Effects of tectonic regime and soil conditions on the pulse period of near-fault ground motions

In this article, the effects of site conditions (rock vs. soil) and tectonic regime (interplate vs. intraplate) on the pulse period of near-fault forward-directivity ground motions are investigated through linear regression analyses, and appropriate scaling laws that relate the pulse period to the earthquake magnitude are derived. The analyses of rock and soil sites are performed using interplate records only. The results show that for earthquakes of smaller magnitude, the pulse period is longer at soil sites than at rock sites. As the earthquake magnitude increases, the pulse period values at rock and soil sites converge. This observation is in agreement with findings reported in previous studies. However, as shown in this article, the effect of soil conditions on the pulse period is not statistically significant at the 10% level based on the available near-fault ground motion datasets. Regression analyses for interplate and intraplate records are also performed, including regressions with and without self-similarity. The results show that the pulse periods of interplate records are significantly longer (factor of 2.45, on average) than the pulse periods of intraplate records with similar magnitude. It is demonstrated that this difference should also be reflected in the rise times of the events originating in the two different tectonic regimes. Using the specific barrier model as a "tool" of analysis, it is shown that the significant difference in the pulse periods also implies a significant difference (factor of 4) in the local stress drops for the events of the two tectonic regimes. This latter conclusion is supported by recent reports in the literature of the reanalysis of eastern North American earthquakes, accounting more carefully for attenuation.