Study of periodic vertical crustal movement in the Omaezaki peninsula, Central Japan, and it's tectonic implications

As a part of the earthquake prediction project in the Tokai district, levelling observations are being carried out four times a year in the Omaezaki peninsula. To analyze this time dependent data, an adjustment method in which the velocity of vertical motions, the amplitude and phase of annual variation of each benchmark are assumed to be unknown is applied. The main results of the analysis are summarized as follows: (1) at reference time 1981.0, the average phase of benchmarks along the levelling route between Kakegawa city and Omaezaki is about 21 degree, (2) at BM140-1 (Kakegawa), relative to BM92305 (Omaezaki) the maximum amplitude of one-year period variation is about 7.55 mm, i.e., the difference between the top and bottom of wave movement may reach 15.1 mm, and (3) the subsidence rate on the levelling route from BM140-1 to BM92305 decreases gradually as the benchmark is farther from the fixed point BM92305 with the maximum subsiding rate of 7.7 mm/yr. The estimated rates of vertical deformation (i.e., after removing the seasonal variation) are inverted to estimate interplate coupling between the subducting Philippine Sea plate and the overriding continental plate. This facilitates understanding of the strain accumulation process in this complicated region. The results indicate that there is a high coupled region extending from 7 to 27 km deep, the maximum back-slip rate in this region reaches 27 mm/yr, and the direction of the plate convergence at the Suruga Trough is N 54 degrees W +/- 5 degrees.