On the Well Water Decreases Preceded the Nankai Earthquake

The water level in wells along the Paci.c coast from the Kii peninsula to Shikoku showed remarkable decreases a few days before the 1946 Nankai earthquake. If pre-slip occurred on a deep portion of the earthquake fault, an uplift .eld would cover the area of the decreased well water. However, the expected uplift is only a few centimeters. Can the drastic changes of well water be explained by the slight uplift? By our field surveys, we confirmed that the wells were located in a small delta. The seawater permeates under the delta which faced the sea, and the fresh water .oats on the seawater due to the different density. The Ghyben-Herzberg’ law shows that the depth between the horizon and the sea-and fresh water boundary is balanced to 40 times of the height between the horizon and fresh water head which is called groundwater table. This law implies that a slight uplift of the fresh water in a delta induces a 40 times drop of the sea-and fresh water boundary. In this case, much fresh water will be required to suspend the slight uplift of fresh water. If the new fresh water does not be supplied from the outside of delta, the much fresh water flows from the upper level of delta to take the new balance. Then, the well water on the upper level of the delta must decrease or dry up. The decrease of well water before the great earthquake was also con.rmed at the time before the 1854 Nankai earthquake (M 8.4). The reproducibility of the decrease of well water level caused by the pre-slip of Nankai earthquakes is fairly high. These understandings are effective for the prediction of the next great Nankai earthquake.