Groundwater anomalies associated with the 1995 Hyogo-ken Nanbu earthquake

There have been many reports on earthquake-induced groundwater anomalies, but the mechanism of these anomalies is not very clear. The 1995 Hyogo-ken Nanbu earthquake, which occurred on January 17, 1995, was one of the greatest shallow inland earthquakes in Japan since the modern Japanese observation system for monitoring seismicity was established. Many groundwater anomalies have been observed following this earthquake. Our purpose was to study these anomalies systematically and to gain some knowledge of their mechanisms. We initiated a questionnaire survey in and around the source region on March 8, 1995. We also investigated the discharging waters which appeared on Awaji Island after the Hyogo-ken Nanbu earthquake. These results were compared with the volumetric strain changes estimated from fault models for the earthquake which stretch in a SW-NE direction. Many post-seismic rises in the level or discharge of groundwater as well as a few post-seismic drops were detected by the questionnaire and the investigation. However, their distribution did not coincide with the distribution of the volumetric strain changes, which are of a quadrantial type because the fault models are of a right-lateral strike-slip type. Several precursory groundwater anomalies were also reported, although few pre-seismic crustal strain changes were observed. Therefore, the volumetric strain changes do not always explain the groundwater anomalies that accompanied the 1955 Hyogo-ken Nanbu earthquake.