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Crustal strains are deduced from geodetic data of triangulations and repeated trilaterations in the southern Fossa Magna district, Central Japan. This area is located very close to the convergent boundary of the Philippine Sea plate, and has been attacked by great earthquakes several times during historical ages.For the investigation of the regional distribution of compressional strains, lines of contraction in the district are obtained from the deduced principal axis of crustal strains in the second order triangles during the interseismic periods. The detected compressional strain field can be interpreted as the drift of the Philippine Sea plate for the south Kanto with N31°W and 4.3cm/yr, and for the Tokai with N45°W and 5.3cm/yr. Representative values of maximum shear strain rate are also deduced from the distribution of maximum shear strain rates in the second order triangles. These values are 0.40μstrain/yr for the south Kanto, and 0.16μstrain/yr for the Tokai. The highest value is found to be 0.53μstrain/yr in and around the Izu peninsula, where contemporary seismic activities are very high.A high strain accumulation is detected along a zone in the Tokai district parallel to the Suruga trough, where microearthquake activities are also very high at present. Another high strain accumulation zone has been known in the Boso Peninsula, south Kanto, but no microearthquake activity has been found there until now.ISHIBASHI (1983) supposed the existence of a triple junction near Kofu, where the North American, Eurasian, and Philippine Sea plates meet together and discussed the possibility of a jump of the above mentioned triple junction to a new position at the north end of the Nishi-Sagami Bay fault. The present author points out that the present day triple junction can be located at the jumped point mainly on the hasis of the intense crustal activity, high strain rate, and the highest level of the Holocene marine terrace at the north end of the Nishi-Sagami Bay fault. The age of the jump is presumed to be approximately 6, 000 years ago.

Crustal Strain in the Southern Fossa Magna, Central Japan, and Their Tectonic Implications