Application of an exploration method using microtremor array observations for high resolution surveys of deep geological structures in the Kanto plains

摘要

To investigate the applicability of an exploration method using microtremor array observations for high resolution surveys of deep S-wave velocity structures in the Kanto plains, we carried out long-period microtremor array observations at 61 sites arranged at an interval of about 5 km in the southern part of Saitama prefecture. In this method, phase velocities of the fundamental-mode Rayleigh waves in microtremors are measured from the spatial autocorrelation method and are inverted to S-wave velocity structures by an inversion technique based on genetic algorithm (GA). To estimate detailed 3-D S-wave velocity structure in the whole exploration area, we always referred to the prior information about underground structures for GA inversions at each site; the contents of the information are accurate S-wave velocity profiles shown by the previous report (MATSUOKA et al., 2000), P-wave velocity profiles from past seismic reflection and refraction surveys, and the gravimetric basement.As a result, we revealed the detailed depth distributions of the S-wave velocity layers that correspond to the basement rock at a depth from about 1.0 to 3.4 km and to the sedimentary stratum divided into 9 layers in maximum. The depth distributions were in agreement with geological structures deduced from deep wells and results obtained by geophysical prospecting in this area.