Simultaneous determination of P and S velocities of the crust and the sub-Moho mantle with earthquake hypocenters in central Honshu,Japan.

Abstract

P and S arrival times of crustal and subcrustal earthquakes, obtained from three-component wide-band seismographs of the 10-station telemetered network of Nagoya University, were inverted by a damped least squares method to obtain the P and S velocity structure of the and the in the Chubu district, Honshu, Japan. The hypocentral parameters and the P and S station anomalies were determined simultaneously. The solutions showed satisfactory stability and resolution. The and the were divided into four layers, based on results of refraction studies. The uppermost of 4km thickness is the sedimentary layer which was not incorporated into the inversion. The second with a thickness of about 20km is the crust for which P and S velocities of 6.1 and 3.6km/sec were obtained with a VP/VS ratio of 1.69. This P velocity is consistent with the results of refraction studies. When this was divided into two parts at a depth of 12km, the sums of squared residuals were reduced dramatically, suggesting a subdivision of the upper curst at this depth. The upper portion possesses P and S velocities of 6.0 and 3.55km/sec and the portion, of 6.3 and 3.7km/sec, respectively. Both portions are characterized by a VP/VS ratio of 1.69. The third of about 10km thickness includes the lower crust there in, for which P and S velocities of 6.9-7.0 and 3.8-3.95km/sec were obtained. The corresponding VP/VS ratio is 1.78-1.81, in marked contrast to the upper crustal VP/VS ratio. The fourth is the sub-Moho mantle for which we obtained P and S velocities of 7.7-7.8 and 4.4km/sec that give a VP/VS ratio of 1.76-1.77, a value similar to that for the crust. The inverted P velocity is in agreement with the results of refraction studies. The Moho boundary probably lies at a depth of around 33km. The boundary between the upper and is not well constrained by the data. The consistency of our P velocity structure with those derived from explosion seismic studies indicates the feasibility of the present method for obtaining an S velocity structure. Poisson's ratio changes significantly across the boundary between the upper and but not across the Moho.