Upper mantle shear-wave velocity structure in the Japan region

abstract The upper mantle shear-wave velocity structure in the Japan region has been determined from S travel times of 101 earthquakes with focal depths varying from 40 to 600 km, using a new analytical method given by Kaila (1969). In southwestern Japan, the S velocity obtained as 4.35 km/sec at a 40-km depth remains almost constant to a depth of about 170 km. The shear velocity in northeastern Japan increases linearly from 4.42 km/sec at a depth of 45 km to 4.62 km/sec at a depth of 145-km. For central Japan, the S velocity determined as 4.41 km/sec at a 40-km depth increases linearly to 4.55 km/sec at a 170-km depth, followed by a slight decrease in the velocity gradient, with velocity still increasing linearly to 4.68 km/sec at a depth of 345 km. At this transition depth, there is a first-order velocity discontinuity, the velocity increasing from 4.68 to 4.92 km/sec. Below this depth, velocity again increases linearly from 4.92 to 5.04 km/sec at a depth of 600 km. The shear velocities at depths between 440 to 640 km in Japan are found to be extremely low in comparison to those of Jeffreys (1939), Gutenberg (1959), and Arnold (1967). These low S velocities can explain satisfactorily the late S arrivals from shallow earthquakes between Δ = 20° to 30° as observed in the Japanese region. Graphs have been drawn to show the variation with depth of Δ*, the epicentral distance to the inflection point, Δ1, Δ2, (Δ2 − Δ1), ptrue =∂T/∂Δ, and αS = (T − pΔ) at the inflection point as obtained from the S-wave travel-time analysis.