A Distorted Distribution of Apparent Velocities Observed With Ultra-sensitive Tripartite Network

Abstract

Several hundred microearthquakes observed with ultra-sensitive tripartite network at Matsushiro are analysed for the purpose of location of their origins. Attentions are drawn to the fact that lower limit of the observed apparent velocities shows a systematic tendency concerning the direction of the wave propagation. It varies broadly from 3.6km/s to over 10km/s. This tendency is interpreted due to the inclination of the surface layer boundary near the observatory. If the lower limit corresponds to the critical case of wave refraction, the expected inclination of the boundary is calculated to be 13 degrees under one of the crustal models proposed by Mikumo et al. (1961). Seventy five percents of analysed shocks are located in the western side of the fossa magna.