1996 M7.9 Adak earthquake
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. 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.
This mainshock was felt with intensity VI at Adak Island, which is situated 73 kilometers (45 miles) northeast of the epicenter, and ayt Atka, situated 237 km (147 miles) northeast of the epicenter. Some damage was reported from these earthquakes, but there were no reports of injuries.
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. 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; 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.
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 mainshock 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.