Japan Integrated Velocity Structure Model Version 1
Kazuki Koketsu & Hiroe Miyake
Earthquake Research Institute, University of Tokyo, Tokyo, Japan

Haruhiko Suzuki
Engineering Division, OYO Corporation, Tsukuba, JapanSUMMARY:
The Japan islands are in a complex tectonic setting with various subducting plates, and most of their urban areas
are located on sedimentary basins. These lead to three-dimensionally complicated velocity structures, whichcause significant effects on the propagation of seismic waves from an earthquake to the urban areas. Accordingly,
it is important for the simulation of long-period ground motion and its seismic hazard assessment to determine
the three-dimensional (3-D) velocity structure of the whole Japan islands. We have already proposed a standard
procedure for modeling a regional 3-D velocity structure in Japan, simultaneously and sequentially using various
kinds of datasets such as the extensive refraction/reflection experiments, gravity surveys, surface geology,
borehole logging data, microtremor surveys, and earthquake ground motion records (Koketsu et al., 2009). We
then applied this procedure to northeastern and central Japan in 2009, and to southwestern Japan in 2011. We
have now constructed the Version 1 of the Japan Integrated Velocity Structure Model by combining theseregional models. Long-period ground motions from future Tokai, Tonankai, Nankai, and Miyagi-oki earthquakes
and their response spectra were computed by using this model, and hazard maps were published in 2009 and2012 based on the results of the computation.
Keywords: 3D velocity structure, long-period ground motion, seismic hazard map

1. INTRODUCTION
Metropolitan areas are usually located over large-scale sedimentary basins. For example, Tokyo, thecapital city of Japan, and its metropolitan area are located in the largest sedimentary basin of Japan,
which is called the Kanto basin with an area of about 17,000 km2 and the maximum sediment
thickness of about 4 km. The basement rocks are almost exposed in mountain ranges, which surroundthe basin on the west and north sides. The Kanto basin is mostly bounded by the Pacific ocean on theother sides as well, so its velocity structure of the Kanto basin is three-dimensionally complicated.
Most other sedimentary basins in earthquake-prone countries are in similar situation to this for theTokyo metropolitan area (TMA). The damage in TMA itself from the 1923 Kanto earthquake, in
Mexico City from the 1985 Michoacan earthquake, and in the Marina district of San Francisco fromthe 1989 Loma Prieta earthquake has clearly illustrated the risks for population centers located inbasins (e.g., Olsen et al., 1995). The sediments filling the basins amplify ground motions and theirvelocity structures complicate the propagation of seismic waves (e.g., Koketsu and Kikuchi, 2000), soit is important for the prediction of strong ground motion and seismic hazard to determine thethree-dimensional (3-D) velocity structures of these urban basins.
This importance motivated various kinds of explorations carried out in and around the urban basins.
However, a study on a single exploration dataset cannot completely define the 3-D velocity structureof an urban basin. We have to simultaneously or sequentially use several kinds of exploration datasetsfor modeling the 3-D velocity structure of an urban basin, because none of single datasets havesufficient resolving power. It is also necessary to verify a resultant model by seismic waveform studies,
since this modeling is carried out mainly for strong ground motion prediction (e.g., Sato et al., 1999;
Magistrale et al., 2000). In particular, long-period ground motion has become an increasingly
important consideration because of the recent growing number of large-scale structures, such ashigh-rise buildings, oil storage tanks, suspension bridges, off-shore oil drilling platforms, andbase-isolated structures (Koketsu and Miyake, 2008). We can calibrate velocity structure models bycomparison of observed and synthetic dominant periods of spectral ratios and time history waveformsof long-period ground motion (e.g., Suzuki et al., 2005). Based on these experiences, we haveproposed a standard modeling procedure (Koketsu et al., 2009), because extensive geophysicalexperiments and geological investigations have been carried out and velocity structure models arebeing constructed all over Japan. We then applied this procedure to northeastern and central Japan in
2009, and to southwestern Japan in 2011. We have now constructed the version 1 of the Japan
integrated velocity structure model by combining these regional models. Long-period ground motions
from future Tokai, Tonankai, Nankai, and Miyagi-oki earthquakes and their response spectra werecomputed by using this model, and hazard maps were published in 2009 and 2012 based on the resultsof the computation.

