Simultaneous determination of the three-dimensional crustal structure and hypocenters beneath the Kanto—Tokai District, Japan

Abstract

A new method is proposed for determining the two-dimensional depth-distributions to the Conrad and the Moho together with station corrections and source parameters from first arrival time data. In the present study, the velocity structure is modelled by a hyperbolic function (tanh) and the configuration of each interface is expressed in a series of Chebyshev functions with coefficients which are unknown parameters. A non-linear inversion method by Tarantola and Valette is applied for inversion. An approximate ray tracing method with sufficient accuracy for a complex region is adopted for computation of seismic ray paths and travel times. The method is applied to P-wave first arrival time data from local earthquakes and explosions gathered in the seismological network by the National Research Center for Disaster Prevention to reveal the crustal structure beneath the Kanto-Tokai District, Japan. The results obtained in this study are very interesting and show the usefulness of the present method for the study of the crustal structure. The distribution of station corrections is reasonable where there is information on the shallow structure. Low-velocity zones appear around Tokyo Bay and the Cape of Omae and high-velocity zones appear in the mountainous regions. The depth to the Conrad is about 12–16 km and its lateral change is gentle. While the lateral change in the depth to the Moho is comparatively large and there is a narrow region where the depth to the Moho exceeds 40 km. Except for this region, the central-mountainous region, the agreement of the crustal structure obtained in the present study with those by the previous refraction studies is fairly good. In comparing our results to fractional velocity perturbation in Layer 1 (0–32 km) derived with Aki and Lee's method by Ishida and Hasemi, our work provides good information on the features causing the crustal velocity anomalies which have been unable to be constrained in Ishida and Hasemi's work.