Abstract
During the 1990-1995 eruption of Unzen Volcano (Fugendake), SW Japan, seismicity near the crater intensified in association with the growth of a dacite lava dome. In this paper, we examined the seismicity recorded there between November 1993 and January 1994, the period when remarkable inflation occurred in the SW fiank of the dome and the upper southern flank of Fugendake. The seismicity in this period was characterized by an increase in the number of high-frequency earthquakes and a gradual increase with time in the maximum magnitude of the earthquakes. On the basis of cross-correlation coefficients, we identified 70 earthquake groups, each having more than 100 earthquakes. This categorization expanded the definition of the well-known earthquake family; the waveforms in each group varied slightly over time. From among the 70, we then analyzed 20 major groups. The cross-correlation of waveforms in each group was higher for the earthquakes that occurred at shorter intervals. In many groups, the maximum magnitudes of earthquakes changed continuously with time, and the frequency distribution of magnitudes became largest near the maximum magnitude at that time. New earthquake groups were activated just after the activity of previous earthquake groups had declined or ceased. Such transitions had often been linked to changes in the rate of ground deformation on the upper southern flank of Fugendake, suggesting that the manner of magma movement and the stress field had changed significantly in and around the shallow conduit just beneath the lava dome.
