The Nojima seismic fault, appeared in the Awaji Island following the Hyogo-ken Nanbu Earthquake, it attracted the interest of many researchers from immediately after the occurrence, since it showed a large displacement crossing a farmland, road and a river embankment. Engineering geological surveyers and researchers energetically investigated and studied the scale and moving mode of the surface seismic faulting and the relation between the surface deformation around the fault and various other kinds of damage. In contrast to the Nojima seismic fault, a seismic surface faulting in the Kobe district was not clear except in a part of the area. Although the presence of faulting particularly becomes of major interest as an expansion factor of damage in the earthquake disaster belt; the remarkable damaged zone from Kobe to Ashiya and Nishinomiya; the results of the surveying were confined to finding out the intermittent shear displacement in some places. However, the results of deep ground prospecting showed that the extended part of known faults and the unknown faults lay under the alluvium or dilluvium. The relations between the lying positions of these faults and the damage on ground surface, ground ruptures and ground deformation, are so important to the engineering geologist. The engineering geological investigation practiced after the earthquake about the crustal movement, the surface seismic faulting and the ground deformation related to the faulting, was unprecedented in scale, and it was the first case that various kinds of investigation methods were applied to the ground after the earthquake. Therefore, this chapter was written from the standpoint that the results of the investigation should be left as a documentary record as much as possible. These records would be expected to become valid material when later evaluation on fault activities, or to estimate any damage during earthquake.
Many structures were damaged in the vicinity of the epicenter, including Kobe-city and Ashiya-city, and severe damage occurred along an east-northeast trending narrow zone, so-called “the linear earthquake damage zone.” Structure damage also extended sporadically farther to Itami-city, Takarazuka-city, Toyonaka-city and Osaka-city where seismic motion with more than 5 of Richter scale was recorded. The damage grade becomes smaller concentrically from the epicenter except the linear damage zone. The occurrence of the narrow severe damage zone has been explained by several hypothesis, e. g. the possible movement of hidden active faults under sediment, geomorphologic effect, amplification of earthquake motion in surface soft sediment, contrast of load-proof strength of structures between damaged and undamaged area, deep geological structure, etc. After the earthquake, deep geological investigations involving dynamic explorations have been performed. and seismic simulation has been carried out by several organizations, which revealed detailed geological structures and dynamic character of the basement. These results are going to lead a true cause of earthquake damages. This chapter first clarifies that earthquake damages caused by the geomorphologic and geological structure. However detailed study on it has already been described in the interim report, so new data was straightened in this paper. Secondly, we focus target structure of the basement and it's particular physical properties from the investigation of geological and geomorphologic consideration and earthquake motion simulation. Finally relationships between damages in the distant places and geological structure, geological distribution and geotechnical characteristics were described.
The ground disasters caused by the Hyogo-ken Nanbu Earthquake occurred in the areas stretching at the foot of the Rokkou Mountains and emerged in various types. We have been collecting data on these disasters, mainly investigating the relationship between some landslide occurrences and topographical-geological conditions. As a result of investigation, it is seen that several common conditions exist to these ground disasters accompanying the Earthquake.
Data on various hydrological and geochemical fluctuations caused by the 1995 Hyogo-ken Nanbu Earthquake were accumulated. We reported fluctuations of the relatively shallow groundwater at the Nuruyu area, the Northern Awaji Islands, and those of the deeper one at the regional (about 300km×300km) scales. Then, we briefly reviewed the precursory geochemical changes in groundwater. Groundwater gushed out soon after the earthquake at around the Nuruyu and Nojima-Tokiwa areas, which are situated to the east of the Nojima fault, and then has dried up within the several months. There has been no sign of the recovery of groundwater level in wells one year after the earthquake. The water springing out at the hanging wall side along the Nojima fault seemed to be moved from the eastern mountainous area where groundwater has dried up. Groundwater in deep underground also fluctuated by the earthquake and their spatial distribution seemed to be related to the distance and the direction from the epicenter of the earthquake. Temperature increase at several hot springs has also been observed after the earthquake. The investigation of the groundwater fluctuations is important not only for the prediction of earthquakes but also for the engineering geological activities, such as planning water supply for the mountainous villages, monitoring and solving groundwater pollution problems, and evaluating long-term stability of deep underground environments. We proposed several plans to monitor and manage data on groundwater fluctuations both in usual times and at earthquakes. We also showed the recommended countermeasure for the fluctuations of hot springs, and pointed out the importance to prepare water supply plans for the places where the drops of water levels have been observed by the previous earthquakes.