Publication Date:
1977-01-01
Description:
On February 22, 1976, a magnitude 5 thrust earthquake occurred at a depth of 25 km in the subduction zone near the Adak microearthquake network in the central Aleutians. The earthquake was located in a previously identified source volume with maximum dimensions of about 10 km. This source volume falls within a larger seismotectonic feature characterized by a continuous band of seismicity along the arc in the depth range 17-27 km. Within this band there was a period of decreased seismicity over a region 50 km wide, centered on the main shock, beginning about 41/2 months prior to the earthquake. During a 5 week period before the earthquake, six small foreshocks occurred in the smaller source volume. High-resolution hypocenter locations showed a clear alignment of the foreshocks, progressing toward the location of the main event. The line of progression was roughly parallel to the direction of slip in the mainshock. The first and last foreshocks were characterized by thrust focal mechanisms common to earthquakes occurring during ‘normal’ periods. The remaining foreshocks were also thrust mechanisms, but with the principal axes of stress rotated nearly 90° on the basis of observed first motions and S/P amplitude ratios. For these same foreshocks there was also a suggestion of a very small P delay to a station with a ray going through the hypocentral region of the mainshock. © 1977, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.
Print ISSN:
0022-3743
Electronic ISSN:
1884-2305
Topics:
Geosciences
Permalink