Global study of seismic wave attenuation in the upper mantle behind island arcs using pPwaves

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Observations of striking and consistent differences in the attenuation of pP phases produced by mantle earthquakes and recorded by the World-Wide Standard Seismograph Network (WWSSN) provide data for mapping variations in the attenuation of high-frequency (0.5- to 2-Hz) compressional waves in the wedge of mantle above nearly all of the inclined seismic zones on earth. The data reveal several zones of high attenuation that in nearly all cases correspond to major presently active tectonic features above or near the inclined seismic zones. Zones of high attenuation behind the Tonga, New Hebrides, Mariana, and Japanese island arcs coincide with zones of presently or recently active crustal extension and creation of marginal basins. In western South America a zone of high attenuation underlies most of the broad uplifted terrain of the Andean Altiplano. In contrast to the zones beneath active marginal basins this zone is not related to crustal spreading. A high-attenuation zone exists beneath the Sea of Okhotsk that, although it is near, is offset from the Kuril-Kamchatka arc and thus may not be directly related to that system. This zone may instead be related to possible offshore continuations of the Baykal-Aldan rift system or the postulated plate boundary between Asia and North America. Where they are determined, the high-attenuation zones seem to exist in the upper 200–300 km of the mantle. Wherever data are available, they show that the zones of high compressional wave attenuation correspond to zones of high attenuation for shear waves, low seismic wave velocities, and high heat flow. These properties, in addition to the geological evidence of crustal extension or uplift, suggest that the compressional wave attenuation results from high temperatures and/or partial melting of the upper mantle material. Although all the high-attenuation zones occur near inclined seismic zones that reach depths greater than 300 km, the absence of such zones in the Indonesian, Philippine, Izu-Bonin, and New Britain-Solomon regions shows that lithospheric subduction to great depths is not a sufficient condition for their occurrence. The zones of high attenuation are also not systematically related to the distributions of active volcanos; this condition, together with other data, indicates that large bodies of partially melted material probably do not exist beneath the volcanos of island arcs.