Temporal changes in coda Q−1 and b value due to the static stress change associated with the 1995 Hyogo‐ken Nanbu earthquake

Abstract

The seismicity in the Tamba region, northeast of the Hyogo‐ken Nanbu earthquake in Japan (January 17, 1995; MJMA 7.2), increased significantly following this earthquake. This increase suggests that the static stress change due to a large earthquake causes a change in the crustal condition or dynamics. In order to reveal the changes quantitatively, we investigate the temporal variation in coda Q−l and b value in the Tamba region. We analyze the waveform data of many shallow microearthquakes (M 1.5–3.0) in the region recorded in a period from 1987 to 1996. Coda Q−1 is estimated in 10 frequency bands in a range of 1.5–24 Hz based on the single isotropic scattering model. At frequencies between 1.5 and 4.0 Hz the temporal variation in coda Q−1 shows significant correlation with the occurrence of the Hyogo‐ken Nanbu earthquake; coda Q−l increases after the event. A variation in b value whose sign is opposite to that of coda Q−1 is recognized. The fracture dimensions of microearthquakes that contribute to the variation in b value are estimated to be 400 m. This scale length is consistent with the characteristic scale length of scatterer, 300–600 m, which contributes effectively to a temporal variation in coda Q−1. The crustal activity in the Tamba region is possibly controlled by the heterogeneity with dominant scale of 102 m. The stress sensitivity of the coda Q−1 change is estimated to be 10 (MPa)−1. This value is an order of magnitude larger than the stress sensitivity of seismic velocity reported before.