Local Effects Arrays

Abstract

It is not possible to give a precise definition of "local effects" as considered herein in contrast to "wave propagation effects" dealt with elsewhere in this report. By one possible definition, local effects might be those which affect the components of ground motion with a frequency greater than about 1 Hz (or, alternatively, wavelengths less than 1000 m). Seismologists have tended to concentrate their attention on frequencies less than 1 Hz and there is known to be a large degree of coherence in such frequency components over local areas. In the present context, local effects refer to geologic and topographic anomalies and the manner in which they affect ground motion in the frequency ranges of concern to engineers. In the most common specification of design ground motions for a building, no consideration is given to possible variations in ground motion over the site occupied by the building. This suffices for many practical purposes. In effect, one uses a single point specification, without consideration of gradients in the motions. However, for some special large structures (such as nuclear power plants, which may experience rocking and twisting input) and for extended structures (such as bridges, dams and pipelines), it is necessary to consider differential motions between points of the earth, that is, to consider horizontal gradients in the motions. The vertical gradient can also be important for embedded structures. In addition, there are other important engineering problems for which knowledge is incomplete, including interaction between soil and structure, soil failure and the nature of very long period motions.

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