The needs for research and investigation with special reference to earthquake observation have been increased according to the advent of high-rise buildings, long-span bridges and nuclear power facilities. The purpose of the earthquake observation is to clarify soil-structure interaction problems and also to get rational and safe aseismic design criteria. Consequently, ever since the observation of underground earthquake motions was started in the 1960's, efforts have been made to obtain exact knowledge of the characteristics of ground motion, the dynamic behavior of structures, and the input motions to structures. These efforts were promoted by the development of bore-hole instrumentation techniques.

Along with the purpose mentioned above the object of the earthquake observation is fundamentally classified into two categories. The one is mainly to obtain the information on the dynamic elastic characteristics of soils and structures during small or moderate earthquakes, including after-shocks, and earthquake swarms. The other, as the project for observation of strong-motion which was started early in the 1950's by installing strong motion accelerographs, is to get the information on the dynamic elasto-plastic characteristics of soils and structures during strong earthquakes that rarely occur.