HOLOCENE ACTIVITY OF THE SUBDUCTION MEGA-THRUSTS ALONG THE SAGAMI TROUGH, NEAR TOKYO, DEDUCED FROM THE HEIGHT DISTRIBUTION AND AGES OF PALEOSHORELINES

Sagami Trough, extending NW-SE off South Kanto, is convergent plate boundary where the Philippine Sea Plate subducts beneath the North American Plate. Two major historical earthquakes, the 1703 Genroku Kanto Earthquake (M 8.2) and the 1923 Taisho Kanto Earthquake (M 7.9) occurred along the trough. As a result of the coseismic crustal movements associated with these earthquakes and repeated pre-historic earthquakes, Holocene emerged shoreline indicators (e.g. emerged wave-cut bench, fossilized mollusk etc.) are observed on 15 levels along the coast of the Miura Peninsula and the Boso Peninsula. The height distributions of the lowest two levels of them in the Miura Peninsula indicate 1.0-1.5 m of uplift during the both historical events. In the Boso Peninsula, although the whole of the peninsula was uplifted (max. 2 m) and tilted toward the northeast during the 1923 event, the 1703 event resulted in northward tilting accompanied with the uplift of more than 5 m in the southernmost area and subsidence of ca. 1 m in the central area. According to above results, it is inferred that the fault source model of the 1703 Genroku Kanto earthquake consists of a combined fault system of fault A and B. The fault A is the same as the model of the 1923 event which has already been estimated by Ando (1974). The uplift of the Miura Peninsula (both the 1703 and 1923 events) and the Boso Peninsula (only the 1923 event) can be explained by about 6.7 m slip of the fault A. The fault B is a low angle dip thrust located off the southeast of the Boso Peninsula. The unique movement of the 1703 event in the Boso Peninsula has been derived from about 12 m slip of the fault B. 14C ages of the older paleo-shorelines above two recent levels indicate that the characteristic earthquake generated from the fault A has occurred repeatedly about every 400 years. Basing on the geometry of emerged shoreline topography, four large uplift events of the 1703 Genroku type occurred several times in addition to several repeated small uplift events of the 1923 Taisho type. Recurrence interval of the 1703 type earthquake accompanied with the slip of fault 2 and 3 is 2000-2700 years. Therefore, the slip rate of these faults are estimated to be 16.8 mm/year (fault A) and 5.2 mm/year (fault B) respectively. These value are greatly different each other and smaller than the recent back-slip rate (30 mm/year) estimated from GPS data.