抄録
In order to investigate the seismicity in and around Suruga Bay, several temporal observation stations with high sensitive seismometers have been set up since June, 1975.
The seismicity is not so simple as to suggest the existence of the sinking Philippine Sea plate in this region. The seismic activity in the northern half of Suruga Bay has been very low throughout the observation period, whereas the activity in the southern half had been low until September, 1976 and then turned out high.
On October 26, 1976, a small earthquake (M≤4) and its aftershocks occurred near Shizuoka City. The foci of these shocks distributed along the western part of the boundary between the seismic and aseismic areas in Suruga Bay, suggesting the existence of a tectonic line. An east-westward principal compressional axis has been suggested from the first motion data of the main shock. This orientation is different from those of major earthquakes in the earlier periods in this region, which show north-southward principal compressional axes.
According to the data by JMA, the Suruga Bay region was active before the Shizuoka earthquake of April, 1965 (M=6.1). The following inactive period lasted until summer in 1976. The subsequent seismicity according to JMA is consistent with our observation. These activity changes seem to be associated with focal mechanism changes of earthquakes in the central Shizuoka prefecture.
