Rupture Processes of the 1987-1988 Gulf of Alaska Earthquake Sequence

Three large earthquakes occurred in the Gulf of Alask• during 1987-1988: one on November 17, 1987 (Mw = 7.2), the second one on November 30, 1987 (Mw = 7.8), and the third one on March 6, 1988 (Mw = 7.7). These major intraplate, strike-slip oceanic events occurred along conjugate trends in a region of no previous seismicity. Body wave modeling places their centroid depths in the upper mantle. The depth of the first two events is approximately 20 km and of the third is 15 km. As compared to other large events, the body wave models for the two largest earthquakes have short rupture lengths, 110 km and 40 km, respectively, and short duration times, 36 and 20 s, respectively, for their magnitude. The latter of the two events has a rupture length much shorter than that inferred from •ftershock seismicity. The short rupture lengths and source durations may reflect differences between the strength of oceanic and continental lithosphere and suggest that events in oceanic lithosphere have a higher moment release per unit area than similar continental events. Most of the moment release occurred near the epicenter and/or regions of apparent structural complexities where seismicity trends intersect.