 28×28 30×30
Mo (N・m) 5.02×1020 1.10×1021 6.43×1019 1.02×1020 2.58×1020
Stress Drop (MPa) 20.4 21.6 15.7 10.5 23.1
Rise Time (s) 3.6 4.5 1.9 2.5 2.7
Ruptur start time (s) 0.0 35.0 57.0 87.0 102.0
2005/12/173:32 M6.1
2011/03/103:16 M6.4
2010/06/1312:33 M6.2
2010/06/1312:33 M6.2
2005/10/1920:44 M6.3
Empirical Green'sfunction
SMGA1: 40km×40km
SMGA2: 50km×50km
SMGA3: 21km×21km
SMGA4: 28km×28km
SMGA5: 30km×30km
70
km
200km
strike (N195E)
dip
SMGA2160km
75
km
225km
140
km
170
km
336km380km
SMGA1SMGA3
SMGA5
SMGA4
161
km
120
km
287km
Rupture starting point
of SMGA
Hypocenter
Origin (144.374°, 39.885°, 6.4km )
Figure 3. Source Model
56
-56
0
7
-7
0
4
-4
0
Obs-NS Max : 50.19gal Obs-EW Max : 36.30gal Obs-UD Max : 28.42gal
Syn-NS Max : 55.98gal Syn-EW Max : 40.09gal Syn-UD Max : 39.41gal
Obs-NS Max : 4.80cm/s Obs-EW Max : 5.91cm/s Obs-UD Max : 6.60cm/s
Syn-NS Max : 5.33cm/s Syn-EW Max : 5.58cm/s Syn-UD Max : 6.18cm/s
Obs-NS Max : 2.21cm Obs-EW Max : 3.31cm Obs-UD Max : 4.25cm
Syn-NS Max : 2.47cm Syn-EW Max : 2.56cm Syn-UD Max : 3.02cm
Acc
. (ga
l)Vel
. (cm
/s)
Dis
. (cm
)IWTH14
0020 100 (s)
203
-203
0
24
-24
0
8
-8
0
Obs-NS Max : 130.28gal Obs-EW Max : 202.76gal Obs-UD Max : 84.51gal
Syn-NS Max : 82.10gal Syn-EW Max : 112.34gal Syn-UD Max : 85.56gal
Obs-NS Max : 17.01cm/s Obs-EW Max : 23.86cm/s Obs-UD Max : 9.23cm/s
Syn-NS Max : 7.99cm/s Syn-EW Max : 11.53cm/s Syn-UD Max : 11.59cm/s
Obs-NS Max : 4.77cm Obs-EW Max : 7.87cm Obs-UD Max : 5.96cm
Syn-NS Max : 2.83cm Syn-EW Max : 4.61cm Syn-UD Max : 4.96cm
Acc. (
gal)
Vel
. (cm
/s)
Dis
. (cm
)
MYGH12
0020 100 (s)
95
-95
0
14
-14
0
6
-6
0
Obs-NS Max : 61.04gal Obs-EW Max : 78.74gal Obs-UD Max : 42.76gal
Syn-NS Max : 94.93gal Syn-EW Max : 74.91gal Syn-UD Max : 68.22gal
Obs-NS Max : 14.13cm/s Obs-EW Max : 7.47cm/s Obs-UD Max : 7.33cm/s
Syn-NS Max : 10.66cm/s Syn-EW Max : 6.76cm/s Syn-UD Max : 8.02cm/s
Obs-NS Max : 6.45cm Obs-EW Max : 2.78cm Obs-UD Max : 2.92cm
Syn-NS Max : 3.02cm Syn-EW Max : 2.69cm Syn-UD Max : 3.17cm
Acc
. (ga
l)Vel
. (cm
/s)
Dis
. (cm
)
FKSH17
0020 100 (s)
125
-125
0
16
-16
0
8
-8
0
Obs-NS Max : 92.47gal Obs-EW Max : 89.61gal Obs-UD Max : 85.08gal
Syn-NS Max : 125.34gal Syn-EW Max : 61.43gal Syn-UD Max : 64.31gal
Obs-NS Max : 16.22cm/s Obs-EW Max : 10.03cm/s Obs-UD Max : 5.93cm/s
Syn-NS Max : 10.50cm/s Syn-EW Max : 6.49cm/s Syn-UD Max : 4.56cm/s
Obs-NS Max : 7.61cm Obs-EW Max : 6.10cm Obs-UD Max : 2.83cm
Syn-NS Max : 3.63cm Syn-EW Max : 2.96cm Syn-UD Max : 2.81cm
Acc. (
gal)
Vel
. (cm
/s)
Dis
. (cm
)
IBRH14
0020 100 (s)
Figure 4. Comparison of Observed (Black Lines) and Calculated (Red Lines) Waveforms for Observation
Stations Using Forward Source Modeling (Band Pass Filter: 0.1–10 Hz)
Fig. 5 shows a comparison between observed waveforms by K-NET of NIED, another observation
stations that were not used for source modeling, and the synthetic waveforms calculated by the empirical Green's function method. Although the synthetic displacement amplitudes at observation
stations in the Kanto basin are smaller than the observed ones, the synthetic velocity and acceleration
waveforms generally reproduce the observed ones.
