June 1996 Adak Earthquakes

At 8:03 p.m. ADT (04:03 6/10 UTC) on Sunday evening, June 9, 1996, a major earthquake occurred in the Andreanof Islands region of Aleutian Islands. This earthquake was felt with intensity VI at Adak Island, which is situated 73 kilometers (45 miles) northeast of the epicenter and Atka, situated 237 km (147 miles) northeast of the epicenter. The magnitude 7.9 earthquake, the largest earthquake to occur in North America in the previous ten years, was preceded by several foreshocks the day before; the largest of which had moment magnitude 6.5. One hundred and ninty-four aftershocks (open circles), magnitude 4.0 or larger, had occurred through the end of June, 1996, the largest of which occurred eleven hours after the main shock and had a magnitude of 7.2. Some damages have been reported from these earthquakes, but no reports of injuries have been received. The main shock generated a tsunami. The following wave heights (peak-to-trough) were recorded at selected tide stations: 102 cm on Atka, 15 cm on Shemya, 12.5 cm at Kodiak and 10.2 cm at Sand Point, Alaska; 46 cm on Midway; 55 cm at Kahului, 38 cm at Hilo, 33 cm at Nawiliwili and 10 cm at Honolulu, Hawaii; 30 cm at Crescent City, California; 10 cm at Port Angeles, Washington. Harvard centroid moment tensor solution indicates thrusting mechanism on a north-northwest dipping plane. Moment tensor solutions for the aftershocks are similar to the main shock.

The earthquakes in the Adak area occur on the convergent boundary between the Pacific and North American crustal plates. This region, where the two plates are being forced directly into one another, is one of the world's most active seismic zones. Over one hundred earthquakes of magnitude seven or larger have occurred along this boundary in the past hundred years. A separate map illustrates the recent earthquakes as circles, superimposed on stippled areas illustrating the rupture zones of major earthquakes this century. Generally speaking, the magnitude of an earthquake is roughly proportional to the area involved in its faulting. Each major rupture is labelled with the earthquake's year and a black dot indicating the epicenter for the associated earthquake. With the exception of the Unalaska and Shumagin seismic gaps, all portions of this plate boundary have ruptured in the past hundred years.

The 1957 Andreanof Islands Mw 8.6 earthquake ruptured an 800 km-long portion of the plate boundary (see map); in 1986 a portion of the western half of this 1957 zone failed again in an earthquake of magnitude 8.0. The 1996 earthquakes appear to have completed re-rupture of the western end of the 1957 zone, immediately westward of the 1986 region.