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The application
of empirically derived site corrections was found to not significantly
improve the predictions. This is due to the fact that the corrections are
derived from observed rock/site ratios and the observed rock records are
likely to be more complex than the rock Green’s functions we use due to
regional structure. Slide 14 compares predicted-to-observed PGV ratios for
the case in which the predicted rock PGV are modified by the QTM corrections
utilized by the southern California TriNet ShakeMaps. Significant under prediction is observed
at stations in the Los Angeles basin.
It may be possible with 3D numerical approaches to develop a set of
3D-to-1D corrections.
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The application
of both hard rock and soft rock Green’s functions to account for site
variability was found to significantly improve the comparison between
observed and predicted ground motions, and is shown in slide 14. Note that the use of multiple 1D velocity
models to account for site variability may not be necessary if a set of
3D-to-1D corrections can be obtained numerically.
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The application
of an empirical attenuation relationship that incorporates directivity
affects was found to greatly improve the ground motion simulations by
providing more rapid preliminary estimates, and providing an empirical base
level that is amenable to different levels of approximation such as strong
motion centroid mapping (TriNet) and source specific conservative shakemaps.
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Alternative
methods of simulating ground motions to high frequency were found to improve
results somewhat but not enough to offset additional computational cost. It may be worthwhile to investigate
improving the computational efficiency of these methods.
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