Surface deformation and geologic structures associated with active blind faults

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We performed seismic reflection profiling and gravity profiling across selected active faults in northeast and central Japan, in order to clarify the mechanism of surface deformation and geologic structures associated with blind active faulting. The following results were obtained :1.Most of presently active faults in northeast Japan were originated from Miocene normal faults, and have been reactivated as thrust faults under compressive stress regime since Pliocene time. Seismic reflection and gravity surveys were performed across the source area of the 2003 Northern Miyagi earthquake of Mw 6.4. We found Miocene geologic structures indicative of a large amount of normal slip on the source fault, although focal mechanism solutions of the earthquake are of thrust type.2.The source region of the 2004 Chuetsu earthquakes is located in a fold-and-thrust belt. Surface and subsurface geologic data suggest that this fold-and-thrust belt is underlain by a large-scale detachment fault, which was responsible for the stretching (rifting) of, and the deposition of thick sediments in, this area in Miocene time. Most of aftershocks occurred below the detachment horizon. Therefore it is likely that the source faults of the 2004 earthquakes were not related to the thin-skinned structure but represent internal deformation of Mesozoic-Paleozoic basement beneath the detachment.3.In contrast to northeast Japan, inversion tectonics is not obvious in central Japan. It has been believed that the Ina Valley fault zone was newly born in early Quaternary time. Our seismic reflection and gravity survey, however, revealed that a blind active thrust fault exists beneath basin-fill sediments in the Ina Valley. On the hanging-wall side of this blind fault is a thick pile of sedimentary rocks, indicating normal slip on this fault possibly in Miocene time. This suggests that crustal structures of southwest Japan also have been affected by Miocene extension.