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

3 16 De-yi Feny, Ming-zhoti Lin, and Ktin-yuan ZhtianyuyuanFigure 2. Intensity of Haiyuan earthquake.By using the fuzzy description method mentioned above, we may determine thecharacters of the mechanical effects of a historical earthquake at all recording points,and therefore, recognize the compressional points to be denoted by the sign “+”,and the rarefactional points to be denoted by the sign ‘I-’’. As an example, Table 3shows the data of compressional and rarefactional points for the 1920 Haiyuan(M = 8.5) great earthquake. These data may be used to determine its sourcemechanism.3.2. MethodUsing the data of compressional and rarefactional points obtained from recordedmacroscopic phenomena of a historical earthquake, we may determine its sourceniechanism, namely, the fault plane and the principal compressional and dilatationalstress axes by applying the method based on the first motion sigris of P waves.However, there may be a range of source mechanisms because of insufficient datapoints. Consequently, we often obtain several possible models of source mechanismfor the same earthquake. For choosing the optimal model, we may apply the methodof fuzzy multifactorial evaluation (Feng et al., 1983~). This method consists of thefollowing main steps.First, we must take several factors for fuzzy multifactorial evaluation. In thispaper, three factors were used:(1) Comparison with regional seismotectonics. Suppose we have n possible faultplane solutions for an earthquake, then we can compare them with the main regionalgeological fault and find the corresponding standard deviations in orientation,(TI, uz, . . . , un. The values l - o,,i = l, 2, . . . , n may be considered as a factorfor fuzzy multifactorial evaluation.