This paper deals with the lessons learned from the Hyogo-ken Nanbu earthquake (M = 7.3) which occurred in Kobe, Japan in 1995 and whose epicenter was shallow, namely, about 15 km in depth. Right after the earthquake, field observations were carried out to investigate the failure mechanism of structures. The results showed that certain structures, particularly a tunnel, were damaged by an unusual failure mechanism which might have been caused by high-frequency impulsive vertical seismic waves. However, in the results of measurements taken by accelerometers, there was no evidence of such high-frequency impulsive vertical seismic waves. Thus, in February of 1995, measurements of the ground vibrations caused by the aftershocks of the earthquake were initiated, and were continued for about two years. To measure the aftershocks, servo-type accelerometers (DC~100 Hz), were used; they differ from conventional earthquake accelerometers (SMAC-MDU) whose measurement range is limited to DC~30 Hz.
The Hyogo-ken Nanbu earthquake (M=7.3) occurred in Kobe, Japan on January 17, 1995. Its epicenter was shallow, namely, about 15 km in depth. Numerous structures, such as bridges, buildings, tunnels etc., located along the earthquake fault, were destroyed. Many people believed that the failure mechanism of the structures might have been due to high-frequency vertical vibrations. However, in the published results of measurements taken by accelerometers, there was no evidence of such high-frequency impulsive vertical seismic waves. Therefore, it was decided that the ground vibrations caused by the aftershocks of the earthquake should be measured. To measure the aftershocks, servo-type accelerometers (DC~100 Hz) were used; they differ from conventional earthquake accelerometers (SMAC~MDU) whose measuring range is limited to DC~30 Hz. The measurements of the aftershocks were taken from February 1995 to May 1997. In this paper, the results of the two-year measurement period are shown and the ground vibrations of the aftershocks are discussed.
