Velocities of earthquake-generated Pn waves in Japan measured by the time-term method

Velocities of upper-mantle P waves (P_n) generated by large shallow earthquakes in the Japan area have been studied by the time-term method in three areas of the Japanese islands. Data reported from routinely recording seismic stations, grouped to act as elongated arrays, were used. The velocities observed correspond to the P-wave velocity in the top layer of the mantle. For a region in southwestern Japan not spanned by active volcanoes the value obtained was 8.0 km/s, while for central Japan and on northern Honshu Island the corresponding values were as low as 7.6–7.7 km/s. After allowance for the influence of systematic earthquake mislocation, the probable error of the measurements is estimated not to exceed 0.1 km/s. The differences in P-wave velocity are attributable to the regional variation of subcrustal temperature in Japan reported by Uyeda and Horai, with high temperatures and low velocities in the volcanic zone.The time terms tend to be larger on the Japan Sea side than on the Pacific side of the Japanese islands. We suggest that this is due to an increase in P velocity in the mantle towards the Pacific, perpendicular to our profiles of measurement.