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508 ENGINEERING GEOLOGY/Seismology Figure 10 Predicted median values of peak ground acceler ation as a function of distance for earthquakes of moment magni tude 5.5, 6.5, and 7.5, using equations derived from western North American (solid lines) and European (dashed lines) data sets The European equation is based on surface wave magnitude, so an appropriate empirical conversion to moment magnitude has been used The equations are based on different definitions of the horizontal component, the European equation using the larger of the two horizontal accelerations, the North American equation using their geometric mean; the former definition, on average, yields values about 1.17 times larger than the latter Different criteria were used in the two studies for selecting records, espe cially at greater distances, for regression In light of these various observations, it is not possible to draw conclusions about differ ences or similarities in strong ground motions between the two regions The North American equation is from Boore DM, Joyner WB, and Fumal TE (1997) Seismological Research Letters 68(1): 128 153; the European equation is from Tromans IJ and Bommer JJ (2002) Proceedings of the 12th European Conference on Earthquake Engineering, Paper no 394, London reason, for site-specific predictions, it is often preferred to predict the bedrock motions first and then model the dynamic response of the site separately (Figure 15) The nature of ground shaking at a particular site during an earthquake is influenced by many factors, including the distribution and velocity of the slip on the fault rupture, the depth at which the fault rupture is located, the orientation of the fault rupture with respect to the travel path to the site, and the geological structure along the travel path and for several kilometres below the site A situation that produces particularly destructive motions is the propagation of the fault rupture towards the site, which produces large-amplitude, high-energy pulses of velocity (Figure 16) The variable most often included in predictive equations after magnitude, distance, and Figure 11 (A) Values of peak horizontal ground acceleration recorded in the 1994 Northridge earthquake in California, plotted as a function of distance from the earthquake source, with an indication of the surface geology at the recording site The solid line is the result of a regression on the data using a typical model employed in ground motion prediction equations (B) Residuals group by site class; the horizontal lines show the mean of the residuals in each class, indicating that there is a tendency for stronger motions on softer ground However, the differences between the mean lines are small compared with the overall dispersion, thus inclusion of site classification in the predictive equation would result in only a modest reduction of the aleatory variability site classification is the style of faulting; equations that include the rupture mechanism as an explanatory variable all predict higher amplitudes of motion from reverse-faulting earthquakes than from strike– slip events The addition of this fourth explanatory variable, however, has an almost negligible impact on the scatter in the equations, with no appreciable reduction of the standard deviation Dense networks of accelerographs now exist in many countries around the world, but for many decades these were limited to a few regions such as California, Japan, Italy, Greece, and Yugoslavia In
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