REGIONALIZED SHEAR-VELOCITY MODELS FOR THE UPPER MANTLE INFERRED FROM SURFACE-WAVE DISPERSION DATA

Abstract

Regionalized shear-velocity models of the upper mantle were inferred from phase and group velocities of mantle Love and Rayleigh waves. Im-portant conclusions are: (1) Major shear-velocity differences among the dif-ferent tectonic regions exist at depths less than 300km. (2) Shear velocities in the upper mantle are markedly higher for the shield areas and lower for the tectonic areas than for the oceanic areas. The velocity difference between the shield and tectonic models is approximately 10% at depths less than 100 km, decreasing gradually to 5% at a depth of 200km. (3) The shield data do not require a pronounced low-velocity channel in the upper mantle, as does the oceanic model, but only a slight low-velocity channel if the Sn velocity is 4.6km/sec. (4) The tectonic data do not necessarily require the presence of a high-velocity lid just beneath the M-discontinuity. (5) Unlike the other two regions, the oceanic data clearly require a pronounced low-velocity channel. (6) The dispersion of Love and Rayleigh waves for the tectonic regions can be explained more easily by different shear-velocity structures rather than by a single isotropic-layered structure. The shear velocities inferred from Love waves are higher by 0.2km/sec than those inferred from Rayleigh waves, the velocity differences being concentrated at depths from 150 to 300km. This discrepancy suggests anisotropy or an equivalent laminar-melting structure, or elliptical magma pockets.
The phase-velocity data were taken from Kanamori's paper. The group velocities were recently determined by Hamada for moderate-size earthquakes recorded at LASA (Large Aperture Seismic Array in Montana) and at a deep mine observatory in Ogdensburg New Jersey.