The Relation between Seismic Activities and Earth Tides in the Case of the Matsushiro Earthquake Swarm

The temporal variations of dilatational strains by solid earth tides were calculated and compared with the variation of seismic activities in the case of the Matsushiro earthquake swarm. It has been found that in the beginning of each stage of the swarm, major earthquake activity has important relation with the expansive peaks of dilatational strains caused by earth tides. The swarm started on August 3, 1965 with a few events a day. Each group of small felt shocks in the first week began around some of the peaks of dilatational strains. There were four periods in which major earthquake activity occurred around the peak of dilatational strains. These correspond to the beginning of new kinds of seismic activities. The first period was the beginning of the Matsushiro earthquake swarm. The second was that of a larger earthquake series. The third was the period in which the epicentral area extended into a wider area. The fourth was the beginning of the expansion of the seismic area, again. In these four periods, the type of the seismic activity which occurred was predominantly a large main shock with aftershocks. It suggests that fracturing was beginning in a new area. Such phenomena show that dilatation causes a decrease in the strength of rocks under the condition of the critical stress level and thus becomes a trigger for the occurrence of earthquakes. Such a synchronization of small events with the dilatational change may be related to the stress level and its changes in the crust