11:30 AM - 11:45 AM
[SSS13-04] Did the Ninna earthquake (887 CE) really occur in the Nankai Trough?
Keywords:Nankai Trough, Ninna earthquake, tsunami deposit, Osaka Bay Fault
Historical records reveal that the Ninna earthquake (887 CE) caused severe seismic shaking in the Kinki district, including Kyoto, as well as a tsunami in Settsu county. Based on these records, the Ninna earthquake is considered to have occurred in the Nankai segment of the Nankai Trough. Moreover, based on the wide area of the affected region implied by historical documents, fault rupture is also considered to have occurred in the Tokai segment. Recently, a sand layer interpreted as a tsunami deposit was found in a natural levee of the Ota River in Shizuoka Prefecture, and the age of the layer overlaps with that of the Ninna earthquake (Fujiwara et al., 2019).
The authors conducted detailed radiocarbon dating on drill core samples obtained from two areas in Mie Prefecture, and found tsunami deposits with ages agreeing with the Meio (1498), Eicho (1096), and Hakuho (684) tsunamis in both areas. On the other hand, we did not find any layers that can be correlated to the 887 Ninna earthquake tsunami. The historical documents show seismic tremor in the Kinki district and tsunami in Osaka Bay in the Ninna earthquake, but no historical documents of shaking, tsunami, and crustal deformation in the Pacific coast have been found at this time. The sand layer found in the Ota River lowland has different characteristics from modern tsunami deposits, and lacks sufficient evidence to be interpreted as a tsunami deposit. Therefore, it is difficult to say that this sand layer is a definite evidence for the existence of the Ninna earthquake tsunami along the Pacific coast.
Acoustic surveys and seafloor coring in Osaka Bay indicate that the Osaka Bay Fault was most recently ruptured 900–1200 cal yr BP, assuming a marine reservoir effect of 400 years (Nanayama et al., 2000). This age overlaps with the year of the Ninna earthquake. If we assume that the Ninna earthquake occurred on the Osaka Bay Fault, this would reasonably explain the severe seismic shaking in the Kinki district and the tsunami in Osaka. This is also consistent with the fact that no definite records of seismic shaking, tsunamis, or crustal deformation have been found in the Pacific coast.
The authors conducted detailed radiocarbon dating on drill core samples obtained from two areas in Mie Prefecture, and found tsunami deposits with ages agreeing with the Meio (1498), Eicho (1096), and Hakuho (684) tsunamis in both areas. On the other hand, we did not find any layers that can be correlated to the 887 Ninna earthquake tsunami. The historical documents show seismic tremor in the Kinki district and tsunami in Osaka Bay in the Ninna earthquake, but no historical documents of shaking, tsunami, and crustal deformation in the Pacific coast have been found at this time. The sand layer found in the Ota River lowland has different characteristics from modern tsunami deposits, and lacks sufficient evidence to be interpreted as a tsunami deposit. Therefore, it is difficult to say that this sand layer is a definite evidence for the existence of the Ninna earthquake tsunami along the Pacific coast.
Acoustic surveys and seafloor coring in Osaka Bay indicate that the Osaka Bay Fault was most recently ruptured 900–1200 cal yr BP, assuming a marine reservoir effect of 400 years (Nanayama et al., 2000). This age overlaps with the year of the Ninna earthquake. If we assume that the Ninna earthquake occurred on the Osaka Bay Fault, this would reasonably explain the severe seismic shaking in the Kinki district and the tsunami in Osaka. This is also consistent with the fact that no definite records of seismic shaking, tsunamis, or crustal deformation have been found in the Pacific coast.