THREE-DIMENSIONAL STRUCTURE BENEATH THE HOKKAIDO-TOHOKU REGION AS DERIVED FROM A TOMOGRAPHIC INVERSION OF P-ARRIVAL TIMES

Abstract

An approximate inversion method, which has been widely used in medical science and called ART (Algebraic Reconstruction Technique), is applied to obtain a 3-D image of the uppermost mantle beneath the Hokkaido-Tohoku region for the period from 1973 to 1981. About 7, 600 P-wave arrival times recorded by seismic stations of the Japan Meteorological Agency are used. We present three types of 3-D models. 1) A model from the 7, 600 data. 2) Models for 1973-1977 and 1978-1981, respectively. 3) Eight models for 2 years moving windowed data. The first model is in good accordance with the results of previous authors who inverted P-arrival time data by damped least-squares inversion methods. The model clearly exhibits a high-low velocity contrast along the strike of the deep-seismic zone below a depth of about 60 km. The contrast is estimated to be about 4%. A subtraction operation of the second type 3-D models does not seem to show any significant differences between the models. No significant variations are apparent in and around the source region of the 1982 Urakawa-oki earthquake (M_S=7.2) in the display of the 2 years windowed results. However, the latter two results do not necessarily deny possible velocity change if we consider the sensitivity of event location process to the expected velocity change in the source area of earthquake.