Vertical variation curves on the Pacific coast of Shikoku before the 1946 Nankai earthquake.

Abstract

The long term vertical variations including 1946 Nankai earthquake have been obtained on the Pacific coast of Shikoku. However, the time of the datum just before the main shock was undefined, because it was obtained by human witness. From tidal record, the time was estimated to be between two high tides of which were the former spring tide and the day before the main shock. Within this time width, we proposed a logarithmic function to represent the vertical variation until the main shock. The time width was also confirmed by the witness testimonies for decrease of well water. We convert the decrease of well water to uplift of the ground. From the data of the time required for well water decrease and of the uplift of the ground, a gradient of the logarithmic function was estimated. The time width was limited from the vertical variation curves which were obtained from the level data of the Geographical Survey Institute and the gradient of logarithmic function. Finally, as a median of the time width, the time of the datum obtained just before the main shock was adopted to be 6.7days before the main shock. On the Pacific coast of Shikoku, the vertical variation curves until the main shock and the logarithmic functions as an approximate formula were determined. According to the function, it is estimated that in one month before the main shock, land has to 0.3m raised at Susaki, Kochi prefecture. This is consistent with witness testimony to say sea level has been reduced in Susaki bay.