A seismic gap past and future



A new analysis of data from a seismically quiet section of the Alaska-Aleutian arc called the Shumagin gap suggests that a large earthquake may be likely to occur there in the next 20 years.

The Aleutian archipelago trails off the end of the Alaska Peninsula like the dots of an ellipsis and lies above one of the most seismically active subduction zones in the world. Some of the largest earthquakes ever recorded have occurred along the arc, which extends from near Kamchatka in the Soviet Union to mainland Alaska and coastal British Columbia (Canada). Earthquakes with magnitudes as large as 9.2 and their aftershocks have caused ruptures along most of the length of t h e Aleutians in the last half-century. However, some segments of the arc, including areas around Komandorski Island in the Soviet Union, Unalaska Island, and the Shumagin Islands south of the Alaska Peninsula (see figure) show historical evidence of great (Ms⩾7.75) quakes in the past but have not ruptured completely during the 20th century. These zones are called seismic gaps and have been proposed as likely sites of future great earthquakes.