The Effect of Geological Site Conditions on Earthquake Intensity : Seismic Zonation

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This research addresses the relationship between surface geology, as described in geological maps, with the earthquake strong ground motions recorded at a site. The motivation is to make more precise the general observation that structures built on rock sites experience the least damage and those on soft soil the greatest damage during an earthquake. The aim is to address a scale which falls between "macrozonation" - seismic regionalization - and "microzonation", that is, site specific intensity modification. The Whittier Narrows earthquake of October 1, 1987 and the Loma Prieta earthquake of October 17, 1989 produced the largest suite of strong ground motion records (accelerograms) ever taken in just two events. In addition, many accelerograms have been recorded on a variety of soil and rock conditions during other earthquakes. This study examines this extensive data library to correlate the intensity characteristics of each instrument site with characteristics of surface geology, as described on surface geology maps. This correlation has application in determining priorities for the rehabilitation of structures, in land use planning, and in determining probable maximum loss estimates prepared for insurers, bankers and business.