Three-dimensional topography of the sediment/basement interface in the Tokyo metropolitan area, central Japan

Abstract

An extensive seismic refraction experiment was conducted in the Tokyo metropolitan area, central Japan, to reveal the three-dimensional basin structure in this area. Sixty-three explosions were fired over 14 years and more than 2000 travel times were picked for several phases.

For our inversion, the basement is divided into two homogeneous regions where the sediment is assumed to be of uniform velocity. The interface separating the sediment and basement is then parameterized by bi-cubic B-splines and a smoothness constraint is imposed to avoid oscillatory artifacts in the solution. The relative weight of the constraint to data residuals is determined by minimizing Akaike's Bayesian information criterion. About 1500 travel times of refracted P waves are inverted tomographically, and the interface shape determined agrees well with the results of other geophysical surveys such as borehole measurements and gravimetric observations. The basement velocities in the northern and southern regions are 5.7 and 5.4 km/sec, respectively.

Previous two-dimensional analyses suggested there is a 4.5 km/sec layer in the uppermost part of the southern basement, but we ignore it for simplicity so that estimated depths are likely underestimated by 1 ∼ 2 km in the southern region. Nevertheless, this study provides the first detailed image of the basin structure in the area, which will help strong motion and earthquake hazard studies as additional constraints.