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|Title:||State-of-the-art for assessing earthquake hazards in the United States. Report 20, The contribution of directivity focusing to earthquake intensities|
|Authors:||University of California, Berkeley. Department of Geology and Geophysics.|
Soils and Pavements Laboratory (U.S.)
Bolt, Bruce A., 1930-2005.
|Publisher:||Geotechnical Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
Abstract: This report describes available evidence for effects of moving sources in earthquakes and analyzes them in terms of the physics of wave emission. The study is addressed mainly to questions arising in geotechnical investigations for engineering purposes and suggestions are made on the significance of the effect in strong ground motion estimation. From a seismological point of view, the effect of the moving source has been clearly demonstrated in numerous studies using seismographs located at both moderate and great distances (i.e., the far field) from the source. Such studies, however, usually concern long-period seismic waves with periods above 2 to 5 seconds. Second, when seismic waves in the near field with a range of wave frequencies characteristic of engineered structures (i.e., 1 Hz to 10 Hz) are considered, there is as yet only limited definitive evidence available, and this is somewhat contradictory. Nevertheless, the likelihood is that the elementary predictions of the magnitude of the effects can be sometimes modified by other features of the source mechanism, the geological variations along the wave paths and within the fault zone. Third, the ratio of peak horizontal ground motion in the forward direction of fault rupture to the peak motion in the backwards direction is probably greatest for ground displacements and velocities and least for peak accelerations. High frequency ground accelerations show variations due to scattering, attenuation, and source asperities that mask directivity effects. The conclusions are based largely on analyses of strong ground motions recorded in the 1979 Coyote Lake, 1979 Imperial Valley, and 1980 Livermore earthquakes, all in California. Each earthquake provides positive evidence for a measurable but variable directivity focusing.
|Rights:||Approved for public release; distribution is unlimited.|
|Appears in Collections:||Miscellaneous Paper|