Earth Planets Space, Vol. 56 (No. 8), pp. 843-858, 2004
Research Center for Urban Safety and Security/Department of Earth and Planetary Sciences, Kobe University, Kobe 657-8501, Japan
(Received February 16, 2004; Revised July 15, 2004; Accepted July 21, 2004)
Odawara City in central Japan, in the northernmost margin of the Philippine Sea (PHS) plate, suffered from severe earthquake disasters five times during the last 400 years with a mean repeat time of 73 years; in 1633, 1703, 1782, 1853 and 1923. In this region, non-volcanic Izu outer arc (IOA), the easternmost part of the PHS plate, has been subducted beneath Honshu (Japanese main island), and volcanic Izu inner arc (IIA) on the west of IOA has made multiple collision against Honshu. I hypothesize eWest-Sagami-Bay Fracture' (WSBF) beneath Odawara, a north-south striking tear fault within the PHS plate that has separated the descending IOA crust from the buoyant IIA crust, through examinations of multiple collision process and the PHS plate configuration. WSBF is considered a blind causative fault of the 1633, 1782 and 1853 M7 Odawara earthquakes, and is inferred to have ruptured also during the 1703 and 1923 great Kanto earthquakes simultaneously with the interplate main fault. A presumable asperity on WSBF just beneath Odawara seems to control the temporal regularity of earthquake occurrence. Though WSBF has not yet been detected directly, it is considered an essential tectonic element in this region, which might be a fracture zone with a few or several kilometer thickness actually. The WSBF hypothesis is the only conceptual model to explain the earthquake recurrence beneath Odawara.
Key words: Odawara earthquake, repeating earthquake, West-Sagami-Bay Fracture, South Fossa Magna, earthquake prediction, seismotectonics, collision zone.