Project Abstract
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Small-scale, near-surface heterogeneities may affect ground motion estimates significantly, in particular as the highest frequency increases, and are currently not included in the SCEC Community Velocity Models (CVMs). We collected more than 700 Vs30 measurements in the greater Los Angeles basin. We then generated semi-variograms for all the collected Vs30 measurements from the greater Los Angeles basin. The semi-variogram for the Vs30 measurements suggests a Hurst number of 0.25-0.43 (fractal dimension 1.57-1.75). We then simulated 0-2.5 Hz ground motions for the 2008 Mw5.4 Chino Hills, CA earthquake in a subset of the SCEC CVM 4.0. We assume a minimum Vs of 200 m/s, and a fractal distribution of the near-surface heterogeneities with a Hurst exponent of 0.1 and standard deviation of 10% is added to the upper 800 m of the subset. We computed ground motions for a lossless model and for various linear relations between Qs and Vs. We find amplification/de-amplification of PGVs by up to a factor of 2, and that twice as large an area experiences de-amplified PGVs as compared to the area that experiences amplified PGVs. We find that the ’usual’ Qs-Vs relation Qs=50Vs (km/s) grossly underpredicts the PGV from the data at this and other basin sites. Results using other Qs-Vs relations (in particular, higher Qs values, such as Qs=250Vs) provide better match to the PGV and duration of the data. This is expected, as the the small-scale heterogeneities substitute some of the effects from the intrinsic attenuation specified by the Qs-Vs relation. |