Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, JapanGROUND MOTION SIMULATIONS FOR GREAT
INTERPLATE EARTHQUAKES AROUND JAPAN:
VARIABILITY OF GROUND MOTION RESULTEDFROM COMPREXITY IN SOURCE MODELS
Masayuki YOSHIMI1, Haruko SEKIGUCHI2 and Haruo HORIKAWA3
1

Researcher, Geological Survey of Japan, AIST,
Tsukuba, Ibaraki, Japan, yoshimi.m@aist.go.jp
2
Associate Professor, Disaster Prevention Research Institute, Kyoto University,
Uji, Kyoto, Japan, sekiguchi.haruko.6u@kyoto-u.ac.jp
3
Senior Researcher, Geological Survey of Japan, AIST,
Tsukuba, Ibaraki, Japan, h.horikawa@aist.go.jp

ABSTRACT: We numerically simulate ground motions in sedimentary basins for
anticipated great interplate earthquakes along the plate boundaries surrounding Japan.
Possible multi-scale heterogeneous rupture scenarios have been modeled combining
characteristic properties of the source area and adequate variation in source parameters.
Ground motions for anticipated Nankai and Tonankai earthquakes to the Osaka basin, andthose for Kanto earthquakes and Off Boso Peninsula earthquakes to the Kanto basin havebeen simulated and variations been investigated.
Key Words: Ground motion prediction, Nankai and Tonankai earthquakes, Kanto
earthquakes, Off Boso Peninsula earthquake, variability analysis

INTRODUCTION
Ground motion prediction is one of the most important issues to be tackled in order to assure seismic
design against earthquakes that might come near future. Uncertainty in the ground motion prediction
depends on how well we know about the future source process and subsurface structure. Thanks torigorous efforts in modeling subsurface velocity structure all over the Japan, reliable subsurface
structure models have become available in wider areas for ground motion computations. On the contrary,large uncertainty still exists about the future source process. Appropriate modeling of complex source
process and its variation is, thus, a key factor especially in ground motion prediction where the variationof the estimation is of great concern as well as the estimation itself.
After the 2011 huge Tohoku Earthquake with horrible tsunami disaster and nuclear crisis we faceurgent risk of other great earthquake all over the country. Importance of the ground motion prediction isincreasing.
This paper aims at stating a series of ground motion prediction works the authors have been
conducted, in which a source modeling procedure has been proposed and applied for ground motion
prediction that takes into account a complex feature of the source process [Sekiguchi and Yoshimi,

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2006; Yoshimi and Sekiguchi, 2006a; Sekiguchi et al., 2008; Sekiguchi and Yoshimi, 2011; Sekiguchi
et al., 2011].
MULTI-SCALE HETEROGENEIOUS SOURCE MODEL
Modeling of earthquake scenario begins with a kinematic source model (we refer it as an initial source
model), which can be either an asperity-background source model [Sekiguchi et al, 2008] or a source
model inverted from waveforms [Sekiguchi and Yoshimi, 2011]. Once the initial source model isprovided, possible source processes of the great earthquakes are modeled to have multi-scale
heterogeneity in both distributions of the slip and the rupture velocity considering characteristic
properties of a source area and adequate variation in source parameters.
Multi-scale slip distribution
Multi-scale heterogeneity is added to the slip distribution with following iterative operations. At first,
uniform size patches are randomly distributed on the fault plane. Then the slip amount of the fault area
inside every patch is increased or decreased by a certain amount keeping the total seismic moment
unchanged. The radius of the set of patches starts from a half of that of the smallest asperity, and then it
decreases with factor of 1.5 as the procedure is iterated. Slip fluctuation amplitude begins with a half of
the average slip on the fault, and then it decreases by factor of 1.5 as iteration. Among the iterative
operations the total area of the patches is kept constant, to be as large as the sum of the asperity areas.
Finally, the amplitude of the slip spectrum is adjusted to decay k-1.75 so as the source model to follow anempirical characteristic obtained by stochastic characterization of earthquake slip complexity frommany published source models [Mai and Beroza, 2002].
Multi-scale rupture velocity distribution
Multi-scale heterogeneity is also introduced to the rupture velocity using the same set of patches used togive fluctuation to the slip distribution. At first, the velocity vector of the rupture propagation at every
point on the fault is picked from the initial source model and stored. Then the rupture speed is fluctuatedwith the similar iterative manner as the slip is fluctuated while the rupture speed is increased if the slip
is increased, and vice versa. Amplitude of the speed fluctuation is constant, independent of the patch
size. Finally, the rupture triggering time at every point on the fault plane is determined referring theinitial velocity vector and fluctuated rupture speed distributions. Amplitude of fluctuations of rupture
velocity is set following Miyakoshi and Petukhin (2005).
In computation, slip velocity time functions are calculated following Nakamura and Miyatake (2000)
for each point on the fault considering the spatially variable slip, stress drop and rise time.

