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seismotec tonic struc ture or source area within the region examined is defined by msgnitude and or intensity, epicentra1 distance and focal depth. These MCEs are candidates for the controlling MCE. 2. Contro11in~ Maximum Credible Earthquake The second part of the investigation is to select the controlling MCE for the site as follows: a. Select the most conservative distance from each seismic source to the site. b. For each candidate MCE select strong motion records of earthquakes which have similar source and propagation path properties and were recorded on a foundation similar to that of the structure or, if these site-matched records are not available, a ttenua te the epicentra1 ground motion parame ters or MM intensity to the site using one or more applicable attenuation relationships. c. Select the controlling MCE based on the most severe ground motion parameters estimated for the site. There may be more than one controlling MCE because of the frequency characteristics on the dam and its components. 3. Maximum Design Earthquake The final se1ec tion of the MDE considers whether or not the dam DUst be capable of resisting the controlling MCE, which is a "worst case" situation. Usually, the MDE is equated with the controlling MCE. However, where the failure of the dam presents no hazard to life, a lesser earthquake for the MDE may be justified providing there are cost benefits and the risk of property damage is acceptable. 4. Operatin~ Basis Earthquake The second level of design earthquake, the OBE, represents the maximum level of ground shaking that can be expected to occur at the site during the economic life of the project, usually 100 years for dams. It reflects the desired level of protection for the project from earthquake-induced structural and mechanical damage and loss of service during the project's economic life, or remaining economic life for existing dams. The OBE should be based on a probabilistic analysis which accounts for the time element involved in the definition of the OBE. A probabi listic ana lysis invo 1ve s deve loping a magni tude-frequency or epicentral intensity-frequency (recurrence) relationship for each seismic source; projecting the recurrence information from regional information and past data into forecasts concerning future occurrence; attenuating the severity parameter, usually either peak ground acceleration or MM intensity, to the site; determining the -12- .