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[S04-1-03] Hot Spring Anomalies Observed in Kumamoto Prefecture Associated with the 1946 Nankai Earthquake
Kumamoto-Ken Saii-shi (Record of natural disasters in Kumamoto Prefecture) published by the Kumamoto meteorological station in 1952 describes the 1946 Nankai earthquake in the chapter for earthquakes, "No. 242, December 21, Showa-21: Great earthquake in the Nankai area. The hypocenter is south of Kii (Wakayama Prefecture). There are anomalies in groundwater at many hot springs." (Only related descriptions are extracted and translated here). The anomalies are summarized in a table. The hot springs where anomalies are observed are located in the areas corresponding to Hitoyoshi, Kumamoto, Amakusa, Yatsushiro, Tamana, Minamata and Oguni, using the present location names. The conditions or changes in flow rate, color, and temperature are listed for 15 hot springs. The times of the changes at the hot springs varies from December 23, 1946 to January 6, 1947. The observation periods are from several days to about a half month. The changes in flow rate, both increases and decreases, began on the day of the earthquake and continued over 10 days for most hot springs. The color changes were observed for 10 hot springs. The color changed to light brown, milky white, rust color, clay red, or black at the time of or after the earthquake.
Groundwater anomalies are generally explained by static strain steps or dynamic strain oscillations i.e. seismic waves. Static volumetric strain steps calculated assuming the model of Sagiya and Thatcher (1999) are extension with order of 100 nano strain, which will produce at most 50 cm of ground water level decrease for usual poroelastic materials. Although the ground water level decrease should produce decrease in flow rate, both increases and decreases in flow rate are reported in the Kumamoto-Ken Saii-shi. It is difficult to explain the anomalies solely by static strain steps and poroelastic effects. Local permeability changes caused by dynamic strain or shaking may cause some of the reported anomalies.
Groundwater anomalies are generally explained by static strain steps or dynamic strain oscillations i.e. seismic waves. Static volumetric strain steps calculated assuming the model of Sagiya and Thatcher (1999) are extension with order of 100 nano strain, which will produce at most 50 cm of ground water level decrease for usual poroelastic materials. Although the ground water level decrease should produce decrease in flow rate, both increases and decreases in flow rate are reported in the Kumamoto-Ken Saii-shi. It is difficult to explain the anomalies solely by static strain steps and poroelastic effects. Local permeability changes caused by dynamic strain or shaking may cause some of the reported anomalies.