Abstract:
Great earthquakes (M: approximately 8) occur every 2000 to 3000 years off of the Boso Peninsula, central Japan, and every 100 to 200 years off Miura Peninsula. These two types of events are generally controlled by the oblique subduction boundary on the Sagami trough, between the Philippine Sea plate under the North American plate, making the Megalopolis of Tokyo and adjacent area an extensively hazardous region for strong earthquakes and tsunamis. The most recent events occurred in 1703 and 1923, respectively, the former as the Genroku-type, the latter is known as the Taisho-type Kanto earthquake. Most of the hypocentral regions (asperity regions) are under the sea along the Sagami trough. Steady-slip and slow-slip areas surround the great earthquake asperity regions off Boso and Miura, creating seismologically and geodetically defined areas collectively called the 'Kanto Asperity'. Well-studied uplifted benches along the coastal lines formed via 'see-saw uplift' and subsidence, accompanied by general exhumation of middle-Miocene and younger accretionary prism materials. The Boso trench-trench-trench type triple junction is the world's only known triple convergence of plates and includes a thick section of Izu forearc volcaniclastic sediments. Down-dragging of these sediments into the trenches creates at least four submarine prisms from the north to south along the two peninsulas. Further study of these hazardous areas should include seismic and tilting observations by monitoring to help anticipate the next great earthquake. Our project aims first to drill and core nine sites for reconnaissance geological and geophysical observation, then to drill deep (>1 km) holes in representative sites for installation of a seismometers and tiltmeters. We intend to monitor the data, hopefully on line, by data-logger every 6 months. These data will be essential for evaluating the seismic hazards to the Kanto region.