The 1981 earthquake swarm off the Kii Peninsula observed by the ocean bottom seismometer array.

Abstract

An earthquake swarm that occurred east of the Kii Peninsula near the Nankai trough was investigated using seismic data from four pop-up type OBSs, four telemetering OBSs, and one land seismic observation station. The addition of the pop-up type OBSs was most effective in the analysis of the swarm. We analysed the daily frequency of earthquakes, the b-value, S-P time distribution, and hypocenter distribution. Here we discuss the swarm activity with regard to recent seismicity around the area, to submarine active tectonic lines, to oceanic topography, and to block boundaries along the Nankai trough. The swarm activity was largely divided into two periods, and seemed to consist of six or seven sequences of foreshocks-mainshock-aftershocks or mainshock-aftershocks. The earlier activity occurred in a deeper region (at a depth of about 17km), the later one in a shallower region (about 5km). The epicenter was distributed in a region no larger than 18km×8km. The b-value seems to have decreased before the largest earthquake. The swarm seems to have occurred at a submarine active tectonic line near the Nankai trough.