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-1 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-1 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. ¿ 2000 American Geophysical Union