Provenance of submerged stone pillars in an earthquake and typhoon hazard zone, coastal Tosashimizu, southwest Japan: A multidisciplinary geological approach

Human-made structures and shaped stones on the seafloor near coasts, possibly submerged by tsunami waves, coseismic subsidence, or flooding during typhoons, can often be linked to folk traditions of ancient natural disasters. Stone pillars that appear to have been artificially shaped are submerged in shallow waters inshore Tosashimizu city, southeast Japan, an area subject to repeated Nankai Trough earthquakes. We performed geophysical and geochemical analyses of samples from these pillars, building stones in use in villages in the Tsumajiro area, and nearby natural rock outcrops. In porosity and grain density, the pillars are similar to sandstone from the Tatsukushi Formation and to foundation stones used in the villages; this similarity was confirmed by a principal component analysis of chemical composition data. The shape and surface morphology of the pillars are also similar to those of the foundation stones and stone steps at nearby temples. Therefore, we inferred that the submerged stone pillars were made from Tatsukushi sandstone and had been utilized as building stones for structures such as steps and foundations in Tsumajiro village possibly >100 years ago. Because radiocarbon ages obtained on calcareous fragments adhering to the pillars were inconclusive, when, and by what process, the stone pillars were transported from the land and submerged in the sea is still uncertain.