Characterization of spatial coseismic response of groundwater levels in shallow and deep parts of an alluvial plain to different earthquakes

Abstract. Coseismic changes in groundwater levels have been investigated in many places throughout the world, but most studies have focused on the effects of one large earthquake. Few studies have looked at the spatial patterns of level changes in response to several earthquakes, or at the relationship of the patterns to shallow and deep groundwater in the same area. The aim of this study was to investigate these patterns and to construct a model of hydraulic responses. We selected the Kumamoto City area in southwest Japan, a region with one of the richest groundwater resources in Japan, as our study site. Data from hourly measurements of groundwater levels in 54 wells were used to characterize the coseismic spatial responses to four large earthquakes that occurred in 2000, 2001, 2005, and 2008. Although the distance to the epicenter (12 to 2573 km), and seismic energy (Mw = 4.8 to 8.0) of these earthquakes varied, systematic groundwater level changes were observed in the range of 0.01 to 0.67 m. The zones where coseismic rises were observed were generally wider for deep groundwater than for shallow groundwater. We observed general trends in the changes in groundwater levels, and calculated pressure changes, in the deep groundwater, but the coseismic increases or decreases in compressive stress in the shallow groundwater were variable, depending on the distance to the earthquake epicenter. We developed a conceptual model of the mechanism underlying this phenomenon and also investigated the importance of Togawa lava, consisting of porous andesite and forming a main aquifer, in determining the pattern of groundwater level change.