[S-SS25_2PM1] Earthquake prediction

[SSS25-07] Recent anomalous groundwater temperature and water level changes and impending great earthquakes at the Nankai trough

*Tameshige TSUKUDA¹ (1.Japan Women's Univ.)

Keywords:dailatation, contraction, groundwater temperature, water level, precursor, earthquake prediction

Earthquakes are generated by the anisotropic principal stress regime in the rock medium. In the preparing process of a large earthquake, the medium would be deformed generating regions of contraction and dilatation around the nucleus of the shearing stresses. According to a hydraulic model, pore fluid flow is driven upward to the ground surface through crack systems serving as flowing pipes by high pressure pumps at a deep spot. The change in the quantity of the upwelling hot water from deep underground causes a change of groundwater temperature(Tsukuda et al., 2005). We have groundwater observation stations for temperature at 12 sites, and for water level at two in the Tokai and Nanki regions, central and southwest Japan, respectively, where are close to the so-called Tokai and Nakai earthquakes. High precision quartz thermometers are installed at Otomi (OT) in Yaizu City and Nakajima (NK) in Shizuoka City. At other stations, platinum resistance thermometers are installed. We use semiconductor pressure sensors for water level. At OT(Yaizu) in the Tokai region, the temperature has been monotonously increasing since the measurement started in 2003. The rate of increase has clearly fallen down since the end of 2012, suggesting weekning of the contraction in the rock medium. At NK(Shizuoka), 14km northeast of OT, the temperature data presented a precursory change from increasing to decreasing trend, one year before the 2009 Suruga-bay earthquake of M6.5 (Tsukuda, 2012). The decreasing rate after the earthquake became much higher than before and had continued till 2012. The temperature changed suddenly into increasing trend since May, 2013. The dilatation of the rock medium under Shizuoka recently changed into contraction. At stations HA and WA in Shionomisaki, Nanki region, Wakayama Prefecture. The long-term trend of the water level is rising, corresponding to the ground subsidence found by levelling and GNSS data (Kobayashi, 2013). The trend of temperature is similarly rising, suggesting contraction of the rock medium under Shionomisaki, the southernmost end of Honshu. At KZ(Kozagawa) in the Nanki region, the temperature is monotonously falling since the observation started in 2002. The decreasing rate is growing during recent two years, suggesting the dilatation turned to be intensified recently. As mentioned above, the deformations of the rock medium are accelerated under the regions close to the source regions of the great erthquakes at the Nankai trough. For prediction studies for the impending great erthquakes, we should start to conduct detailed and multidisciplinary observations.