The discovery of emerged boring bivalves at Cape Omaezaki, Shizuoka, Japan: Evidence for the 1361 CE Tokai earthquake along the Nankai Trough

Abstract

This study found several in situ fossil shells of the boring bivalve Penitella gabbii exposed in mudstone at an elevation of 1.05–1.35 m above mean sea level in a wave-cut platform at Cape Omaezaki, Shizuoka, central Japan. This region faces the Nankai Trough where great earthquakes (M ~8) occur with a recurrence interval of 90–200 years. Based on the upper limit of living P. gabbii (−0.8 m above mean sea level) and geodetic data, the total uplift of shells since the time of their active is estimated to be up to 2.5 m. Using 14C dates from the uplifted terrace deposits around the study area and shell fossils, we propose that the emerged fossils provide new evidence for the pairing of the 1361 CE Shohei (Koan)–Tokai earthquake (rupture of Suruga Trough and the eastern half of the Nankai Trough) with the 1361 CE Shohei (Koan)–Nankai earthquake (rupture of the western half of the Nankai Trough).