Comprehensive Study of the Seismotectonics of the Eastern Aleutian Arc and Associated Volcanic Systems. Annual Progress Report, April 1, 1985-March 31, 1986

摘要

In the five-week period between October 9 and November 14, 1985, a sequence of 5 moderate-sized events with magnitudes m/sub b/ = 6.4, 5.0, 5.2, 5.0, and 5.6 occurred in the Shumagin Islands seismic network region of the Eastern Aleutian arc. In addition to this cluster of moderate-sized events the microseismicity rate in the Shumagin region has increased by 35 to 80% for at least a 6-months period beginning in April 1985. While we cannot be sure that these unusual seismicity patterns are part of a precursory activity to a large impending Shumagin earthquake, such a possibility must at least be considered. In addition to these observations of recent seismicity in the Eastern Aleutians, we have thoroughly reviewed the entire historic and instrumental seismicity, the tectonics and seismic hazards for most of the Alaska-Aleutian seismic zone, and have redetermined the probabilities for the occurrence of great earthquakes at this active plate boundary for the twenty year period 1985-2005. The aftershock sequence, amount of slip and magnitude Mwgreater than or equal to 8.6) of the great 1957 earthquake in the central Aleutians, have been reanalyzed and its effect on the seismic potential for the Unalaska seismic gap is assessed. We find this questionable gap at present capable to sustain a Mwapprox. =8.6 event. 11 refs., 4 figs. (ERA citation 11:017618)