Analysis of the Seismic Risk on Firm Ground for Sites in the Central Boston Metropolitan Area. Optimum Seismic Protection for New Building Construction in Eastern Metropolitan Areas

摘要

The aim of this study was to develop a curve which estimates the risk that any given level of ground motion intensity will be equaled or exceeded in some future time period in the cities of Boston and Cambridge. The results were obtained by first taking instrumental and historical data from which the parameters such as the relative frequency of occurrence of various sizes of earthquakes in various area in the neighborhood of Boston were estimated. This information was then coupled with an attenuation law. The computational method used is described. The approximate zones of repeated earthquake activities in the New England area are shown on a map. The sources which surround Boston and which were considered in the main analysis are shown. The largest contribution to the risk in the final results came from the source north of Boston in the Cape Ann area and to a lesser degree from the sources south of Boston. This is because their frequency of occurrences is relatively high compared with other sources but primarily because their distance from the site, and therefore, the attenuation of the intensity, are relatively small. Because of the short distance to the site, the geometric definition of the Cape Ann source has a great influence on the risk. This fact led to three different models of that source and to two different sites, yet significant differences were obtained only under deliberately conservative assumptions. Therefore, it is felt that a single curve for the cities of Boston and Cambridge is justifiable.