AFTERSHOCKS OF THE ANDREANOF-ISLANDS EARTHQUAKE OF JUNE 10, 1996, ANDLOCAL SEISMOTECTONICS

Three questions have been addressed in connection with an Mw = 7.7 ear thquake that occurred in the Andreanof Islands, Alaska, on June 10, 19 96. First, was there any variation in seismic activity rate during the previous five years that might have been diagnostic of the imminent o ccurrence of this event? The answer, based on the USGS PDE catalog, is ''No.'' Second, though there were abundant pre-event earthquakes and aftershocks on both sides of Adak Canyon, why was there very little ac tivity under the canyon itself, a property that has been observed prev iously?. It is not possible to resolve the cause of the behavior with existing seismological data. Finally, how does one explain the eastern group of aftershocks, which fell within the zone of abundant aftersho cks of the 1986 Mw = 8 Andreanof Islands earthquake, when very little of the 1996 mainshock moment was released under or to the east of Adak Canyon? One explanation of these aftershocks is that the recent after shocks are late aftershocks of 1986 whose occurrence within a short ti me was induced by the stress pulse, but not seismic slip, from the 199 6 earthquake. Though large earthquakes in the Aleutians occur in disti nct tectonic blocks, the boundaries of these blocks may not always be defined reliably by aftershock zones.