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Dynamic stress drops of moderate earthquakes of the Eastern Aleutians and their relation to a great earthquake

LAMONT-DOHERTY GEOLOGICAL OBSERVATORY OF COLUMBIA UNIVERSITY, PALISADES, NEW YORK 10964

Abstract

Short-period WWSSN records are used to calculate dynamic stress drops of moderate earthquakes (m_b = 5.5 to 6.1) in the Eastern Aleutians, Alaska. For short-period pulses where scattering effects often distort the pulse shape, estimates of dynamic stress drop using the formula of Boatwright (1980) have fewer sources of uncertainty than does the formula for static stress drop given by Brune (1970, 1971). Earthquakes of several tectonic settings within the subduction zone were studied: shallow (5 to 8 km) normal faulting events near the trench; earthquakes in the main thrust zone (30- to 50-km depth), and intermediate-depth (100 to 200 km) earthquakes in the Benioff zone. Dynamic stress drops of the intermediate-depth earthquakes ranged from 20 to 70 bars while the stress drop of earthquakes in the main thrust zone varied over a considerably larger range, 10 to 300 bars. The two normal events had stress drops around 100 bars. Evidence from the great 1964 Alaska earthquake and its aftershocks also suggests an inhomogeneous stress field in the main thrust zone which may produce large amounts of high-frequency radiation when ruptured by a large earthquake.

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