For Concrete Structures: OBE (operational basis earthquake) = 144-yr Return Period (50 percent chance of being exceeded in 100 years).
Max. Design EQ: for Structures Defined As Critical use MCE, In general for other structures, MDE = 950-yr return period (10 percent chance of being exceeded in 100 years).
MCE is defined as the greatest earthquake that can reasonable be expected to be generated by a specific source (which comes from a deterministic site hazard analysis).
USACE references: Stability Analysis of Concrete Structures, Earthquake Design and Evaluation of Concrete Hydraulic Structures, Earthquake Design and Evaluation for CW Projects. Safety of Dams – Policies and Procedures.