224
-224
0
20
-20
0
13
-13
0
Obs-NS Max : 118.20gal Obs-EW Max : 158.53gal Obs-UD Max : 53.64gal
Syn-NS Max : 190.73gal Syn-EW Max : 224.49gal Syn-UD Max : 58.03gal
Obs-NS Max : 17.59cm/s Obs-EW Max : 19.54cm/s Obs-UD Max : 5.07cm/s
Syn-NS Max : 16.77cm/s Syn-EW Max : 13.07cm/s Syn-UD Max : 4.74cm/s
Obs-NS Max : 10.36cm Obs-EW Max : 13.42cm Obs-UD Max : 3.16cm
Syn-NS Max : 6.22cm Syn-EW Max : 5.05cm Syn-UD Max : 2.17cm
Acc. (
gal)
Vel
. (cm
/s)
Dis
. (cm
)
TKY025
0020 100 (s)
220
-220
0
22
-22
0
12
-12
0
Obs-NS Max : 167.83gal Obs-EW Max : 164.04gal Obs-UD Max : 50.66gal
Syn-NS Max : 191.31gal Syn-EW Max : 220.25gal Syn-UD Max : 62.66gal
Obs-NS Max : 22.38cm/s Obs-EW Max : 22.26cm/s Obs-UD Max : 8.55cm/s
Syn-NS Max : 18.48cm/s Syn-EW Max : 21.51cm/s Syn-UD Max : 8.01cm/s
Obs-NS Max : 12.36cm Obs-EW Max : 10.59cm Obs-UD Max : 4.76cm
Syn-NS Max : 8.31cm Syn-EW Max : 8.03cm Syn-UD Max : 3.48cm
Acc. (
gal)
Vel
. (cm
/s)
Dis
. (cm
)
SIT010
0020 100 (s)
Figure 5. Comparison of Observed (Black Lines) and Calculated (Red Lines) Waveforms for Observation
Stations That Was not Used for Source Modeling (Band Pass Filter: 0.1–10 Hz)
4. GROUND MOTION SIMULATION
4.1. Simulation method
Ground-motion simulations were performed using the 3D finite-difference procedure presented by
Pitarka (1999). This approach employs a staggered-grid formulation and is applicable to arbitrarily
complex 3D elastic media. The algorithm is accurate to fourth order in space and second order in time and allows for the implementation of a finite source with a complex rupture history. We set an
absorbing region outside the finite computational region and applied the non-reflecting boundary
condition of Cerjan et al. (1985) and the A1 absorbing boundary condition of Clayton and Engquist (1977) to the absorbing region. Anelastic attenuation is accounted for in the simulations by using the
technique described by Graves (1996). This approach uses a spatially variable Q operator having a
linear dependence on frequency. The surface exposure of the region modeled in the 3D
finite-difference simulations is shown in Fig. 6. The finite-difference model covers an area of 412 km (east–west direction) 471 km (north–south direction), and extends to a depth of 100 km. The grid
spacings were 0.3 km horizontally and 0.1 to 0.6 km vertically, and the time step was 0.0075 sec. The
absorption region was 20 grids.
The 2012 version subsurface structure model was used in the Long-Period Ground Motion Hazard
Map published by HERP (HEAP, 2012). We used the subsurface structure model presented on the HERP website (HERP model). Table 3 shows the physical parameters of the HERP model, and Fig. 7
shows the depth to the top of the basement. The physical parameters for Layer 1 in Table 3 were
replaced with the parameters for Layer 2 to carry out the finite-difference calculation.
We used the source model described in Section 3, but the depth of the source model was modified so
that the source location would be at the plate boundary in the HERP model. The values for strike and
dip angle in Table 2 were used, and the rake angle was assumed to be 90°. The slip velocity time function was calculated using the approximation proposed by Nakamura and Miyatake (2000).
