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

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.

Spatial variation in coda Q and stressing rate around the Atotsugawa fault zone in a high strain rate zone, central Japan

“…Historically, large earthquakes have occurred in this zone. This zone is also characterized seismically by a low velocity in the lower crust (Nakajima and Hasegawa, Copyright Matsubara et al, 2008), low coda Q at lower frequencies (Jin and Aki, 2005), and a stress-induced shear wave polarization anisotropy (Hiramatsu et al, 2010). Jin and Aki (2005) showed that low coda Q for 1-2 Hz and 2-4 Hz frequency bands corresponded spatially to the high strain rate zone from an analysis of Hi-net data.…”

“…from Coda Q Hiramatsu et al (2010) have proposed a new method to estimate a spatial variation in the stressing rate in the crust using a spatial variation in shear wave splitting or coda Q. They obtained a variation in the stressing rate of 3±0.6 kPa/year in the brittle upper crust, and that of 13±3.5 kPa/year in the ductile part below the brittle-ductile transition zone in the crust, in the NKTZ.…”

“…stress change, and T C is the characteristic time of the stress accumulation in the crust (Hiramatsu et al, 2010). This formulation is based on the following two ideas: a constant stressing rate is approximated by a successive stepwise stress change with a constant time interval, and a variation in the condition of cracks attributable to the step-wise stress change recovers to a steady-state during the constant time interval.…”

“…This formulation is based on the following two ideas: a constant stressing rate is approximated by a successive stepwise stress change with a constant time interval, and a variation in the condition of cracks attributable to the step-wise stress change recovers to a steady-state during the constant time interval. In other words, a steady state crack condition, which is represented by the value of coda Q, is controlled by the amount of the step-wise stress change and the constant time interval, which is the stressing rate (Hiramatsu et al, 2010). Therefore, we can estimateσ from Q C /Q C .…”

Numerical Study on a Relationship between Scattering Seismic Wave and Crustal Stress Filed, and its Possibility of Application to Field Data

“…Recently, Hiramatsu et al (2010) 1) stressed that coda-Q (denoted as Q c , hereafter) is a parameter of the differential strain rate, which relates with the stressing rate of shear stress, from the observation of the Q c in the vicinity of the 1995 Southern Hyogo Prefecture earthquake before and after the main shock event. Sugaya et al (2009) 2) discussed a change in Q c after the 1995 Southern Hyogo Prefecture earthquake in connection with a property change in the crust, particularly in the lower crust.…”