1965 M8.7 Rat Islands Earthquake

Mainshock and Aftershocks: 

Large earthquakes in the Aleutian arc 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 year.

The great Mw=8.7 Rat Islands underthrusting earthquake (larger star on the above map) of February 4, 1965, ruptured a 600-km segment of the western end of the Aleutian Islands (black solid line). The waveform analysis indicates unilateral rupture from east to west and three large pulses of moment release located along the fault (Beck and Christensen, 1991). The Rat Islands event was closely followed by a large tensional outer-rise event of March 30, 1965 (Ms 7.5, smaller star), which was located oceanward of the largest moment release associated with the Rat Islands mainshock rupture. This event may have been triggerred by the large displacement near the mainshock epicenter. The overriding plate along the western Aleutian subduction zone is laterally segmented into a series of rigid tectonic blocks separated by fault controlled canyons and extensional basins. This tectonic setting controlled the rupture behavior of the M8.7 event.

The 1957 Andreanof Islands Mw 8.6 earthquake ruptured an 800 km-long portion of the plate boundary (see map) to the east of the 1965 event rupture.

Tectonic Setting: 

Large earthquakes in the Aleutian arc 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 year.

The great Mw=8.7 Rat Islands underthrusting earthquake (larger star on the above map) of February 4, 1965, ruptured a 600-km segment of the western end of the Aleutian Islands (black solid line). The waveform analysis indicates unilateral rupture from east to west and three large pulses of moment release located along the fault (Beck and Christensen, 1991). The Rat Islands event was closely followed by a large tensional outer-rise event of March 30, 1965 (Ms 7.5, smaller star), which was located oceanward of the largest moment release associated with the Rat Islands mainshock rupture. This event may have been triggerred by the large displacement near the mainshock epicenter. The overriding plate along the western Aleutian subduction zone is laterally segmented into a series of rigid tectonic blocks separated by fault controlled canyons and extensional basins. This tectonic setting controlled the rupture behavior of the M8.7 event.

The 1957 Andreanof Islands Mw 8.6 earthquake ruptured an 800 km-long portion of the plate boundary (see map) to the east of the 1965 event rupture.