Probability of earthquake occurrence as estimated from crustal strain |
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| Authors: | Tsuneji Rikitake |
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| Institution: | Earthquake Research Institute, University of Tokyo, TokyoJapan |
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| Abstract: | Statistics of ultimate strain of the earth's crust are obtained on the basis of levelling and triangulation data over earthquake areas. The mean value of ultimate strain e0 is obtained as 5.3 · 10?5 with a standard deviation σ amounting to 3.3 · 10?5 on the assumption that the deviation from the mean value is described by a Gaussian distribution.Assuming that crustal strain increases linearly with time t from an approximately zero value immediately after a large earthquake, which occurred at t = 0, the probability of having a crustal rupture or an earthquake occurrence during a time-interval from 0 to t can be calculated from e0 and a along with the data for strain accumulation over the area concerned as brought out by repetitions of geodetic survey.Applying the above theory to an area southwest of Tokyo, where an earthquake of magnitude 7.9 took place in 1923, the probabilities for repetition of an earthquake there are estimated as 0.2, 0.5 and 0.8 respectively for periods 1925–1980, 1925–2030, and 1925–2080.Similar studies are made for the areas off eastern Hokkaido and the Tokai district in Central Japan. No geodetic data over focal regions are available in these cases because observations are made only on land more than 100 km distant from epicentral area off the coast. In the circumstances theoretical land deformations caused by a plate subduction, which is believed to be taking place at the trench axis, are compared to the deforma tions actually detected by repeated surveys. Although the reliability of probability calculated on the basis of such processes may be substantially lower than that based on data taken in an area immediately covering a focal region, it is striking that the probabilities of reoccurrence of a large earthquake for a time-interval from the last shock to the present are so high that they exceed 0.8 ~ 0.9 for reasonable values of parameters involved. |
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