On well water and sea level changes witnessed before the 1946 Nankai earthquake on the Pacific coast of Shikoku, Japan

Abstract

This study investigates the vertical crustal movement in the period approaching the 1946 Nankai earthquake. To obtain data for just before the main shock, we examined witness testimonies on well water and sea level change that might relate to vertical crustal changes. The testimonies were taken into consideration along with existing documents and local knowledge of the inhabitants of the Pacific coast of Shikoku. The water in a few wells decreased or dried up about 1 week before the main shock. Abnormal changes in sea level were witnessed several days before the main shock. From several hours before the main shock, fishing boats returning to port could not dock at the port because the sea level was too low. On the contrary, there were also testimonies of those who noticed no lowering of sea level. The generation of a tsunami is proposed as the explanation of the abnormal changes of sea level and conflicting testimonies.