CRUSTAL AND UPPER MANTLE STRUCTURE IN JAPAN FROM AMPLITUDE AND PHASE SPECTRA OF LONG-PERIOD P-WAVES : PART 2. KANTO PLAIN

Abstract

Crustal and upper mantle structure in the Kanto plain has been investigated by the same method as described in Part 1 of this paper. Taking the amplitude ratio of the vertical component to the horizontal one of long-period P-waves registered at Tsukua and the phase difference between them, we obtain two observational curves related only to the structure beneath the Kanto plain. The curves for waves incident from almost due south are conspicuously different from those of the other directions, suggesting a difference in structure. Comparing seven sets of these curves corresponding to seven regions classified by the incident direction with the theoretical curves calculated by varying layer parameters of probable models derived so far from other studies we have obtained several models for each region. The most probable model has been selected on the assumption that the structure does not vary greatly with azimuth. This model has a crustal thickness of about 29 km and a thick 7.4 km/sec intermediate layer as much as 20 km or more. A comparison of travel-time residuals at Tsukuba with those at Matsushiro has revealed that at Tsukuba P-wave arrivals from the south to southwest direction are as much as 1 second earlier than those from the south to southeast direction. This has been reduced to a higher velocity in the upper mantle under the southwest part of the Kanto plain, compared with the velocity under the southeast part.