Seismic regionalization or zoning: revision

Abstract

This note amplifies and revises my 1959 paper on seismic regionalization, which followed the USSR practice, with tentative application to the United States and to California. Strictures on the former mapping policy of the USCGS (now under NOAA) do not apply to the 1969 Seismic Risk Map of the United States, prepared by Dr. Algermissen and assistants. This map does not represent risk in the actuarial sense; like my maps of 1959, it indicates maximum expectable events, independently of their frequency of incidence. However, its zones are numbered 0 to 3, instead of indicating intensity on the Modified Mercalli or other scale. With small local deviations, which were duly explained, my 1959 regionalization for California depended largely on surface geology and soil conditions. Situation with respect to the main active faults is of less consequence in the long run, because of eventual exposure of any locality to shaking from one source or another. Subsequent experience, including instrumental recordings, has largely confirmed this with regard to expectable effects on ordinary construction. However it appears that different maps on different principles would be required with reference to large or tall construction, especially with foundations penetrating far below the surface. The M M intensity scale is applicable to effects on ordinary structures, and the commonly used correlation of the scale with accelerations has the same reference. Thus the association of about 0.1 g with intensity VII represents the general peak level during the earthquake, lasting long enough to accomplish damage to weak structures. The San Fernando earthquake of 1971 finally established the previously indicated conclusion that in areas of high intensity, especially during large earthquakes, much higher transient accelerations may occur. These larger motions, often associated with relatively long periods, were responsible for conspicuous damage to large engineering structures.

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