Periodic Change of Seismic Activity Observed in the Kinugawa Cluster Area, Southwestern Ibaraki Prefecture, Central Japan

Abstract

The seismic activity of the Kinugawa cluster in the southwestern Ibaraki Prefecture, central Japan, shows a clear periodicity of 10 years. It is considered to reflect the average time necessary for the asperities between the Philippine Sea plate and the continental plate to reach the breaking point. Such periodicities, however, are not observed in the activity during the period of 1926 through 1950. This may be due to the inaccuracy of hypocenter determination as earthquakes belonging to the Kinugawa cluster seem not to be well separated from those of the Tsukuba cluster. Another possible cause of that is the influence of the 1923 great Kanto earthquake. Since the mid 1990's is supposed to become an active period, it is expected that several M 4.5-5 earthquakes will occur in the forthcoming few years.