Aftershocks of the Andreanof Islands Earthquake of June 10, 1996, and local seismotectonics

Three questions have been addressed in connection with an Mw = 7.7 earthquake that occurred in the Andreanof Islands, Alaska, on June 10, 1996. First, was there any variation in seismic activity rate during the previous five years that might have been diagnostic of the imminent occurrence 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 activity under the canyon itself, a property that has been observed previously?. 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 aftershocks 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 aftershocks are late aftershocks of 1986 whose occurrence within a short time was induced by the stress pulse, but not seismic slip, from the 1996 earthquake. Though large earthquakes in the Aleutians occur in distinct tectonic blocks, the boundaries of these blocks may not always be defined reliably by aftershock zones.