Strain Event in 1985-1987 in the Tokai Region, Central Japan

Abstract

Electro-optical distance measurements (EDM) for the baseline networks of Sangane and Awagatake in the Tokai region began in 1978. The rangings of two baselines, Sangane-Zaoh and Awagatake-Magami, have been repeated four times a year on average. The directions of the baselines are parallel to that of the convergence of the Philippine Sea plate. In order to minimize the EDM error, distance measurements are carried out at sunset. The results of distance measurements for the 13 years are summarized as follows: (1) The accuracy of EDM around sunset is generally better than 5×10-7. (2) A NW-SE contraction and a NE-SW extension are common to both networks. A shear strain at a rate of 1 to 3×10-7/year is consistent with the geodetic surveys carried out in the last 100 years. (3) Both baselines, 80 km apart from each other, indicated a similar strain event, suggesting the accelerated contraction amounting to 1 to 2×10-6 in 1985-1987. The contraction is concordant with the change in geodetic tilt near Zaoh and the change in seismic activity in the Ise Bay area. The result obtained from observations might be evidence of the remarkable strain event that occurred in the Tokai region in 1985-1987.