Anomalous Changes in Groundwater Chemistry

Possible Precursors of the 1995 Hyogo-ken Nanbu Earthquake, Japan

Abstract

A post-earthquake investigation revealed statistically significant changes in the chemical composition of groundwater pumped from wells located about 20 km east of the epicenter within the aftershock region, preceding the disastrous M7.2 Hyogo-ken Nanbu earthquake in southwestern Japan on January 17, 1995. The chloride ion concentration had increased steadily after August 1994, compared to the constant level since July 1990, and reached an excess of about 10% just before the earthquake. Groundwater sampled after the earthquake showed much higher Cl^- concentrations. Together with the increase in Cl^-, sulfate and alkaline earth metal ions (Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) showed similar increases in their concentrations, while sodium ion and silica contents showed decreases. These changes may be the result of shallow groundwater inflow, enriched in chloride, sulfate and alkaline earth metal ions, to the artesian layer of the wells, due to permeability enhancement in the shallow stratum. The observed changes in groundwater chemistry before and after the earthquake are useful clues to understanding the mechanism of the present earthquake.