Modeling of earthquake-induced hydrological changes and possible permeability enhancement due to the 17 January 1995 Kobe Earthquake, Japan

Abstract

Hydrologic changes associated with the 17 January 1995 Kobe Earthquake occurred in Awajishima Island very close to the epicenter. These included: (1) large drop of water table in the mountainous area; (2) rapid increase of discharge along active faults; and (3) change of chemistry of discharged water. A simple horizontal flow model was constructed to explain the observed changes; and optimal sets of specific yield and the change of hydraulic conductivity were estimated. Results suggest that this model can explain the observed phenomena consistently. The hydraulic conductivity is estimated to increase at least five times than that before the Earthquake; however, quantitative measurement of the increase of discharge just after the earthquake would constrain better the change of the hydraulic conductivity. The specific yield of the unconfined aquifer is between 0.3 and 1.7% depending on the assumed recharge rate but independent of the assumed depth to the impermeable basement. The change in chemical composition of the discharged water could be explained by the upward movement of deeper water due to the invasion of saltwater into the aquifer.