GROUND MOTION SIMULATION OF GREAT EARTHQUAKES
ALONG NANKAI TROUGH
One application of the multi-scale heterogeneous source modeling is the great earthquakes along the
Nankai trough, namely Nankai and Tonankai earthquakes, whose probability of occurrence arerespectively about 60 percent and 70 percent in the next 30 years [Headquarters of Earthquake Research
Promotion, 2012]. Site of our special concern has been Osaka basin, west Japan, where over ten
millions of people live and numerous factories have been accumulated. These earthquakes will bringabout strong ground motion especially at longer period to the basin.
Source models for the Nankai and Tonankai earthquakes
For the anticipated Nankai and Tonankai earthquake, a set of source models proposed by the Central
Disaster Management Council (CDMC) [CDMC, 2002] had been chosen as initial source models (Fig.

2641a). Since their source spectra lack high frequency component (>0.1 Hz) compared to omega-square
spectra (Fig. 1b), we modified them by adding multi-scale heterogeneity into slip and rupture velocity
so as the resultant ground motions show omega-squared spectral decay [Sekiguchi and Yoshimi 2006].
A number of multi-scale heterogeneous source models had been generated with various sets ofrandom patches. Also, macroscopically varied source models had been made with varying macroscopic
source parameters (e.g. the average stress drop, average rupture velocity, hypocenter location). The
former is aimed at investigating the influences of small-scale fluctuation of slip and rupture velocity,while the latter is aimed at grasping total variation of the ground motion of the earthquakes.
Subsurface structure model
Subsurface structures are modeled as follows. The computation domain is set to be large enough forwave propagation simulation. It extends from deep upper mantle to the ground surface and includes
source areas and Osaka basin. Hence, the subsurface structure should be composed of the subducting
plate, crust, deep sedimentary basins and accretionary wedges. However, we excluded the accretionary
wedge and sediment layers other than the Osaka basin, because those soft layers were found slightly toaffect the computed ground motion in the Osaka basin for the Nankai earthquake [Yoshimi and
Sekiguchi, 2006]. Thereby subsurface structure model is comprised of two parts: Osaka sedimentarybasin and the crust structure. For the velocity structure of the Osaka basin, a 3D velocity model (GSJ
model) was employed. The GSJ model had been constructed based on geological/geophysical dataset
over the area [Horikawa et al., 2003] expressing realistic basement shapes and gradually changing
material properties (Fig. 2). The other part, a 3D crust structure, is roughly modeled incorporating large
structures, the Conrad and the Moho discontinuities and shape of the Philippine Sea slab (PHS)[Furumura, 2002] allocating appropriate velocity and density to each member. The shape of the PHS
slab is used as the shape of the fault surface. Source nodes are distributed on the upper boundary of the
PHS slab resulting in a complex curved surface as a fault surface (Fig. 1a, For detail, see Sekiguchi etal., [2008]).

