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The St. Elias, Alaska, earthquake of February 28, 1979: Regional recording of aftershocks and short-term, pre-earthquake seismicity

U.S. GEOLOGICAL SURVEY, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025
EARTH PHYSICS BRANCH DEPARTMENT OF ENERGY, MINES AND RESOURCES, OTTAWA, Canada, K1A OY3

Abstract

The St. Elias, Alaska, earthquake (M_s 7.1) of February 28, 1979 occurred beneath the Chugach and St. Elias Mountains of southeastern Alaska and southwestern Yukon Territory. The main shock and aftershocks were recorded at regional high-gain, high-frequency seismographs operated by the U.S. and Canada. Hypocenters and magnitudes are presented for 308 aftershocks that occurred between February 28 and March 31, 1979. These data contain a nearly complete record of events of magnitude 3.5 and larger starting about 20 min after the main shock. The largest aftershock has a poorly determined magnitude slightly above 5, and the frequency-magnitude distribution has a b value of 1.36. A t^–p inverse power law with an unusually low value of 0.93 for p adequately describes the decay with time in the frequency of occurrence of large aftershocks. The aftershocks occurred in a broad zone that extends about 115 km southeast from the epicenter of the main shock. Events tend to form clusters within this zone. One of the most remarkable features in the distribution of epicenters is that relatively few aftershocks were located near the epicenter of the main shock, and the highest rate of activity was centered about 50 km southeast of the epicenter of the main shock. Within the accuracy of the data, the depths of the aftershocks are all less than about 20 km. In the few areas where good depth control is available, the seismicity appears not to extend to the Earth's surface. Additional data from temporary stations operated in the aftershock zone during July and August 1979 indicate that the seismicity in some areas may be confined to a zone less than 6 km in vertical thickness. Focal mechanisms determined from p-wave first motions for some of the larger aftershocks all indicate northward-directed compression, which is consistent with the focal mechanism of the main shock.

A review of the regional seismicity during the 6-month period preceding the St. Elias earthquake indicates that, relative to a comparable 6-month period 1 yr earlier, there was a 45 per cent increase in the rate of activity for events of magnitude 1.8 and larger, and possibly a decrease in the b value during the same period. Also, a prominent cluster of events with magnitudes less than 3.3 occurred at the southeast corner of the aftershock zone during the 6 months prior to the earthquake. The seismic record from the USGS network since 1974 is not yet complete in time, so it is not possible to determine how unusual the seismic activity preceding this earthquake has been.

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