Concealed Quaternary Faults Found in the Central Tokyo Metropolitan Area

The Central Disaster Management Council presents several scenarios of hazardous earthquakes that might strike the Tokyo metropolitan area in the near future. After the Great Hanshin Earthquake(January 17, 1995, M 7.3 on the JMA scale) which caused devastating damage to both human lives and economic activities, studies on major active faults throughout Japan were accelerated to evaluate their potential for producing serious earthquakes. Within the past decade, three inland earthquakes of M 6.9 to 7.3 occurred in Japan without obvious surface ruptures. The faults that caused those earthquakes, had not been identified previously by active fault researchers. Since historic times, the Tokyo metropolitan area has been heavily inhabited and artificially modified by various constructions; therefore its original geomorphologies, with which active faults are deciphered, have been almost lost to date. The authors summarized data on Quaternary faults found at three construction sites and twelve records of seismic profiles, and reexamined borehole data on restricted places in the metropolitan area. This revealed four concealed faults displacing middle to late Pleistocene deposits in Chuo Ward and one in Koto Ward. These concealed Quaternary faults are classified as Class C active faults with an average slip rate of 0.1 to 0.01 m/1000 years. Active faults, however, have not been studied in the central metropolitan area for some reasons. The authors would like to call for an immediate full-scale active fault study to prepare for earthquake disasters in the heart of Tokyo.