Precise seismometric observations in the epicentral region of local shocks

摘要

The precise observations of local earthquakes were carried out in Wakayama District with many sets of high magnification seismometer, for the purpose of studying the crustal structure and the nature of local shocks. The travel-time anomalies were detected on the time-distance curves when the focus was determined in either case, both from the arrival times and from the P-S times at each station. The anomalous travel- time curves were separable into two groups which seemed to be related with the recorded directions of initial motion of the local shocks at their respective stations. These anomalies may safely be regarded as being attributed to the nature of hypocentre and mechanism of earthquake occurrence. With regard to the structure of the crust in the district concerned under the assumption of horizontal parallel layers, the velocities in the surface layer, second, and third layers were estimated to be 4.3, 5.4-5.6 and 6.0 km/sec for the P-waves and 2.5, 3.2 and 3.4 km/sec for the S-waves, and the thickness of surface layer about 4 km, respectively.