The effective period of the simulation was greater than 3 sec because of the values of the
finite-difference grid spacing and the physical parameters of the subsurface structure model. Since the effective period of our source model was 0.1 to 10 sec, the effective period of the synthetic waveforms
was 3 to 10 sec.
138˚ 140˚ 142˚ 144˚
34˚
36˚
38˚
40˚
MYGH12
FKSH17
FKSH19
IBRH14IBRH15
IBR016SIT010
TKY025KNG001
KNG002
139˚ 140˚ 141˚ 142˚ 143˚
35˚
36˚
37˚
38˚
39˚
−2−1
0123456789
10(km)
Figure 6. Locations of Finite-Difference Simulation
Area, SMGAs and Observation Stations
Figure 7. Depth to the Top of Basement
for HERP Model.
4.2. Simulation Results
Fig. 8 compares the observed waveforms with the synthetic waveforms. Overall, the propagations of
seismic ground motion (such as the arrival time and duration) from the north into the Kanto basin were reproduced. A more detailed look at these results indicates that the amplitude of the principal motions
and shape of the wave packet are reproduced from station MYGH12 in Miyagi Prefecture to IBR016
in Ibaraki Prefecture, but the amplitude of the later phase of the synthetic waveforms is somewhat lower than that of the observed ones. The principal motions of the synthetic waveforms correspond
Table 3. Physical Parameters for Subsurface Structure Model (HERP Model)
Qs
1 1.7 0.35 1.80 702 1.8 0.5 1.95 1003 2.0 0.6 2.00 1204 2.1 0.7 2.05 1405 2.2 0.8 2.07 1606 2.3 0.9 2.10 1807 2.4 1.0 2.15 200 Accretionary Wedge8 2.7 1.3 2.20 2609 3.0 1.5 2.25 30010 3.2 1.7 2.30 34011 3.5 2.0 2.35 40012 4.2 2.4 2.45 40013 5.0 2.9 2.60 40014 5.5 3.2 2.65 400 Basement (Upper Crust 1)15 5.8 3.4 2.70 400 Upper Crust 216 6.4 3.8 2.80 400 Lower Crust17 7.5 4.5 3.20 500 Mantle18 5.0 2.9 2.40 200 Oceanic Crust 2 (Philippine Sea Plate)19 6.8 4.0 2.90 300 Oceanic Crust 3 (Philippine Sea Plate)20 8.0 4.7 3.20 500 Oceanic Mantle (Philippine Sea Plate)21 5.4 2.8 2.60 200 Oceanic Crust 2 (Pacific Plate)22 6.5 3.5 2.80 300 Oceanic Crust 3 (Pacific Plate)23 8.1 4.6 3.40 500 Oceanic Mantle (Pacific Plate)
RemarksLayer Num.Vs
(km/s)Vp
(km/s)ρ
(g/cm3)
15
-15
0
Obs-NS Max : 7.44cm/s Obs-EW Max : 14.78cm/s Obs-UD Max : 8.34cm/s
Syn-NS Max : 6.83cm/s Syn-EW Max : 9.95cm/s Syn-UD Max : 12.26cm/s
Vel
. (cm
/s)
MYGH12
0030 150 (s)
16
-16
0
Obs-NS Max : 11.32cm/s Obs-EW Max : 15.61cm/s Obs-UD Max : 6.11cm/s
Syn-NS Max : 10.09cm/s Syn-EW Max : 10.98cm/s Syn-UD Max : 4.11cm/s
Vel
. (cm
/s)
IBR016
0030 150 (s)
8
-8
0
Obs-NS Max : 8.36cm/s Obs-EW Max : 8.23cm/s Obs-UD Max : 5.83cm/s
Syn-NS Max : 4.24cm/s Syn-EW Max : 3.08cm/s Syn-UD Max : 5.28cm/s
Vel
. (cm
/s)
FKSH17
0030 150 (s)
14
-14
0
Obs-NS Max : 13.86cm/s Obs-EW Max : 12.12cm/s Obs-UD Max : 6.66cm/s
Syn-NS Max : 10.69cm/s Syn-EW Max : 10.90cm/s Syn-UD Max : 7.34cm/s