Figure 1. (a) Source models of hypothetical Nankai and Tonankai earthquake (left: initial model, right: modified
model, up: fault slip, bottom: rupture time, green contour: depth to the PHS slab). (b) Source spectra at Osakabasin for 100 source models (broken line: for initial model, red: for mean of modified model) and their standard
deviation (blue line in the bottom panel) [Modified from fig.1 of Yoshimi and Sekiguchi (2010)]

265Figure 2. 3D Osaka basin model. Left: depth to the basement, right: depth profiles of S-wave velocity (Vs) along
S-N (A-A’) and W-E (B-B’) lines. [Modified from fig.2 of Yoshimi and Sekiguchi (2010)]

Computed ground motions in the Osaka basin
Ground motions are computed with a 3D FDM [Pitarka et al. 1999] incorporating the 3D velocity
structure and the complex source process. Computed ground motions in the basins, in general, aredominated by direct pulses and prolonged surface waves with dominant period varying with the velocity
structure beneath (Fig. 3a). Multi-scale heterogeneous source model produces stronger ground motion
than the initial source model (Fig. 3b), as is expected from comparison of the source spectra (Fig. 1b).
Velocity amplitude in the basin stations ranges around 50 to 100 cm/s.
Ground motions are also computed with parameter variation both of the microscopic parameters(fluctuation within heterogeneous source models) and the macroscopic source parameters (average
stress drop, rupture velocity and the hypocenter location etc.) to obtain possible variation of the groundmotion level.
Effect of small-scale fluctuation on ground motion variation
Varying microscopic parameters, ground motions from 100 source models have been computed. Fig. 4aand 4b respectively displays mean values and the coefficient of variation of the peak ground velocity(PGV) and the velocity response spectra (h=0.05) at period T [s] (Sv(T)) for the Osaka basin (redrectangle in Fig. 1a). In general, the deeper the basin, the larger is the PGV. The largest PGV isobserved around the deepest portion of the basin and the PGV attenuates towards basin edges especiallytoward the east. As for Sv(T), spatial pattern differs among natural periods. For the period 3.5 sec and5.5 sec, peaks are aligned along lines parallel or perpendicular to the basin edges to form mesh-likespatial patterns while at period 7.5 sec relatively smooth pattern is seen, which indicates wave
interference and period dependent surface wave excitation control the patterns. Variations of the PGVand Sv’s are also controlled by the nature of the wave propagation in the basin structure. as they are
dependent of the basin shape. These suggest that even in the statistic analysis of the ground motion, we
have to pay attention to the multi-dimensional effects of wave excitation.
Effect of macroscopic source parameters on ground motion variation
Varying macroscopic parameters (see Fig.5), ground motions have been also computed. Figure 6
displays variation of the PGV and Sv(6.0) at eleven selected stations on the Osaka basin (Fig2).
Compared with the variation due to microscopic parameter, variation of the macroscopic parameter266

considered here changes drastically the ground motion. Among the cases shown in Figure. 5, the case of
50 percent larger average stress drop on asperity produces largest ground motions in the Osaka basin.
The PGV and velocity response spectra at basin sites become twice as those of the base case. On thecontrary, the case of deeper hypocenter gives quite small ground motion to the basin. It is because therupture propagation from the deeper hypocenter does not produce forward directivity effect.

Figure 3. (a) Example of surface velocity waveforms for hypothetical Nankai earthquake (EW component) along
the S-N (upper) and W-E (bottom) lines (Fig.2). (b) Comparison of velocity waveform in/around the Osaka basinwith and without multi-scale heterogeneity in source model.

Figure 4. (a) Mean value (among 100 simulations) distribution of PGV, Sv(3.5), Sv(5.5) and Sv(7.5) for EW
component. (b) Distributions of the coefficient of variation of PGV, Sv(3.5), Sv(5.5) and Sv(7.5).

