Late Holocene paleoseismicity of the Yoro fault system, central Japan-geologic evidence of the last two historical large (M7.8 to 8) earthquakes of Tensyo jishin (earthquake) in 1586 and Tenpyo jishin (earthquake) in 745

Toshihiko Sugai (Univ. Tokyo), Yuichiro Fusejima, Yasuo Awata, Takashi Azuma, Yoshihiko Kariya and Yasuhiro Suzuki (Aichi Pref. Univ.)

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Abstract:The Yoro Fault system, which extends for 55 km along the western boundary of the Nobi tilted-basin, has been considered to be composed of three reverse-fault segments named Yoro, Kuwana, and Yokkaichi. The time-space diagram of the fault system, which shows the timing of faulting events revealed at several sites, however, implies that the whole Yoro fault system ruptured simultaneously and produced a single large earthquake at least twice in the last 2000 years. The diagram also indicates that the last two events are correlated with the two historical earthquakes of Tenshyo in 1586 and Tenpyo in 745. Based on the vertical offset shown by the depositional surfaces of shallow marine sediments, the vertical slip per event was estimated about 5 m at Hazawa area, the central part of the Yoro fault, while it is about 6 m at Yuriage area, the northern part of the Kuwana fault. The Yoro and Kuwana faults display similar slip per event along with the simultaneous timing of the faulting event, suggesting that the Yoro and Kuwana faults form a single behavioral segment. Moreover the Yokkaichi fault probably belongs to the same behavioral segment. It is highly likely that the Yoro fault system will generate Mw 7.5 single earthquake in future, which should give destructive damage to the Chukyo metropolitan area, although the day is not so imminent because of the relatively short duration (about 410 years) after the last faulting event in comparison with the average recurrence interval of about 1000 to 2000 years.