Active reverse faulting deduced from flexural deformation of marine terraces around southern area of the Shimokita Peninsula, northeast Japan

概要

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Marine terraces well developed along the coast of the Shimokita Peninsula are good indicator of late Quaternary crustal deformation. We decipher active reverse faulting on the basis of deformation of the marine terrace surface formed in MIS 5e around Rokkasho village in the southern part of the peninsula. The marine terrace surfaces in the area are classified into two groups of H and M. M1 surface covered by Toya tephra is correlated with that formed in MIS 5e. A 1-2 km wide flexural scarp tilting toward east is found on MI. surface. Vertical offset of M1 surface is over 30 m in the north and 20 m in the south. The flexure extending NNE-SSW for 15 km is consistent with the prevailing Tertiary monocline structure well recorded in seismic reflection record. M1 surface and the Tertiary system have been folded convincingly owing to the activity of west-dipping blind active thrust extending to the depth of several hundred meters beneath the structure. This fault may merge into the extensive submarine fault along shelf edge in the north. It is possible that they compose an over 100-km long active fault.