Abstract
A multi-disciplinary synthesis was made on the tectonics and seismotectonics in the northwestern Sagami Trough and adjacent areas, where the plate boundary between the Izu-Bonin and Northeast Japan arcs passes and historical M7-8 inter- and intraplate earthquakes frequently have occurred. In a convergent boundary between plates, a displacement between plates is usually accommodated along a broad belt of active imbricated thrusts or folds, which develop in the wedge of an overriding plate. In such a situation, we can easily identify two kinds of plate boundaries: a deformation front and a subduction entrance. The deformation front is a line connecting the thrusts or folds nearest to a trench/trough axis. The subduction entrance is a line connecting the edges of trench/trough filling coarse deposits, which cover a surface of a subducting plate. In Sagami Bay, the deformation front in the overriding Northeast Japan arc is estimated to be located along the northern and eastern edges of the Sagami Basin, while the subduction entrance of the subducting Izu-Bonin arc is estimated to be located along the southern edge of the Basin.
Many geomorphological, geological, geophysical evidence needs and supports the existence of the West Sagami Bay Fracture (WSBF), an intraplate propagating fracture between the Izu-Bonin outer and inner arcs, proposed by Ishibashi (1988), whereas these evidence needs the slight modification of the estimated geometry around the WSBF. Frequent dike intrusions in the Higashi Izu monogenetic volcano field cause spreading of the upper crust and probably move the Manazuru "microplate" (MNZ) to the NNE direction against the Izu block. The Tanna-Hirayama tectonic line is proposed to be a transform fault, which accomodates the relative movement between the MNZ and the Izu block. In the MNZ hypothesis, the Kozu-Matsuda fault, located to the northeast of the Izu Peninsula, is interpretated as a deformation front between the overriding Northeast Japan arc and the MNZ, which is buoyantly subducting beneath the Oiso Hill and Tanzawa Mountains. The proposed new geometry around the WSBF and the hypothesis of the MNZ can explain (1) the crustal structure and tectonic features around the Kozu-Matsuda fault, (2) tectonic implication of the "Oiso-type" earthquakes, which are estimated to displace the Kozu-Matsuda fault periodically, and (3) the rapid upheaval of the Tanzawa Mountains since 1 Ma.