Three-dimensional velocity structure model of Nara Basin

Abstract

We constructed a three-dimensional (3-D) velocity structure model of the Nara basin, Japan, combining geologic knowledge, geophysical surveys, and seismological observations to contribute to the improvement of ground motion prediction for future earthquakes. Few geophysical surveys have been conducted on the underground structure in the Nara basin. However, geologic studies suggest that the sediments filling the Nara basin and those filling the Osaka basin have commonality. Therefore, we utilized the methods and knowledge acquired through modeling the velocity structure of the Osaka basin to construct a velocity structure model of the Nara basin. First, an isochronous surfaces depth model was constructed by combining the bedrock-depth distribution estimated from gravity anomalies and the geologic profile at a borehole near the center of the Nara basin. Then, the isochronous surfaces depth model, which contained the 3-D structure of depositional age, was converted to a model of the 3-D structure of seismic velocity and density. Microtremor observation results were used to constrain the 3-D velocity structure model with respect to the seismological response of the ground. The ability of the velocity structure model to reproduce ground motion was assessed through a ground motion simulation for a small earthquake.