Geologist Views of the Predicted Tokai Earthquake
Fuji-Tokoha University, 325 Oobuchi, Fuji City, Shizuoka 417-0801, Japan
1. The hypothesis of the Tokai earthquake is based on assumption of elastic and robust slab of crust (wider than 9,000 km2 and 15 km thick) for the overriding mass of the underthrust, to which the Philippine Sea Plate is subducting. In seismology the crust is assumed elastic, homogeneous and strong enough to sustain strain energy of the gigantic earthquake, M>8. Geology of the region suggests the crust is neither elastic nor strong enough. It consists of compound slabs of folded strata of accretionary sediments. It is uncommon to consider such robust slab in this area. 2. The Fossa Magna is a large strike slip fault running from Suruga Bay to Itoigawa to the north. It follows a wide drag zone in the west, in which the focal area of the predicted Tokai earthquake is situated. The drag zone has been produced by slow creeping over 6 Ma. The creeping of the left-lateral fault is still active. Part of strain energy by subducting plates may be released by the creeping. 3. The Nankai trench is considered to extends into the Suruga Bay. In geology the bay is a canyon, seaward extension of the Fossa Magna, by which the trench is displaced and dragged to north to the south of Izu peninsula.
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