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

<jats:p>The seismicity in the Tamba region, northeast of the Hyogo‐ken Nanbu earthquake in Japan (January 17, 1995; <jats:italic>M</jats:italic><jats:sub>JMA</jats:sub> 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 <jats:italic>Q</jats:italic><jats:sup>−l</jats:sup> and <jats:italic>b</jats:italic> value in the Tamba region. We analyze the waveform data of many shallow microearthquakes (<jats:italic>M</jats:italic> 1.5–3.0) in the region recorded in a period from 1987 to 1996. Coda <jats:italic>Q</jats:italic><jats:sup>−1</jats:sup> 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 <jats:italic>Q</jats:italic><jats:sup>−1</jats:sup> shows significant correlation with the occurrence of the Hyogo‐ken Nanbu earthquake; coda <jats:italic>Q</jats:italic><jats:sup>−l</jats:sup> increases after the event. A variation in <jats:italic>b</jats:italic> value whose sign is opposite to that of coda <jats:italic>Q</jats:italic><jats:sup>−1</jats:sup> is recognized. The fracture dimensions of microearthquakes that contribute to the variation in <jats:italic>b</jats:italic> 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 <jats:italic>Q</jats:italic><jats:sup>−1</jats:sup>. The crustal activity in the Tamba region is possibly controlled by the heterogeneity with dominant scale of 10<jats:sup>2</jats:sup> m. The stress sensitivity of the coda <jats:italic>Q</jats:italic><jats:sup>−1</jats:sup> change is estimated to be 10 (MPa)<jats:sup>−1</jats:sup>. This value is an order of magnitude larger than the stress sensitivity of seismic velocity reported before.</jats:p>