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Figure 5. Variation of source models for anticipated Nankai earthquake with varied macroscopic parameters:+50 % stress drop (left), -30 % stress drop (middle), and deeper hypocenter (right).

Figure 6. Variation of PGV an Sv(6.0) with micro parameter (left) and with MACRO parameters (right) atselected sites in the Osaka basin. Red lines: case of larger stress drop, blue lines: smaller stress drop, green lines:
case of deeper hypocenter, gray lines: base heterogeneous model.

GROUND MOTION PREDICTION OF GREAT EARTHQUAKES NEAR KANTO REGION
Great interplate earthquake in Kanto region is another application of the multi-scale source modeling.
As two oceanic plates are subducting beneath the Kanto Basin, there are two possible zones for greatinterplate earthquakes close to the Kanto region. One is the boundary between the northward subducting
Philippine Sea (PHS) slab and the crust, namely northeastern dipping source plane from the Sagami
trough beneath the Kanto region. The other is the boundary between the westward subducting Pacific(PAC) slab and the crust or PHS slab, westward dipping source plane from the Japan trench toward the
Kanto region.
The former zone extends beneath the Kanto region at a depth ~30km making the interplate
earthquake here potentially destructive. This zone is the source area of the historically well-known

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devastating interplate earthquakes, 1923 Taisho Kanto earthquake and 1703 Genroku Kanto earthquake.
Another great interplate earthquake is likely to occur in the reasonably near future because therecurrence period is about 200 years simply derived from the interval between the latest two events.
The latter zone spreads along the eastern margin of the Kanto region. Northern portion of this zone,
where the Pacific Plate subducts beneath the crust, has ruptured during the 2011 Tohoku earthquake andits aftershocks. Contrary, the southern portion, where the PAC slab subducts beneath the PHS slab, Off
Boso Peninsula region, did not ruptured at the events. Although earthquakes of this region observed
since Meiji era (1868) do not excess magnitude 7.5, a tsunami generating M8-class earthquake issuggested to have ruptured in 1677 (Enpo era) from historical record [Usami, 2003]. Considering thisregion is next to the source area of the huge M9 events and shear stress is accumulated by 10 cm/yr
subduction of the PAC slab since the 1677 event, interplate earthquakes are highly expected.
Ground motion and its variance due to the anticipated Kanto and Off Boso earthquakes has been
investigated with various earthquake scenarios with adequate variation of source parameters. On
constructing the source models, we assume that repeating great earthquakes share some inherent
characteristics that closely related to its tectonic setting, namely the asperity distribution, with certain
variation in source parameters substantial for nonlinear process. Therefore, for the Kanto earthquake,asperity distribution of the 1923 event is assumed as inherent characteristics and the other source
parameters (e.g., stress drop, rupture velocity, etc.) are variable. For the case of Off Boso Peninsula
earthquakes, no information about asperity being available, asperity distribution of the 1923 Kanto
earthquake is adopted.

Figure 6. (a) Subducting plates beneath Kanto region and source faults for anticipated great interplate
earthquakes: Kanto earthquake and Off Boso earthquake. Contours represent depth to the PHS and PAC
slabs. (b) Altitude distribution of seismic basement of the 3D velocity model used in computation
modified from CDMC model with gravity anomalies.

Source models for the Kanto earthquakes
A bunch of source scenarios have been constructed fixing the area of the asperities based on the source
model of the 1923 Kanto earthquake [Sato et al., 2005], considering possible combination of theasperity activation and variation of the source parameters within a certain ranges supported bystochastic analysis of source properties [for detail see Sekiguchi et al. 2011a].
Source area assumed for the Kanto earthquakes comprises of two parts: fault A and B (Fig.6a). The
fault A is the source area of the 1923 Kanto earthquake [Sato et al., 2005]. The fault B is part of thesource area of the 1703 Kanto earthquake needed for explaining terrace uplift [Shishikura et al, 2004].
While the source area of the 1703 event is sum of the fault A, B and far off the Boso Peninsula [e.g.
Matsuda et al. 1974], the third part is omitted because it gives small contribution to the strong groundmotion in the Kanto region.

