Upper mantle P structure on the ocean side of the Japan--Kurile Arc

Abstract

Slowness measurements are made for the first and later arrivals of P waves from about seventy Kurile-Kamchatka earthquakes (13° < Δ < 30°) at the Wakayama Micro-Earthquake Observatory, Japan. The experimental error of dT/dd is not more than 3 per cent and the data points suffice to draw a dT/dΔ curve fairly uniquely. In the distance range 4° < Δ < 15° a travel-time curve, including a later arrival branch, is constructed for a specific Kurile event using the records along the Pacific coasts of the Japanese-Kurile islands. The results are inverted to obtain a velocity model for the upper 800 km of the mantle beneath the trench side of the Japan-Kurile Arc. The model includes a high-velocity lid extending down to 85 km depth. The low-velocity zone is of relatively high speed (8.1 km/s) and is terminated by a high-velocity gradient zone at depths 165-200 km, just below which the velocity is nearly constant with depth. The velocity increases very sharply near 400 km by about 6 per cent. An abrupt change in the slope of velocity occurs near 520 km. A major transition zone in the depth range 630-670 km consists of about 7 per cent increase in velocity and is sharp, especially near the base. A minor transition zone is tentatively suggested to exist around 740 km. Relative arrival times, crossover distances and qualitative amplitude behaviour calculated for this model are consistent with the observed data. Station residuals for the least-squares determination of dT/dΔ closely correlate with the local seismicity.