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.
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