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Two inherent asperities are assumed on the fault A (asperities a1 and a2), which is common feature
among inversion source models of the 1923 earthquake [Wald and Sommerville, 1995; Kobayashi and
Koketsu, 2005; Sato et al., 2005]. For the fault B, since no inversion model is available other than
average slip estimated from terrace uplift [Shishikura, 2004], characteristic of the asperity a2 is adopted
to an asperity on the fault B (asperity b) with modification of the total slip and slip direction.
The variable source parameters are the average stress drop (average over the fault), the averagerupture velocity, the hypocenter location, and multi-scale heterogeneities. Variation range of the average
stress drop is set following the empirical relation between magnitude and the rupture area [Murotani etal., 2008]. Variation range of the average rupture velocity is set referring the average and standard
deviation (SD) value for intraplate earthquakes [Yamada et al., 2007, Miyakoshi and Petukhin (2005)].
As for the hypocenter position, there is no reliable data to locate hypocenters of the future Kanto
earthquakes other than the hypocenter of the 1923 Kanto earthquake. Another hypocenter is assumed atsoutheast of the asperity a2 so to cause the largest forward directivity effects to the Kanto region.
Source models for the Off Boso earthquakes
For the Off Boso region, source models are constructed supposing the repetition of the 1677 earthquakeof M8. As no information about exact rupture area or asperity distribution is available, the inverted
source model of the 1923 Kanto, M8, earthquake is used as the initial model; consequently, source
models with two asperities are assumed while the geometry and slip direction are modified as to beappropriate for the source region. As for the location of the source area, at least the source should benorth to the triple junction of the PHS slab, PAC slab and North American plate. Then, as the source
length is comparable to the north-south distance of the unruptured zone, there only remains uncertaintyof the depth of the source area. Here, three source locations are assumed: shallow, middle and deep partof the upper boundary of the PAC slab. The hypocenter location is set so as the forward directivity
effects on the land to become the largest [Sekiguchi et al., 2011b].
Subsurface structure model for Kanto area and computation method
A 3D velocity structure model is constructed combining a 3D Kanto basin velocity model [CDMC,
2004], PHS slab geometry [Ishida, 1992; Sato et al., 2005; Toda et al., 2008; Uchida et al., 2009], PAC
slab geometry [Ishida, 1992; Noguchi, 1998], and Moho discontinuity [Ryoki, 1999]. The sedimentary
structure is extrapolated from the CDMC model to the east using gravity anomaly data (Fig. 6b).
The ground motions are computed with a four-step hybrid technique. We first calculate
low-frequency ground motions at the engineering basement with a 3D finite difference method [Pitarka,
1999]. We then calculate higher-frequency ground motions at the engineering basement using astochastic Green’s function method modified after Onishi and Horike (2000), and combine the lowerand higher-frequency motions using a matched filter. We finally calculate ground motions at the surface
by computing the response of the alluvium-diluvium layers with the equivalent linear method [DYNEQ:
Yoshida and Suetomi, 1996] to the combined motions at the engineering basement.
Computed ground motions on the Kanto basin and their variation
Kanto earthquakes
Among all the combination of the asperities, we found the 1923 Kanto earthquake, namely combinationof asperities a1 and a2, generates close to the largest ground motion, while the strongest is given by thelargest rupture area (al, a2 and b) and east hypocenter (Fig. 7a). For these strong scenarios, intense
ground motions with PGV exceeding 50 cm/s are obtained in the area above the fault plane. Above thesouthern, or shallow, portion of the fault plane, PGV on the surface reaches 200-250 cm/s.
Figure 8 shows comparison of the velocity response spectra at CHB014 and TKY020 between thesimulated Kanto earthquake (combination of al+a2 and a1+a2+b) and observed ground motions during
the 2011 Tohoku M9 earthquake. Predicted ground motions are much stronger than the observation of
the Tohoku earthquake for wide period range.

