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|Title:||State-of-the-art for assessing earthquake hazards in the United States. Report 8, Duration, spectral content, and predominant period of strong motion earthquake records from Western United States|
|Authors:||Chang, Frank K.|
Krinitzsky, E. L.
Earthquake resistant structures
Western United States
|Publisher:||Soils and Pavements Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
Abstract: The purposes of this investigation were principally to assess the duration and spectral content of strong-earthquake accelerograms and, indirectly, to consider their applicability in earthquake design. Correlations of duration with MM intensity for the near and far fields and for Richter magnitude have been obtained. Difference in durations for soil and rock sites was determined. A set of relations between the duration and distance for soil and rock sites was established from records of the San Fernando Earthquake of 9 February 1971 (magnitude of 6.5). Values for other magnitudes were extrapolated. Duration is taken to be the time interval between the first and last peaks of acceleration equal to or greater than 0.05 g. The spectral content in the range of 0.1-10 Hz for strong-motion records in western United States for acceleration level equal to or greater than 0.05 g was processed with the modified Nigam and Jennings' response spectra computer programs. The corrected accelerograms on the digital magnetic tapes of NIS 130, 131, and 132 provided by the California Institute of Technology were the input data for this study. The critical damping ratios of 5.0, 7.5, and 10.0 percent were assigned to the soil (or soft, alluvial), intermediate (firm sediments), and rock (or hard) sites, respectively. The relative response spectral amplitudes of acceleration, velocity, and displacement were reduced to the ground surface by dividing the relative response spectral amplitude by the dynamic amplification factor of 1/2h , where h is the critical damping ratio. The frequency-amplitude spectra were then plotted as a function of magnitude, epicentral distance, and site conditions. The characteristics of duration are as follows: (A.) duration is greater in the near field than in the far field and greater in alluvium than in rock with the duration-ratio of alluvium to rock approximately 2 to 1; (B.) duration increases with magnitude and intensity, as expected; and (C.) the maximum duration at a source in rock for magnitude 8.5 was extrapolated to be about 43 sec. The duration is twice as long where there is magnification by alluvium or soil. Frequency content and spectral shape have the following characteristics: (A.) the predominant frequencies of the strong-motion earthquakes for the magnitude of 5.3-7.7 are in the range of 0.1-6.67 Hz; (B.) the maximum acceleration, velocity, and displacement levels are within the ranges of 1.5-5.0, 0.5-1.5, and 0.1-0.5 Hz; (C.) the predominant period of the acceleration spectra does not increase with distance within the range of 0-90 km, but the predominant period of displacement does; (D.) the spectral mode shape depends on the source spectrum function (magnitude), distance, and local geological conditions; (E.) generally, the average or upper-bound peak amplitudes have a uniform envelope, but the possible maximum acceleration was found to be about 0.5 g near surface faulting for the discrete frequency range of 4-5 Hz; and (F.) the summation of the maximum amplitude level of the predominant frequencies equals approximately the total amplitude of the ground motion in time history if the selected damping ratio corresponds to the geological condition of the recording site. An assumption is made that the amplitudes of all discrete frequencies on the envelope are in phase.
|Rights:||Approved for public release; distribution is unlimited.|
|Appears in Collections:||Miscellaneous Paper|
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