Seismic-wave attenuation beneath the central Aleutian Arc

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.