Abstract
There were significant changes in seismicity preceding a large earthquake which occurred off Shikotan Is., one of the Kuril Is., on 4 October 1994. The 1994 Hokkaido-Toho-Oki earthquake, m = 8.1, was an intra-plate event that ruptured through a substantial part of the subducting oceanic lithosphere. It is the largest event in a data set of 3, 714 earthquakes beneath the southern corner of the Kuril Is., in the latitude range 42.5°N to 44.5°E, longitude range 145.5°E 148.5°E during the period January 1984 through 4 October 1994 from the digital network run by the Research Center for Earthquake Prediction (RCEP) of Hokkaido University. We applied to this data set an objective monitoring procedure that was developed for investigating the changes in seismicity preceding a large earthquake. We found a seismic quiescence of a period of 3 years for events of magnitudes ≥ 3.5 preceding the main shock, in the vicinity, but not including the immediate epicentral area. The magnitude dependence of the precursory seismicity patterns may come from a departure from self-similarity, one of the currently prevalent assumptions in earthquake source models. The usual assumption that quiescence should be most evident for small magnitudes of <3.0 is not appropriate here. While it is likely that the RCEP catalog is not complete for events as small as m=2, the detectability threshold of the network was constant for the period studied, and so both the lack of change in the seismicity of smaller events and the quiescence for large (m >3.5) events can be confidently assumed to be real. This procedure for alerting one to the possible existence of a developing anomaly has potential value as a general monitoring tool.
