Expulsion of geopressured hydrothermal system along active faults and its relation to the occurrence of earthquakes in the Shinanogawa seismic belt, Japan

The 1995 Northern Niigata Earthquake (M 6.0) occurred at a shallow depth in the Niigata seismic gap. The anomaly areas in temperature, electrical conductivity and Cl− concentration of groundwater trend northeast as linear distribution in the epicentral area and are approximately coincident with the area of the seismic intensity 6 (JMA scale). The distributions of seismic intensity 6 and groundwater anomalies convincingly imaged the presence of a buried active fault beneath the epicentral area. The occurrence of this earthquake and the anomalies of groundwater were related to the expulsion of geopressured hydrothermal system (GHS). All epicenters of the destructive earthquakes along the Shinanogawa seismic belt are actually located in the buried active fault zones characterized by the areas of temperature and geochemical anomalies of groundwater. These earthquakes might have been triggered by the activity of GHS. The expulsion of GHS along an active fault in combination with the thermal softening of fault zone attributed to high rock temperature may reduce fracture strength of the rock, and trigger earthquake occurrence. The dimension of the anomaly area of groundwater temperature at the epicentral area reflected the scale of the earthquake fault. The linear anomaly areas of groundwater resulting from the expulsion of GHS and having no historical destructive earthquake are of the predicated areas of future destructive earthquakes. Monitoring of groundwater conditions in these areas may provide useful information regarding the future occurrence of earthquakes.