Historic Earthquakes: Tectonic Summary

Magnitude 7.8 RAT ISLANDS, ALEUTIAN ISLANDS, ALASKA
2003 November 17 06:43:07 UTC

EARTHQUAKES IN THE RAT ISLANDS
The region surrounding the Rat and Buldir Islands portion of the Aleutian arc is highly seismic. The largest earthquake known to have affected this region was the magnitude 8.7 event of February 1965, which was one of the largest earthquakes of the twentieth century. The 1965 earthquake ruptured an approximately 400 kilometer-long patch on the Pacific-North American plate boundary, and generated a tsunami that was 11 meters high in the Aleutians. Despite the large magnitude of the 1965 earthquake, there were no fatalities and only minor damage and flooding due to the sparseness in population in the epicentral area.

TECTONICS
The Rat and Buldir Islands are part of a volcanic island arc formed over the past 55 million years by magma generated from the subduction of the Pacific plate beneath the North American plate. The Pacific plate thrusts northwestward beneath the North American plate at the Aleutian trench at a rate of about 7 cm/year. The forces generated by the motion and interaction between the two plates leads to a build up of stress within the surrounding rock. An earthquake occurs when some of this stress is suddenly released, causing rapid movement along a fault.

Like the magnitude 8.7 earthquake of 1965, this recent earthquake occurred as the result of thrust faulting on the interface between the North American and Pacific plates. The thrust interface is active to depths of about 40 km. Other types of quakes occur in the region. Beneath the thrust interface, for example, earthquakes occur within the descending and deforming Pacific plate to depths of about 300 km. Closer to the surface and seaward of the Aleutian trench, earthquakes occur as a result of stresses generated in the Pacific plate as it bends beneath the continental margin. Shallow crustal earthquakes also occur in the overriding North American plate as a result of relative plate motion that is not accommodated by slip on the plate interface.

Small tsunamis generated by this earthquake were observed at Shemya and Adak. However, no tsunami damage has been reported. Earthquakes generate tsunamis when the sea floor near the epicenter abruptly deforms and displaces the overlying water as the result of earthquake faulting or undersea landslides generated by the shaking. Great thrust-fault earthquakes are particularly effective at generating large tsunamis, because they produce large vertical deformations of the Earth's crust. The Aleutian Islands have a high potential for generating destructive local and Pacific-wide tsunamis because of the high number of large magnitude earthquakes in the region, most of which are due to thrust faulting.