Source Parameters Of the May 7, 1986 Andreanof Islands Earthquake

Abstract

Source characteristics of the May 7, 1986 Andreanof Islands earthquake (51.412°N, 174.830°W, NEIC) are investigated from WWSSN, GDSN and IDA records. First motions from over 60 stations determine one steeply dipping nodal plane. We constrained this nodal plane and inverted long-period surface waves at a period of T=256 sec and determined the second nodal plane to be dip 18°, rake 116°, and strike 257°. This shallowly dipping thrust mechanism is consistent with plate motions in this region. Seismic moment from surface-wave inversion is 1.3×10^(28) dyne-cm corresponding to M_w=8.0. Amplitudes of body and surface waves from short-period instruments yield magnitudes of equation image and M_s=7.7. The teleseismic average P-wave moment rate spectrum from 17 short- and intermediate-period instruments is slightly lower than that of an average M_w=8.0 subduction-zone event. We constrained the fault plane as determined above to deconvolve the first 90 secs of the long-period body wave at 11 teleseismic stations to determine the source time function and the spatial distribution of moment release. The source time function consists of 4 moment-releasing episodes which have a total moment release of 9.4×10^(27) dyne-cm. The fault ruptured bilaterally with the largest moment releasing subevent occurring between 30-45 sec. This subevent nucleates approximately 75-90 km west of the determined epicenter. This region corresponds to the epicentral area of the 1957 Great Aleutian earthquake which is one of the largest earthquakes in recorded history.