Assessment of earthquake hazard in the Tokyo area, Japan

Abstract

The probability of occurrence of an inland earthquake originating beneath the Tokyo area is evaluated for a 10-year period from 1991. On the basis of an earthquake catalogue containing some 40 earthquakes, which caused some damage to Edo (now Tokyo) over a period of 400 years, the parameters of the Weibull and lognormal probability distributions applied to recurrence interval statistics are estimated. With these parameters, it is concluded that an earthquake directly beneath the capital, the magnitude of which is equal to or greater than 6.0, occurs with a probability of 0.40 for the period in question. When the magnitude threshold is assumed as 6.4 and 7.0, the probabilities become 0.17 and 0.049, respectively. These probability evaluations are extended to those for the seismic intensity at typical sites in the area, although no accurate evaluation is possible because the exact focal depths of historical earthquakes are not known. On the assumption that these earthquakes occur near the upper surface of the down-going Philippine Sea plate, however, a typical depth of 30 km or so is suggested. Should an earthquake having a mean magnitude of 6.4, say, and at a mean location of past earthquakes occur, the probability of seismic intensity at typical sites in the capital area exceeding 5 on the Japan Meteorological Agency (JMA) scale would amount to 0.17. The possibility of having a highly damaging earthquake of intensity 6 on the JMA scale also cannot be ruled out, judging from earthquake disasters in the past, although the probability does become lower. To summarize, it may be said that the probability of Japan's capital area being hit by a damaging earthquake in the foreseeable future is not particularly low.