Abstract

Five earthquakes (M > 4.0) that occurred near Swift Reservoir on the southern St. Helens seismic zone (SHZ) in southwestern Washington in 1960 and 1961 were located using a master-event technique. These five earthquakes include the 17 September 1961 Siouxon Peak earthquake of magnitude 5.1 (ML) as well as the Swift Creek earthquake on 11 October 1960 (Mcoda = 4.4). The Siouxon Peak earthquake focal mechanism exhibits right-lateral strike-slip faulting on a north-south fault plane and is very similar to focal mechanisms calculated for recent earthquakes along the SHZ. The sense of motion for the more poorly determined Swift Creek earthquake focal mechanism is consistent with either left-lateral strike-slip on a northerly striking fault plane or thrust, both of which are anomalous for southwestern Washington. Based on first-motion differences in polarity (which results in a different focal mechanism) and the temporal separation between the Swift Creek earthquake and a swarm that preceded the Siouxon Peak earthquake, we have divided the earthquakes near Swift Reservoir into two sequences: the Swift Creek earthquake sequence beginning in early 1959 and culminating in the October 1960 Swift Creek earthquake, and the Siouxon Peak sequence that began in January 1961 and included the Siouxon Peak main shock 8 months later.

The temporal and spatial pattern for the 1961 Siouxon Peak sequence agrees well both with that predicted by an asperity model and that observed for the 1980 to 1981 Elk Lake earthquake sequence along the SHZ about 35 km to the north. The temporal similarities of these two sequences coupled with their similar moment release suggest an empirical model which may permit forecasting the next moderate earthquake along the SHZ. The Swift Creek activity with its unusual (for southwestern Washington) focal mechanism may have been induced by the impounding of the nearby Swift Reservoir.

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