Historical Seismograms - Evidence from the AD 2000 Izu Islands ...

Historical Earthquakes of Thailand, Burma, and Indochina 265By using b-value, a-value can be determined by using the formula of Welkner(1965) as follows:a = log N(M15.0) + log(b In 10) + 5.0 b (3)where N(M>5.0) is the number of earthquakes having magnitude 5.0 or more.The annual number of earthquakes can be calculated theoretically by dividing Nby the period of observation, To. Puttingthe annual frequency of earthquake is equal toa1 = a - log To, (4)the mean return period is equal to4. Results and ConclusionFrom the magnitude-frequency analysis, the values of a and b are 6.654 and 0.75respectively. This means that there are about 54,000 earthquakes having magnitudegreater than zero and ten earthquakes having magnitude greater than 5.0 occurringannually in this region on the average. The value of b obtained in this paper agreeswell with S. Hattorri's (1974) result.Table 3 lists the mean return period (per month) and the frequency (per year)for earthquakes in the magnitude range 5.0 to 7.5. Figure 10 shows the relationbetween magnitude and the mean return period. It can be seen that an earthquakehaving magnitude 5.0 may occur approximately every month for this area, but forthe earthquakes having magnitude 7.0 and 7.5, the recurrence periods are 3 and 8years respectively.In conclusion, the above analysis of historical earthquake data indicates that theregion encompassing Thailand, Burma and Indochina is one of the seismically activezones of the world.Table 3. Mean Return Period and Frequency of Earthquakesfor Magnitude 5.0 to 7.5Magnitude Return period Frequency(Mb) (month) (Per year)5 .O 1.2 9.6595.5 2.9 4.0736 .Q 7.0 1.7186.5 16.6 0.7247 .O 39.3 0.3057.5 93.0 0.129