On April 27, 17th year of Tempyo era in the Japanese traditional calendar (June 1, 745 in the Julian calendar) a large earthquake occurred in central Japan, causing serious damage to the local capital and its surroundings of Mino Province (southern half of Gifu Prefecture at present). In the most authoritative catalog of Japanese damaging earthquakes, its magnitude (M) has been estimated at around 7.9, and an active fault study inferred that the entire Yoro fault system of 55 km long had generated this earthquake. However, I show in this paper that these views are probably overestimation of this earthquake because (A) the catalog’s summary came from (1) Omori’s misreading of Shoku Nihongi (Chronicle of Japan, Continued), the only historical record of this earthquake, in 1913 and (2) Kawasumi’s overestimation of M in 1951, and (B) the active fault interpretation was based on the catalog’s M of 7.9. If the earthquake had been the entire rupture of the Yoro fault system with M 7.9, the disaster should have been devastating not only in Mino Province but also in wide areas of Ise, Owari and Ohmi Provinces (present-day Mie, Aichi and Shiga Prefectures, respectively). However, no description in Shoku Nihongi suggests such disaster. Therefore, the earthquake disaster is considered to have been restricted in and around the Mino Provincial capital. Although an archeoseismological investigation reported that liquefaction had occurred at Jizogoshi ruins in Owari Province due to this event, the judgement of the occurrence time was affected by the overestimated magnitude. Meanwhile, Shoku Nihongi says that aftershocks were felt for about two weeks in the capital, Shigaraki-no-miya (SM) in Ohmi Province, about 65 km southwest of Mino Provincial capital, though the damage in SM is not written. Taking all these features into consideration, I infer that the source region of this earthquake was located around Mino Provincial capital, and estimate its M at roughly 7.0 to 7.3. If we assume that the Yoro fault system, which is reported to have ruptured around the 8th century based on several surveys, generated this event, it is reasonable that only its northern half, 20 to 30 km long, ruptured. The seismic intensity (on the JMA scale) at SM resulting from this inferred source is estimated at about 4 by an empirical attenuation relation of seismic intensity, which harmonizes with the historical record. The seismic intensities at SM due to aftershocks of M 5.0 to 6.0 are estimated at 2 to 3, which also harmonizes with the historical record. However, besides aftershocks in the source region, induced seismic activity may have occurred in Ohmi Province causing seismic intensity of 5 at some limited places near SM, because Shoku Nihongi suggests that liquefaction occurred around SM several times. Interdependency among immature study results of historiographical seismology, active fault study and archeoseismology has erroneously fixed the overestimation of this earthquake. For fruitful interdisciplinary cooperation among these fields, at first, investigation in each field should be carried out as deeply and independently as possible.
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