P-Wave Velocity Structure in the Upper Mantle beneath the Japanese Islands

Abstract

P-wave velocity structure in the upper mantle beneath the Japanese Islands was studied by improved Gutenberg's method. Improvement of Gutenberg's method was made by fitting a series of segments to a travel-time curve and obtaining the slope dT/dΔ for each segment by the least-squares method. Maximum value was obtained from the curve of dT/dΔ versus epicentral distance for each earthquake, and corresponding velocities at depths in the upper mantle were obtained.
The velocity distribution, obtained from 227 nearby deep earthquakes beneath the Japanese Islands, represents the velocity situation of the descending slab and the upper mantle above it. Average velocities are extremely lower by 0.4km/sec than those under the oceans. Velocity difference between the slab and the upper mantle above it reaches to 0.8km/sec, even to the end depth of the slab. Features of the low-velocity layer and “20°-discontinuity” were clearly obtained. The low-velocity layer beneath the Japanese Islands is characterized by the range of depth from 120 to 170km, velocity contrast of 0.5km/sec, and minimum velocity of 7.5km/sec. “20°-discontinuity” beneath the Japanese Islands is at the depth of 364km and has the velocity jump of 0.4km/sec. Shallowness of the “20°-discontinuity” is consistent with the shallowness of deep seismic activity beneath the Japanese and Kuril Islands arcs.
Extremely low P-wave velocity in the upper mantle beneath the Japanese Islands is consistent with the fact that the shear velocities obtained from the studies of the surface wave dispersion are lower by 0.4km/sec than those under the ocean (KANAMORI, 1970), and that the ratios V_p/V_s are almost constant and equal to 1.77 in the upper mantle (UTSU, 1969).