High shear velocity layer in the upper mantle of the Western Pacific

Abstract

SHEAR waves have been only rarely observed in explosion seismology studies at sea, so their precise velocities have not been extensively studied. An inversion method from seismic surface wave studies has been used to measure the shear wave velocities of the oceanic upper mantle^1–3. But, the values obtained, generally 4.6 km s⁻¹, are inherently an average of a path from a seismic source to a seismic recording site on land, so that the method can not give a ‘pure’ value for the upper oceanic mantle. Shear wave velocity is, however, important for the physical property of the upper mantle as well as for discriminating the minerals which make up the oceanic upper mantle. We describe here our measurements of apparent shear wave velocities, using 61 natural earthquakes with epicentral distances ranging from 400 to 2,900 km. Results indicate that the lower part of the oceanic lithosphere contains a large amount of garnet.