The AIST researcher and his coresearchers of Iwata Minami High School and Akita University discovered the tsunami deposits at the late 7th century and the late 9th century in the Otagawa Lowlands in western Shizuoka Prefecture and verified the occurrence of two Tokai earthquakes that were unconfirmed by historical records.

Recurrence history of the great (M8 class) earthquakes along the Nankai Trough have been reconstructed mainly based on historical documents. The Nankai earthquakes have historical records after the 684 Hakuho earthquake, but there was a gap in historical records for the Tokai earthquakes between the 7th and 11th centuries. There was no evidence of tsunamis due to the Tokai earthquakes during this gap, which hindered the discussions on the timings and linkage of the Tokai and Nankai earthquakes. This called for the discovery of traces of huge earthquakes that occurred in the Nankai Trough in the 7th to 11th centuries.

Four tsunami deposits were found from the Otagawa Lowlands in this study. Based on numerous radiocarbon ages, the four tsunami deposits were estimated to date to the end of 7th century, end of 9th century, 11th to 12th century, and latter 15th to early 17th century, respectively. The younger two deposits are attributed to the 1096 CE Eicho earthquake and the 1498 CE Meio earthquake, respectively. The tsunami deposit at the end of 9th century, together with historical records, confirms that a Tokai earthquake occurred at the same time as the Nankai earthquake in 887 CE. The tsunami deposit from the late 7th century also indicates the occurrence of a Tokai earthquake, but it is unclear whether this was coincidental with the 684 CE Nankai earthquake.

In order to reconstruct the size of each earthquake and tsunami, the researchers intend to reconstruct the tsunami inundation distance in the Otagawa Lowlands and other areas, and the distribution patterns of coastal uplift and subsidence associated with the earthquakes around the Enshu-nada coast.