Spatial distribution of shear wave anisotropy in the crust of the southern Hyogo region by borehole observations

Abstract

SUMMAR YIn order to estimate the detailed orientation and density of cracks around an active fault area, we investigate the spatial distribution of shear wave anisotropy in the crust of the southern Hyogo region with high-quality waveform data recorded by a borehole network operated by the Geological Survey of Japan. We introduce the vertically aligned crack model of Hudson (1980Hudson ( , 1981 to explain its anisotropic structure. At stations more than 10 km away from the earthquake fault zone (i.e. Ikeda and Inagawa), crack orientation is nearly consistent with the direction of the regional maximum stress in this area (WNW-ESE). Cracks are distributed at depths below 5 km with a nearly constant crack density, e, of 0.015 at Ikeda, and below 0 km with e=0.009 at Inagawa. These upperlimit depths of crack distribution are roughly comparable with those of microearthquakes there. In contrast, cracks are aligned nearly parallel to the strike of the earthquake fault (NE-SW) at stations at both ends of the fault system (i.e. Takarazuka and Ikuha), and their intensities of anisotropy are three or four times larger than those at Ikeda and Inagawa. At Takarazuka in particular, cracks are distributed at a depth shallower than 4.5 km with a large crack density, e, of 0.06. This result suggests that major stress release by the faulting of the main shock dominates the present stress field at both ends of the fault system. In the middle of the fault system (i.e. Hirabayashi), cracks are aligned parallel to its regional maximum stress and the intensity of anisotropy is twice as large as those at Ikeda and Inagawa. This result contrasts with that of Tadokoro et al. (1999), who determined the crack orientation at a station close to Hirabayashi to be parallel to the fault direction for events in a period earlier than this study by about a year, suggesting that the stress field in this area may have changed over the timescale of a year. Around Inagawa, the distribution of anisotropy seems to have strong azimuthal variations. This result indicates small-scale (of the order of several kilometres) variations of anisotropic structure in the upper crust, probably due to strong heterogeneities in the fluid and/or constitutive materials.

Spatial variation in the crustal anisotropy and its temporal variation associated with a moderate-sized earthquake in the Tokai region, central Japan

“…For a fluid‐saturated crack, we set λ′= 2.25 × 10 9 kg m −1 s −1 and μ′= 0 as Lamé constants for the crack inclusions (e.g. Mizuno et al 2001). We search ɛ in the range 0.0005–0.02 with an increment of 0.0005 and d in the range 0.005–0.2 with an increment of 0.005.…”

Shear wave polarization anisotropy in and around the focal region of the 2005 West off Fukuoka Prefecture earthquake

“…Therefore, it seems unlikely that the origin of anisotropy on Nokono-shima Island is the fractures of shear faults. Mizuno et al (2001) found that the LSPDs change near the end of the fault system, indicating that the LSPDs should change from parallel to the axis of maximum horizontal compressional stress to parallel to the fault strike. However, the LSPD on Nokonoshima Island is not parallel to the fault strike.…”

“…Since crustal anisotropy is caused by stress-induced alignment of vertical microcracks, we can use shear wave splitting as a tool to investigate and monitor the stress beneath the seismic stations. Previous studies (e.g., Tadokoro et al 1999Tadokoro et al , 2002Mizuno et al 2001) investigated the structure of the rupture zone or its temporal changes by analyzing shear wave anisotropy. Saiga et al (2003) found temporal variation Copyright c The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB.…”

A numerical analysis of seismic waves for an anisotropic fault zone

“…This intensive damaging causes velocity reduction and also anisotropy, and in some cases the degree of anisotropy in the fault zone is higher than that of the surrounding rocks (e.g., Mizuno et al, 2001;Watanabe et al, 2001). have found the spatial change of anisotropy in and around the aftershock region from analyzing the shear-wave splitting recorded near the source fault of the 2000 Tottori-ken Seibu earthquake MW6.6.…”