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(Left) Figure 7. PGV distribution (top) for the hypothetical Kanto earthquake similar to the 1703 eventwith its multi-scale heterogeneous source model (bottom).
(Right): Figure 8. Velocity response spectra of the computed ground motions (red lines) compared withthe records of the 2011 Tohoku M9 earthquake (black lines).

Figure 9. Variation of the PGV at selected site in the Kanto basin due to variation of source parameters.

Figure 9 displays variation of the PGV at multiple sites in the Kanto region (Fig.7) due to all the
combination of the source models considered in this study. The source parameter whose variation gives
largest variation of ground motion is the average stress drop. The average + 1SD of stress drop is twiceof the average and makes PGV almost twice larger. The PGV variation due to rupture area depends
mainly on the activation of the fault A, while influence of the activation of the fault B is limited tosoutheastern Boso Peninsula. The relocation of the hypocenter causes large difference in PGV
distributions in some areas like to the northwest of the fault. On the other hand, the variation of the271average rupture velocity causes small differences in the ground motions. This indicates that the forward
directivity effects are not significant in wide area for low-angle thrust faults, and that in the area justover the source fault, high frequency radiation from just below is dominant.

Off Boso Peninsula earthquakes
The scenario deepest and closest to the land, Boso-f3, generates largest PGV among three Off Boso
earthquake scenarios (Figure 10). PGV is tens of cm/s over the Boso Peninsula reaching 50 cm/s in thesoutheast of the peninsula. Ground motion is not severe but can be strong especially on the Boso
Peninsula.
Figure 11 shows comparison of the velocity response spectra at CHB014 and TKY020 between thesimulated Off Boso earthquakes and observed ground motions during the 2011 Tohoku M9 earthquake.
Though they are much smaller than those predicted for the Kanto earthquake, simulated ground motions
for the Off Boso earthquakes are as strong as the ground motion of the Tohoku earthquake in the Kanto
region at longer period range over several seconds. Note that these results are to be validated in terms ofwave propagation because the shallow subsurface structure of the Off Boso region is just a extrapolationfrom a proposed subsurface model (Figure 6b), still, much precaution should be taken against,
especially, long period ground motion of these earthquakes.

(Left): Figure 10. PGV distributions for three hypothetical Off Boso earthquakes and their multi-scale
heterogeneous source models.
(Right): Figure 11. Velocity response spectra of the computed ground motions (red lines) comparedwith the records of the 2011 Tohoku M9 earthquake (black lines).

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CONCLUSIONS
Ground motions have been simulated in sedimentary basins for anticipated great interplate earthquakes
along the plate boundaries surrounding Japan. Possible multi-scale heterogeneous rupture scenarios
have been modeled combining characteristic properties of the source area and adequate variation insource parameters. Ground motions for anticipated Nankai and Tonankai earthquakes to the Osaka basin,
and those for Kanto earthquakes and Off Boso Peninsula earthquakes to the Kanto basin have been
simulated and their variations been investigated to ascertain importance of considering source processcomplexity and consequent variation in ground motion prediction. Thus, future works should emphasize
on validation of source process modeling for great interplate earthquakes, for which the 2011 Tohoku
M9 earthquake and its large aftershocks might provide precious information.

ACKNOWLEDGMENTS
We thank Dr. Reiji Kobayashi for providing us with the inversion results. We thank the Central Disaster
Management Council for providing us with the subsurface structure models of the Kanto Basin. We arealso grateful to Dr. Nozomu Yoshida, Dr. Masanori Horike, and Dr. Arben Pitarka for providing us withcomputation codes. Observed ground motion records by K-NET NIED are used. Some figures weredrawn using Genetic Mapping Tool v.4 (Wessel and Smith, 1998).

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