The left column compares histograms of the log of predicted-to-observed PGV ratios for the Northridge earthquake.The predicted ground motions were computed using rock Greenís functions and site corrections derived empirically.In this case we used the QTM site corrections employed by the southern California TriNet ShakeMap. The histograms compare the performance for quaternary, tertiary and Mesozoic sites.The red bars show that stations located in the Los Angeles basin are systematically under predicted by factors of 2-3.Slide 14 illustrates the predicted-to-observed amplitude ratios in map view.

The right column compares the predicted-to-observed ratios, where the predicted ground motions were computed with site specific (hard and soft rock) Greenís functions with no additional site corrections applied.The ratios were then classified according to the Geomatrix scheme.The red bars again show the Los Angeles basin sites and it is observed that by using site specific Greenís functions there is a considerable improvement in the amount of under prediction (compare the quaternary and class D histograms).