GROUNDWATER ANOMALIES ASSOCIATED WITH THE 1995 HYOGO-KEN-NANBU EARTHQUAKE

There have been many reports on earthquake-induced groundwater anomali es, but the mechanism of these anomalies is not very clear. The 1995 H yogo-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 earthqua ke. Our purpose was to study these anomalies systematically and to gai n some knowledge of their mechanisms. We initiated a questionnaire sur vey in and around the source region on March 8, 1995. We also investig ated the discharging waters which appeared on Awaji Island after the H yogo-ken Nanbu earthquake. These results were compared with the volume tric strain changes estimated from fault models for the earthquake whi ch 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 d etected by the questionnaire and the investigation. However, their dis tribution did not coincide with the distribution of the volumetric str ain changes, which are of a quadrantial type because the fault models are of a right-lateral strike-slip type. Several precursory groundwate r anomalies were also reported, although few pre-seismic crustal strai n changes were observed. Therefore, the volumetric strain changes do n ot always explain the groundwater anomalies that accompanied the 1955 Hyogo-ken Nanbu earthquake.