Modeling of Subsurface Velocity Structures from Seismic Bedrock to Ground Surface in the Tokai Region, Japan, for Broadband Strong Ground Motion Prediction

Abstract

For sophistication of strong ground motion prediction in terms of disaster mitigation, one of the principal issues is to model subsurface velocity structures so that characteristics of earthquake ground motions can be reproduced in the broadband range 0.1 Hz to 10 Hz. In recent years, subsurface structures have been modeled in sedimentary layers on seismic bedrock for a few regions of Japan, in a national project. In this study, subsurface velocity structures were modeled from seismic bedrock to the ground surface for the Tokai region. These models were constructed in accordance with the subsurface velocity structure modeling scheme published by the Headquarters for Earthquake Research Promotion. To begin with, initial models were constructed based on existing bore-hole data, geological information, etc. Next, they were improved based on results of microtremor explorations which had been conducted in recent years. It was found that the new model had different characteristics to the conventional model. This paper will present the modeling process and characteristics of distribution maps for velocity structures and amplification index.

S-Wave Site Amplification Factors from Observed Ground Motions in Japan: Validation of Delineated Velocity Structures and Proposal for Empirical Correction

“…To that end, the National Research Institute for Earth Science and Disaster Resilience (NIED) has developed a unified velocity model (UVM) by integrating shallower-and deeper-parts of the structures above the seismological bedrock in the Kanto and Tokai regions [16][17][18]. The procedure to develop the model was based on the "concept of creating a subsurface structure model" released by the Observed horizontal site amplification factor, hSAF, extracted from strong motions at K-NET, KiK-net, and JMA Shindokei network by GIT after [7].…”

“…Examples of the important features of the resultant UVM are shown in Figures 2 and 3. Figure 2 shows the depth to the seismological bedrock with the Vs of 3.1 km/s (Z3.1) in the Tokai region, while Figure 3 shows the depth to the engineering bedrock with the Vs of 0.35 km/s (Z0.35), which is the interface between the shallower-and deeper-parts of the UVM in the Tokai region [18]. A similar map can be seen for the Kanto region in Senna et al [16,17].…”

“…The major reasons for discrepancy are twofold; one is due to an inevitable inaccuracy of the derived velocity structure, and the other is due to a too simplistic assumption of the 1D horizontally-flat layered model. In the former there are two possible causes; one is the inaccuracy of the referenced values to delineate the Assumed layer profiles for the deeper part of UVM after [18].…”

“…However, other than the contributions of basin-induced surface waves through the modeling of WSR on top of hSAF as proposed by Nakano [8], it is difficult to account for the physical cause of the discrepancy between observation and theory at every sites as seen in Figure 4. We have been attempting to fill the gap Observed hSAF extracted by GIT and 1D theoretical hSAF from UVM after [18]. Fundamental characteristics are well reproduced, however, the theoretical hSAF tends to underestimate the observed hSAF.…”

“…Therefore, we decide to use a direct spatial interpolation scheme as Nakano [8] proposed for WSR. In this scheme, we employ first GMT's "surface" function [31,32] in which the curvature minimization scheme is used together with the Modified hSAF (HSAF mod ) by using FMR and AMR in comparison with observed hSAF (HSAF obs ) extracted by GIT and 1D theoretical hSAF (HSAF the ) from the velocity structure taken from UVM after [18] at the same sites in Figures 1 and 4. The optimal values of FMR and AMR for each site are shown in the upper-right corner.…”