Report - PEER - University of California, Berkeley

5. DISCUSSION AND CONCLUSIONFrom the preceding discussion it is clear that the basic information required for allrepresentations of the earthquake ground-motions to be considered in design is themagnitude and distance of the dominant earthquake scenario. For most current designcodes, however, there is no facility for performing disaggregation and the controllingearthquake scenarios are effectively invisible to the design engineer. Providing insightinto dominant earthquake scenarios not only enables comprehensive definition of theground motions but can also facilitate communication regarding seismic risk(Merovich, 1995).Clearly there is much to be gained by providing some information regarding thedisaggregated hazard in addition to the hazard maps in a code. There is, however,another possibility, which is to substitute the maps of ground-motion parameters for agiven return period with maps of the magnitude and distance pairs that dominate thehazard at a given return period for each ground-motion parameter (e.g., Harmsen etal., 1999). There is also the possibility to go one step further and present maps of M-D pairs corresponding to the dominant source for each site, or if the hazard is affectedby two different types of source, by a pairs of maps for each source (Bommer, 2000).The third element of the disaggregated hazard, the number (ε) of standard deviationsaway from the logarithmic mean, needs to be accommodated in the calculation of theresulting ground motions. There is no way to prescribe a procedure for drafting suchmaps, since they would need to be adapted to the characteristics of the seismicity ofeach region; Bommer and White (2001) present an illustrative application for CentralAmerica. These maps are not unlike the scenario ground motion maps proposed byAnderson (1997) but rather than presenting a single ground-motion parameter on eachmap, two maps (one of magnitudes, one of distances) provide the informationrequired to determine all the required features of the design ground motions. At thispoint it is important to make a clarification: to represent the hazard from a givenseismic source by a single M-D pair, coupled with a constant value of ε, is anapproximation, since the hazard at different response periods will be dominated bydifferent M-D combinations and different degrees of aleatory variability. Theacceptability of this approximation, however, needs to be assessed not in absoluteterms but in comparison with the much cruder approximations made in current codeformats for expressing earthquake actions for design.A key element in improving code representations of seismic actions is discardingthe UHS concept — which many consider appropriate because of the inherentproblems that it presents for any representation of earthquake actions beyond thespectral response at the structure’s fundamental period of vibration — and alsodropping the insistence on the total probability theorem (which the codes of Portugaland China have already done). This does not imply, however, total abandonment ofprobability: adjustments to the design return periods on each map can take account ofthe reduction of exceedance frequencies caused by separating seismic sources. Giventhe rather arbitrary basis of the return periods currently used in design codes, and the478