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.
Swarm-like seismic activity including six moderate events (Mj = 5.1-5.4) occurred in 1989, 1990 and 1997 in the same area as the 2000 Western Tottori Earthquake (Mj = 7.3). For each time period, we carried out temporary seismic observations in and around the source area and processed the data together with data from permanent stations, to determine the hypocenters precisely. In this study we also redetermined the earthquake locations in each seismic activity using a two-step master event technique with common master events, so that the accuracy in the relative locations of the events was improved. The purpose of this study is to clarify the relationship between the preceding seismic activity and the mainshock in 2000 by comparing the hypocenter distributions. The relocated hypocenter distributions show that the three preceding swarms occurred in different parts of the same fault plane as the 2000 Western Tottori Earthquake. The b-values of the preceding swarms were low (0.51-0.67), suggesting a high stress level in the area. The mainshock initiated in the area of the preceding swarms. The rupture propagated with relatively small slip (∼1 m) in the area for the first three seconds. Then, it developed to main rupture with large slip (2-4 m) outside the area toward the southeast.
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