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A test of foreshock occurrence in the central Aleutian Island Arc

COOPERATIVE INSTITUTE IN ENVIRONMENTAL SCIENCES UNIVERSITY OF COLORADO/NOAA, BOULDER, COLORADO 80309
DEPARTMENT OF GEOLOGICAL SCIENCES UNIVERSITY OF COLORADO, BOULDER, COLORADO 80309

Abstract

A statistical method for estimating the occurrence of foreshocks to larger earthquakes using local seismicity data is applied to the catalog of earthquakes from a local seismograph network near Adak, Alaska. Foreshocks are defined in a restricted sense as earthquakes preceding larger earthquakes within a critical time calculated from the mean time between successive earthquakes in the volume surrounding the target event. The threshold of completeness of the earthquake catalog is estimated as duration magnitude M_d2.2. A low-magnitude cutoff on the catalog of M_d2.5 was used to minimize potential bias from variations in weather and instrumentation condition. About 6 to 10 per cent of 51 main shocks of m_b 4.5 in the Adak thrust zone were preceded by detectable earthquakes satisfying this statistical definition for foreshocks. As many as 14 percent of m_b 5.0 earthquakes may have been preceded by detectable earthquakes satisfying this definition. An estimate of foreshock occurrence using as background the seismicity rate in the 4 months preceding larger earthquakes indicates a slightly higher rate of occurrence. The volume around the target events which showed foreshock activity is used to infer the size of a zone of preparation for larger earthquakes in the Adak area. The data are consistent with a zone of preparation whose size is a linear function of target-event magnitude. The size of the zone of preparation ranges from 20 to 45 km in radius for body-wave magnitudes 4.0 to 5.7.

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