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Identification of a Possible Seismic Gap Near Unalaska Island, Eastern Aleutians, Alaska

2011; American Geophysical Union; Linguagem: Inglês

10.1029/me004p0081

2329-0307

Leigh House, Lynn R. Sykes, John N. Davies, Klaus Jacob,

High-pressure geophysics and materials

A portion of the eastern Aleutians, between about 168°W and 160°W, appears to be a major tectonic transition between the oceanic arc structure to the west and the continental arc structure to the east. The 1200 km long aftershock zone of the 1957 Andreanof-Fox Islands earthquake, Mw = 9.1, extends 200 km into the western portion of this zone. This 200 km long segment, which is located near Unalaska Island, underwent deformation quite different from that of the remaining 1000 km long zone. It experienced aftershocks along only a narrow (20 km width) zone at its arcward (northern) edge, in remarkable contrast to the 80 km width of the aftershock zone to the west. Since 1957, the interior of this segment has produced only two earthquakes of magnitude 5 or larger. Two clusters of events occurred prior to the 1957 main shock, one near the western end of the entire aftershock zone (180°W), the other, a more dense cluster, occurred near the western edge of the Unalaska segment (168°W). Travel times of the 1957 tsunami to tide gauges in western North and South America indicate that the eastern extent of the tsunami generating area was situated at or near the western boundary of the Unalaska segment. If the Unalaska segment slipped in 1957 it must either have undergone delayed rupture, so that seismic and tsunami energy were concealed in the coda of the main shock, or have slipped in a rupture process so slow as to be an inefficient tsunami source. Another possibility clearly exists, however, namely that the Unalaska segment did not rupture in 1957. If it did not, the Unalaska segment is a seismic gap, and could also have a high potential for producing an earthquake as large as magnitude 8. This possibility has serious implications for the evaluation of seismic and tsunami hazards in the eastern Aleutians.