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a. Strong ground motion data, where available, should be evaluated to develop acceleration-attenuation relationships. Families of curves have been developed for areas of the U.S. where strong motion data is available. [ , b. Isoseismal maps constructed from historical earthquake data provide regional intensity attenuation data when strong motion information is unavailable. These maps have been produced from many of the high magnitude events in the U.S. The maps IlIJst be used with concurrent consideration for the geology of the site to be most effective. The maps have illustrated significant differences in intensity attenuation in the U.S. between areas east of the Rocky Mountains versus those west of the mountains. Intensity attenuation is much slower in the area east of the mountains. 6. Re servoir- Induced Se ismic i ty Reservoir-induced seismLcLty should be considered in establishing seismic loadings for high dams if the proposed reservoir contains active faults within its hydraulic regime and if the regional and local geology and seismic record within the reservoir's hydraulic regime may be judged to indicate potential for reservoir-induced seismicity. If all the faults within a reservoir are considered tec tonically inac tive, the possibility of reservoir-induced seismicity should not be totally ruled out. The potential should still be considered to exist if the local and regional geology and seismicity suggest that the area is subject to reservoir-induced seismicity. The magnitude of reservoir-induced earthquakes is established using the guidelines for likelihood of occurrence similar to those used for the design earthquakes. E. Selection of Design Earthquakes When the evaluation of the earthquake factors described in section D is completed, the maxilllJm design earthquake (MDE) and the operating basis earthquake (OBE) are selected on the basis of an integrated evaluation of the earthquake factors. The MDE is the largest earthquake used in the seismic analysis of the dam and is generally equated to the controlling maxilllJm credible earthquake (MCE) for the site. The OBE, usually smaller than the MDE, represents the maximum level of ground shaking that can be expec ted to occur at the site during the economic life of the dam. It may not be possible to show that all possible tec tonic features have been discovered. Based on investigations, gaps of information may exist. If so, conservatism may be desirable dependent upon the potential hazards associated with the dam. 1. Maximum Credible Earthquakes The first part of the investigation for selec ting the MDE is to estimate the hypothetical MCE for each potential earthquake source, judged to have a significant influence on the site, from the information developed in section D. The hypothetical MCE for each -11- ..