Vel
. (cm
/s)
SIT010
0030 150 (s)
6
-6
0
Obs-NS Max : 3.09cm/s Obs-EW Max : 5.80cm/s Obs-UD Max : 5.33cm/s
Syn-NS Max : 4.17cm/s Syn-EW Max : 4.73cm/s Syn-UD Max : 5.88cm/s
Vel
. (cm
/s)
FKSH19
0030 150 (s)
14
-14
0
Obs-NS Max : 14.32cm/s Obs-EW Max : 12.26cm/s Obs-UD Max : 4.47cm/s
Syn-NS Max : 10.83cm/s Syn-EW Max : 9.68cm/s Syn-UD Max : 4.81cm/s
Vel
. (cm
/s)
TKY025
0030 150 (s)
11
-11
0
Obs-NS Max : 10.94cm/s Obs-EW Max : 7.77cm/s Obs-UD Max : 7.07cm/s
Syn-NS Max : 6.61cm/s Syn-EW Max : 9.20cm/s Syn-UD Max : 5.93cm/s
Vel
. (cm
/s)
IBRH14
0030 150 (s)
17
-17
0
Obs-NS Max : 17.00cm/s Obs-EW Max : 10.64cm/s Obs-UD Max : 5.02cm/s
Syn-NS Max : 9.98cm/s Syn-EW Max : 9.69cm/s Syn-UD Max : 6.43cm/s
Vel
. (cm
/s)
KNG001
0030 150 (s)
11
-11
0
Obs-NS Max : 11.23cm/s Obs-EW Max : 6.38cm/s Obs-UD Max : 6.92cm/s
Syn-NS Max : 6.74cm/s Syn-EW Max : 9.83cm/s Syn-UD Max : 5.23cm/s
Vel
. (cm
/s)
IBRH15
0030 150 (s)
17
-17
0
Obs-NS Max : 16.86cm/s Obs-EW Max : 10.83cm/s Obs-UD Max : 7.20cm/s
Syn-NS Max : 7.42cm/s Syn-EW Max : 9.75cm/s Syn-UD Max : 5.41cm/s
Vel
. (cm
/s)
KNG002
0030 150 (s)
Figure 8. Comparison of Observed (Black Lines) and Calculated (Red Lines)
Waveforms for Finite-Difference Simulation (Band Pass Filter: 0.1–0.33 Hz).
well to the observed values for all of the observation stations in the Kanto basin south of the SIT010
station in Saitama Prefecture. In addition, not only the amplitudes but also the phases of the UD
components of the observed waveforms are well reproduced by the synthetic waveforms. However,
the later phase amplitudes of the synthetic waveform are lower than the amplitudes observed at the stations in Kanto basin. One possible reason for this is that the source model was composed of only
five SMGAs, the radiation of ground motions from the other source region was not assumed. It is also
possible that the attenuation parameters in the sedimentary basins might be incorrect. These factors will be investigated in a future study. Fig. 9 shows the maximum ground velocities observed by
K-NET and KiK-net (surface) stations, and Fig. 10 shows the estimated maximum velocities. It can be
seen that there are large horizontal velocities from the Sendai basin to the coastal region of south Fukushima Prefecture and the Kanto basin, whereas the vertical velocities are relatively small in the
Kanto basin. These trends are reproduced in the synthetic waveforms shown in Fig. 10. But the
amplitudes of the synthetic waveforms are somewhat underestimated. One possible reason for this
underestimate is that only subsurface structures with S-wave velocities above 500 m/s were considered in the finite-difference calculation, while the amplification at the surface structures were neglected.
