Application of High-Precision Questionnaire Intensity Survey Method to the Modified Mercalli Intensity Scale

Seismic intensity is an important parameter measuring earthquake shaking severity, especially in regions where few strong motion instruments are in operation. This study aims to apply high-precision questionnaire intensity method widely used in Japan to other earthquake countries.Based on the Modified Mercalli (MM) Intensity Scale, an intensity questionnaire form was prepared and survey was conducted for two California earthquakes and the 1988 Nepal-India border earthquake. First, intensity coefficients are assigned to each item category based on the definition of the scale. For each questionnaire, “average” item intensity is calculated by taking average of intensity coefficients as marked, and also “maximum” item intensity by taking maximum intensity coefficient among responses.In California, intensities of the USGS isoseismal maps are found larger than the “average” item intensities and is comparable to the “maximum” item intensities, presumably because the USGS guideline of intensity assessment is to choose the maximum damage phenomena.In order to solve this discrepancy, we introduced fuzzy set theory and expressed intensity coefficients as distribution of likelihood belonging to continuous intensity level. Accumulating membership functions corresponding to selected item categories, the maximum value of distribution suggests the most probable intensity.New method for questionnaire intensity evaluation is examined for previous data. Questionnaire intensities reasonably correlate with intensities locally reported by the USGS and Nepalese agency. Simple adjustment based on age of buildings is found satisfactory for the case of California but not so for the Nepal-India region.