What is to be learned from the Hanshin-Awaji earthquake?
Earthquakes are major natural disasters which are studied worldwide, aiming at improved safety. Recently, the Hanshin-Awaji great earthquake strongly reminded people of the lessons to be learned for increased safety in the future. This large earthquake of Richter-magnitude 7.2 occurred on 17 January 1995, and resulted in damage in the Kobe area. This earthquake is to be classified as a 'directly-under-city' type. The earthquake resulted in very peculiar phenomena of instantaneous destruction of structures, such as tensile brittle fracture at the top of vertical steel columns and steel-reinforced concrete columns at ground floor level, or buckling. The facts show that the effects of this earthquake were quite different from those of ordinary ones coming from a very distant seismic source. These phenomena taught engineers that a longitudinal stress wave pulse caused by the impact coming from a seismic source located directly below the surface is responsible for the destruction of established structures on the surface. Based on these facts a new method is reported for analysing the stress-time relationship for critical locations in engineering structures erected on the surface. It is suggested that this newly proposed longitudinal stress wave analysis method should, in cases of directly-under-city earthquakes, be used in place of the conventional analysis method based upon lateral vibration only. In addition, a scheme is proposed for minimizing the damage in future directly-under-city earthquakes.