Accession Number : ADA132893

Title :   State-of-the-Art for Assessing Earthquake Hazards in the United States. Report 20. The Contribution of Directivity Focusing to Earthquake Intensities.

Descriptive Note : Miscellaneous paper,


Personal Author(s) : Bolt,Bruce A

PDF Url : ADA132893

Report Date : Aug 1983

Pagination or Media Count : 98

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.

Descriptors :   *Ground motion, *Earthquake engineering, *Seismic waves, Sources, Near field, Displacement, Intensity, Far field, Faults(Geology), Peak values, Horizontal orientation, Acceleration, Directional, Focusing, Hazards, Threat evaluation, Mathematical prediction, Seismic data, California

Subject Categories : Seismology

Distribution Statement : APPROVED FOR PUBLIC RELEASE