房総半島南西部岩井低地の離水海岸地形からみた大正型関東地震の発生年代と再来間隔

抄録

Two major historical earthquakes of the 1703 Genroku Kanto Earthquake (M 8.2) and the 1923 Taisho Kanto Earthquake (M 7.9) occurred along the Sagami Trough, where the Philippine Sea Plate subducts beneath the North American Plate. As a result of the coseismic crustai movements associated with these earthquakes and with the repeated characteristic earthquakes of two types (the Genroku-type Earthquake and the Taisho-type Earthquake), Holocene emerged shoreline topography has developed along the southern coast of the Boso Peninsula. We investigated the emerged shoreline topography and beach sediments in the Iwai Lowland with help of a new sampling method using the long geo-slicer to clarify the timing and recurrence interval of the Taisho-type Earthquake. A series of ten narrow emerged beach ridges, which are named the BR-I to BR-X in descending order, are distributed below the highest emerged shore platform, 15.5m high above sea level in the lowland. At least eleven times of coseismic events, probably the Taisho-type Earthquake, can be recognized from the number of the emerged shoreline topography. Based on the ¹⁴C ages and sedimentary sequences related to the BR-I-IX, the Taisho-type Earthquake occurred at least three times between 6825-6719 cal yrs BP and 5304-5055 cal yrs BP. After the other three non-dated events succeeded, four events are confirmed at 3300-3100 cal yrs BP, before 2306-2118 cal yrs BP, 1292-1264 cal yrs BP and around 900 cal yrs BP. The lowest BR-X was judged to be emerged during the 1923 event by comparing between the topographic map issued in 1886 and the present one. Analyzing intermittent emergent age of beach ridges, the recurrence interval of the Taisho-type Earthquake is within 990 years, and is 380 years at shortest.