丹波と丹波西方域激び和歌山付近の地震活動の相関と兵庫県南部地震

抄録

Correlation between seismic activities in the focal region of the Hyogo-ken Nanbu earthquake and in the surrounding regions were investigated. We found that seismic activity in the Tanba region had been well correlated with that of the Wakayama region, but the correlation has disappeared in 1990s. Activity to the west of Tanba region shows a tendency to be delayed to the activity in the Tanba region and this tendency is concordant with the f eature shown by Yoshida and Takayama (1992) that activity in the Mino region is advanced a few years against the activities in the Tanba and Wakayama regions. Such seismic correlations with time lags between surrounding areas of the Kinki triangle are considered to mean that tectonic stress in that district has a tendency to propagate or diffuse to the west at the velocity of 10 km/year. We present some evidence which decrease of the seismicity in the Tanba region in 1992-1993 period and increase in 1994 might be precursory changes for the Hyogoken Nanbu earthquake. Just after the Hyogo-ken Nanbu earthquake seismic activity in the Tanba region increased remarkably and an activity of small earthquakes was observed near the Yamasaki fault located to the west of the Hyogo-ken Nanbu earthquake. However, no unusual change of seismicity was seen in the Wakayama active region. Because a disastrous intraplate earthquake is likely to be followed by another large earthquake, and the most probable site of the succeeding large earthquake is the place where seismicity becomes active after the first earthquake (Yoshida and Ito, 1995), the Tanba region and its surrounding areas should be watched intensively in the following several to ten years.