長野県松代における地球潮汐と地震発生の関係

抄録

We investigate the correlation between the occurrence of earthquake and the earth tide by analyzing the tail of the Matsushiro earthquake swarm that is recorded by the Matsushiro Seismic Array System during the period of November 1984 to December 1994. The seismic activity of the tail exhibits many bursts of earthquakes. We remove the bursts, because they affect our aim to investigate whether the correlation exists or not. We use the tidal strain data recorded by EW- and NS-component extensometers located in Matsushiro Seismological Observatory, Japan Meteorological Agency. We apply Schuster's test to the Matsushiro earthquake swarm. The hypothesis that the earthquakes in the swarm take place randomly is tested and is rejected for the EW-component. The earthquakes tend to occur when the tidal strain of the EW-component is in a compressional state. This state is consistent with the stress field at Matsushiro obtained by using focal mechanisms or in situ stress measurements.
We also investigate the spatial variation of the correlation. A significant correlation is seen in the northwestern part of Matsushiro. The largest earthquake with magnitude of 5.2 occurred on 30 December 1986 near the region. We further investigate the temporal variation of the correlation in this region. The correlation becomes high right after the occurrence of the largest earthquake, whereas the correlation is low before the occurrence of it. This suggests that the strain changes caused by the largest earthquake make the correlation high in this region.