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The Use of Coda Waves for Characterizing the Site Effect on Strong Ground Motion

Keiiti Aki

Published 1994, SCEC Contribution #77

The well established separability of source, path and site effects on coda waves of local seismic events offers the most effective means for determining the site amplification factor empirically. Many studies have confirmed that the coda amplification factor represents those of S waves averaged over various directions of wave approaches. A systematic relation has been found between the coda amplification factor and the age of surface geology at the recording site in California, and has been used to interpolate coda amplification factors observed at selected sites to construct maps of amplification factors for various frequencies in California. We found that, in general, the coda amplification factor is applicable to strong ground motion at rock sites. It is also applicable to soil sites, if the basement acceleration level is lower than 0.1 g. We found, however, the coda amplification factor overestimates the observed strong motion amplification at soil sites when the basement acceleration level exceeds 0.1-0.2 g. These discrepancies can be explained by the increased absorption due to the non-linear histeresis in soil response. We present here encouraging results of successfully testing a preliminary method by M. Mahdyiar and others (1993) for correcting the coda amplification factor for the non-linear effect using the strong motion data from the Landers earthquake of 1992.

Aki, K. (1994). The Use of Coda Waves for Characterizing the Site Effect on Strong Ground Motion. Poster Presentation at Structures Congress XII.

Related Projects & Working Groups
American Society of Civil Engineers, Structural Division