Fig. 11 and 12 respectively show the observed and synthetic pseudo-velocity response spectra for
periods of 4, 7 and 10 sec. The spectra in Fig. 11 indicate large amplitudes for periods of 7 and 10 sec
139˚ 140˚ 141˚ 142˚
35˚
36˚
37˚
38˚
39˚
139˚ 140˚ 141˚ 142˚
35˚
36˚
37˚
38˚
39˚
139˚ 140˚ 141˚ 142˚
35˚
36˚
37˚
38˚
39˚
0
20
40
60
80
100
120(cm/s)
Figure 9. Peak Ground Velocity Observed at the K-NET and KiK-net Stations (Band Pass Filter: 0.1–0.33 Hz)
139˚ 140˚ 141˚ 142˚
35˚
36˚
37˚
38˚
39˚
139˚ 140˚ 141˚ 142˚
35˚
36˚
37˚
38˚
39˚
139˚ 140˚ 141˚ 142˚
35˚
36˚
37˚
38˚
39˚
0
20
40
60
80
100
120(cm/s)
Figure 10. Peak Ground Velocity for Strong Ground Motion Simulation (Band Pass Filter: 0.1–0.33 Hz)
139˚ 140˚ 141˚ 142˚
35˚
36˚
37˚
38˚
39˚
139˚ 140˚ 141˚ 142˚
35˚
36˚
37˚
38˚
39˚
139˚ 140˚ 141˚ 142˚
35˚
36˚
37˚
38˚
39˚
0
20
40
60
80
100
120
140
160
180
200(cm/s)
Figure 11. Pseudo-Velocity Response Spectra Observed at the K-NET and KiK-net Stations (EW comp., h=5%)
139˚ 140˚ 141˚ 142˚
35˚
36˚
37˚
38˚
39˚
139˚ 140˚ 141˚ 142˚
35˚
36˚
37˚
38˚
39˚
139˚ 140˚ 141˚ 142˚
35˚
36˚
37˚
38˚
39˚
0
20
40
60
80
100
120
140
160
180
200(cm/s)
Figure 12. Pseudo-Velocity Response Spectra for Strong Ground Motion Simulation (EW comp., h=5%)
in the Kanto and Sendai basins, and this is fairly well reproduced in the calculated results shown in Fig. 12. However, careful examination of the spectra for a period of 10 sec shows that the calculations
overestimate the observed ones in the west side of Tokyo Bay. This result suggests that the velocity
and attenuation in the subsurface structure model needs to be reconsidered. In addition, Fig. 11 shows that for a period of 4 sec, large amplitudes were observed at many stations in regions other than the
Kanto and Sendai basins, whereas this behavior was not replicated by the simulation. This is also most
likely due to the fact that our simulation model did not account for the amplification of the surface
structure.
5. CONCLUSIONS
In this study, the source model of the 2011 Tohoku earthquake was estimated, and seismic ground
motions with periods of 3 to 10 sec were simulated using our source model and the subsurface structure model provided by HERP, and then the results were compared to the observed ground
motions during the 2011 off the Pacific coast of Tohoku earthquake. The main findings were as
follows:
For Source Modeling
(1) We proposed a source model composed of five SMGAs located in the sea off Miyagi, south Iwate,
Fukushima and Ibaraki Prefectures. Using this source model,
(2) The observed waveforms are well reproduced by the synthetic waveforms calculated by the empirical Green's function method.
For Strong Ground Motion Simulation
(3) The propagation of seismic ground motion (such as the arrival time and duration) from the north into the Kanto basin were reproduced by the strong ground motion simulation.
(4) The amplitude of the principal motions and shapes of wave packet are reproduced from station
MYGH12 in Miyagi Prefecture to IBR016 in Ibaraki Prefecture, but the later phase amplitudes of the synthetic waveforms were somewhat lower than that of the observed waveforms.
(5) At observation stations in the Kanto basin south of SIT010 in Saitama Prefecture, the amplitudes
of the predicted waveforms are well reproduced by the synthetic waveforms. In addition, not only
the amplitudes but also the phases of the UD components of the observed waveforms are well reproduced by the synthetic waveforms. However, the later phase amplitudes of the synthetic
waveform are lower than the amplitudes observed at the stations in Kanto basin.
(6) In the horizontal component, large ground velocities were observed from stations in the Sendai basin and south Fukushima Prefecture along the coast into the Kanto basin. However, in the
vertical component, the maximum velocities were relatively high in the Sendai Basin, whereas they
were relatively low in the Kanto basin. The calculated results generally reproduced this trend. (7) The observed pseudo-velocity response spectra showed large amplitudes in the Kanto and Sendai
basins for periods of 7 and 10 sec, and the synthesized spectra generally reproduced this trend.
However, the spectra for a period of 10 sec shows that the synthesized spectra overestimate in the
west side of Tokyo Bay. (8) The synthesized 4 sec response spectra underestimated in locations other than the Kanto and
Sendai basins. Just as for the velocity spectra, this is possibly due to the fact that the subsurface
structure model did not account for the wave amplification in the surface structure. In this study, the subsurface structure model and our source model consisting of five SMGAs were
employed to simulate ground motions. In future work, this simulation will be further refined by
considering five SMGAs and the other regions of the source area, and reconsidering the subsurface
structure model. It is hoped that this will provide a more accurate representation of the amplitude of the later phase, the phase of the principal motions, and other earthquake parameters.
ACKNOWLEDGEMENT
We thank the National Institute for Earth Science and Disaster Research (NIED), Japan for strong motion
records, the Japan Meteorological Agency for the focal mechanism information, and the Headquarters for
Earthquake Research Promotion of Japan for the subsurface structure model.
REFERENCES
Cerjan, C., Kosllof, D., Kosllof, R. and Reshef, M. (1985), A nonreflecting boundary condition for discrete
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