1990 年 43 巻 1 号 p. 13-26
Average horizontal crustal strain rates during interseismic period in the Japanese Islands, except the Hokkaido district, are deduced from precise geodetic survey, which has been conducted since the Meiji era. First, distance change rates between the first order triangulation stations are calculated using the weighted least squares method. In regions which suffered from disastrous earthquakes or volcanic eruptions, data before the events are neglected in the above calculation. Second, strain rates in arbitrary triangular regions are estimated.
The estimated strain rates ate less than 6×10-7/yr, which is greater than those derived from seismic moment release rate or displacements of active faults. The Japanese Islands may be divided into several provinces on the basis of disstribution of strain rates. In the Tohoku district, a N-S to NE-SW extension is prevailing, and there is a zone of small strain rates which runs parallel to the Oga-Ojika tectonic line proposed by MOGI (1985). This zone may be a boundary between crustal blocks. In the northern Kanto district, strain rates are very small, while a NW-SE compression is predominant in the southern Kanto district. These facts indicate that compressional forces due to the subduction of the Philippine Sea plate may not be transmitted to the northern Kanto district. There is a zone of large compression which runs from Niigata to Gifu through Matsushiro. These high strain rates show a good correlation with seismic activity in the crust. Although compressional strain rates are prevailing in both the Chubu and the Kinki-Chugoku districts, principal axes abruptly change their directions from NW-SE to E-W at around lake Biwa. The westernmost Chugoku and the northern Kyushu districts are characterized by small extension, which suggests that compressional stress predominant in the Chubu and the Kinki districts may fade out. In the Shikoku district, NW-SE compressional strain rates are prevailing. However, temporal variations in baseline lengths show that the elastic rebound associated with the 1946 Nankaido earthquake did not occur in the western part of shikoku. It is considered that the coupling between the subducting Philippone Sea and the overriding Eurasia plates may vary significantly from east to west. Large N-S trending extensions appear in the central Kyushu district, which is consistent with the spreading of the Beppu-Shimabara graben.