Earthquakes beneath the central Aleutian Arc have been examined at stations in the Amchitka seismic network, including one station on the active volcanic island of Semisopochnoi [SSI]. Earthquakes between depths of 100 and 250 km, beneath and slightly north of SSI, show strong attenuation of both P and S phases while stations farther south on islands without active volcanoes show no attenuation. Shallow earthquakes [ < 25 km deep] near SSI show no obvious attenuation. The high-attenuation zone may correlate with a zone of low shear velocities beneath the Arc based on surface-wave studies [Jacob and Hamada, 1972] and a low-density zone suggested by gravity models [Grow, 1973]. Although the narrow frequency response of the Amchitka seismic network prevented an absolute measurement of Q by spectral methods, the complete attenuation of P and S phases from the deeper earthquakes, combined with lower than normal shear velocities, suggests that the zone has a Q lower than in a normal aesthenosphere. The low-Q zone does not appear to continue beneath the Bering Sea and is probably restricted to a narrow zone within 20 km north and south of SSI. Its upper boundary is not well defined and could lie anywhere between a depth of 25 and 125 km. The maximum depth of the low-Q zone could extend to the upper surface of the descending Pacific lithosphere, but the attenuation could also be explained by a smaller and more isolated magma chamber at intermediate depths.

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