Spatial variation in shear wave splitting of the upper crust in the zone of inland high strain rate, central Japan

Yoshihiro Hiramatsu,Shingo Ueyama,Koichi Iwatsuki,Takashi Iidaka

doi:10.5047/eps.2010.08.003

Abstract

We investigate a detailed spatial variation in shear wave splitting in the zone of inland high strain rate, called the Niigata-Kobe Tectonic Zone (NKTZ), central Japan. Most observations show stress induced anisotropy, that is, the orientation of the faster polarized shear wave is parallel to the axis of the maximum horizontal compressional strain rate estimated from GPS data. Others show structure induced anisotropy, that is, the orientation is parallel to the strike of active faults. For the stress induced anisotropy, time delays normalized by the path length in the anisotropic upper crust is proportional to the differential strain rate. We estimate a spatial variation in stressing rate of the upper crust beneath the high strain rate zone based on a response of the normalized time delay to a step-wise stress change caused by a moderate-sized earthquake. The variation in the stressing rate of 3 kPa/year estimated from shear wave splitting is coincident with that from GPS data. We conclude, together with other seismological features in the NKTZ reported previously, that the high strain rate in the NKTZ is attributed to the high deformation rate below the brittle-ductile transition zone in the crust.

Highlights

The accumulation process of stress on faults is one of the most important issues to understand the generation process of earthquakes
To understand the state of the stress in the high strain rate zone, called the Niigata-Kobe Tectonic Zone (NKTZ), and the cause of the NKTZ, we investigate a detailed spatial variation in shear wave splitting in the crust using waveform data at dense seismic stations
We find that the orientation of the faster polarized shear wave is parallel to the axis of the maximum horizontal compressional strain rate estimated from GPS data at most stations

Summary

Introduction

The accumulation process of stress on faults is one of the most important issues to understand the generation process of earthquakes. In central Japan, Sagiya et al (2000) reported a concentration zone of high strain rate from Niigata to Kobe, called the Niigata Kobe Tectonic Zone (NKTZ), from a dense network of GPS observation (Fig. 1). Provides useful information to reveal the cause of the high strain rate zone. We report here a spatial distribution of shear wave splitting in and around the NKTZ to investigate the stress condition in the upper crust and show that shear wave splitting is controlled by the strain rate. We obtain a total of 501 shear wave splitting measurements at 67 stations from 477 events (Fig. 2 and Fig. 5)

Spatial Variations in the Splitting Parameters in and around the NKTZ

Discussion

Conclusions