2. STANDARD MODELING PROCEDURE
In earthquake engineering and engineering seismology, two kinds of basement (bedrock) are definedin a velocity structure model. ‘Seismic basement (bedrock)’ is usually assigned to the uppermost partof the crust, whose S-wave velocity (VS) is around 3 km/s, while ‘engineering bedrock’ with a VS of
400 to 700 m/s is located just below surface layers. Regions between seismic basement andengineering bedrock greatly influence long-period ground motion, so our modeling procedure mostly
targets these parts of subsurface velocity structures. Models for the crustal structures from the seismic
basement to the Moho discontinuity and the velocity structures of subducting plates must also beincluded to simulate strong ground motions from subduction-zone earthquakes. If there are various
kinds of exploration datasets and observed seismograms in such basins as the Kanto basin, thefollowing standard procedure proposed by Koketsu et al. (2009) is applicable for modeling theirvelocity structures in common.
Step 1: Assume an initial layered model consisting of seismic basement and sedimentary layers fromcomprehensive overview of geological information, borehole data, and exploration results.
Step 2: Assign P-wave velocities to the basement and layers based on the results of refraction andreflection surveys, and borehole logging. Assign S-wave velocities based on the results ofborehole logging, microtremor surveys, spectral-ratio analyses of seismograms, and empirical
relationships between P- and S-wave velocities.
Step 3: Obtain the velocity structure right under engineering bedrock from the results of microtremor
surveys referring to the results of borehole logging, since among 2-D or 3-D surveys only
microtremor surveys are sensitive to shallow velocity distributions and the shapes of shallow
interfaces.
Step 4: Compile data and information on faults and folds. Convert time sections from seismicreflection surveys and borehole logging into depth sections using the P- and S-wave velocities
in Step 2.
Step 5: Determine the shapes of interfaces between the layers and basement by inversions ofgeophysical-survey data (e.g., refraction traveltimes and gravity anomalies). In case of
insufficient data, forward modeling is carried out. The depths of faults and folds in Step 4 areintroduced into the inversions as constraints, or additional data to the forward modeling.
Step 6: Calibrate the P- and S-wave velocities in Step 2 and the interface shapes in Step 5 by inversion
or forward modeling of spectral features of observed seismograms such as dominant periodsof H/V (horizontal/vertical) spectral ratios.

Step 7: Adjust the velocities and interface shapes using inversion or forward modeling of time history
waveforms of observed seismograms.
We start from the result of Steps 1 to 2 calling it a 0th-grade model. This model is then revised into a
0.5th-grade model through Steps 3 to 5, as the final result after Steps 6 to 7 is called a 1st-grade model.
Although sufficient exploration data may not be available except for major metropolitan areas,sufficient observed seismograms are always available for all the important sedimentary basins in Japan,
thanks to the K-NET and KiK-net arrays. In this case, stronger emphasis should be placed on Steps 6
to 7 than on Steps 3 to 5, and this is a reason for calling the result before Step 6 only a 0.5th-grade
model. Earthquake Research Committee (2008) has already adopted this standard procedure as arecipe of constructing velocity structure models for “National Seismic Hazard Maps for Japan (2008).”

3. JAPAN INTEGRATED VELOCITY STRUCTURE MODELFigure 1 shows three target regions of the 1st-grade modeling, where all the steps of the standard
procedure were carried out (e.g., Petukhin et al., 2012). For northern Japan, Earthquake ResearchCommittee (2006) constructed 0.5th-grade models for the National Seismic Hazard Maps for Japan(2005). Therefore, the Japan islands are now covered by the 1st- and 0.5th-grade regional models. We
then combined these regional models into the Version 1 of the “Japan Integrated Velocity Structure
Model.”

Figure 1. Target regions of the 1st-grade modeling by the standard procedure.

Long-period ground motion simulations were performed using this version of the Japan Integrated
Velocity Structure Model and the finite difference schemes of Pitarka (1999) and Hayashi and Hikima(2000). Such indexes as peak ground velocities, response spectra at various periods, and groundmotion duration were retrieved from simulated ground motions, and they were mapped onto the“Long-Period Ground Motion Hazard Maps 2009 and 2012” of the Earthquake Research Committee(Koketsu et al., 2008; Kagawa et al., 2012).

AKCNOWLEDGEMENT
This study was supported by the MEXT Program for Long-Period Ground Motion Hazard Maps andGrants-in-Aid for Scientific Research (A) No. 19201034 from the Japan Society for the Promotion of Science.

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