太平洋プレートの沈潜に伴う反流について (2)

抄録

In the Kanto District, Japan, the Pacific Plate dips beneath the aseismic front at an angle of approximately 33°beneath the land, and about 40° beneath the sea where the aseismic front disappears. The boundary between these two segments underlies the Sagami Trough, suggesting that the Pacific Plate at its subduction, splits into two parts along the trough. Further, the marginal line of the upper plate passes beneath Oshima Volcano, Izu Peninsula, Shizuoka City, and then continues in a northwest direction.
The Toyama Deep Sea Valley, about 500km long in the Sea of Japan, starts at Toyama Bay and provides the gate for another type of countercurrent to the subducting Pacific Plate. It is a slab-type one of about 280km in length. The tip of the slab, where it runs against the subducting plate surface, is converted to magma. This, in turn, exerts the pressure needed to enhance separation at the boundary between plate and crust.
Rock breakage and/or crustal block movement result and cause earthquakes. An example is the movement of an isolated tertiary block with the dimensions of 120×75×50km³, located off south Kanto which is known to be the source of the 1923 Kanto Earthquake.
Using the hypothesis of perfect elasticity, the fault plane of an earthquake would never be expected to strike in a direction parallel with that of the rock stream. This peculiar situation in the distribution of the direction of principal pressure for earthquake mechanism around Izu Peninsula, can thus be explained by the influence exerted by the comparatively large shearing stress of the rock stream.