Response spectrum is a plot of the maximum responses (acceleration, velocity, or displacement) of idealized single-degree-of-freedom oscillators as a function of the natural frequencies of the oscillators for a given damping value. The response spectrum is calculated for a specified vibratory motion input at the oscillators’ supports.
Operational Basis Earthquake (OBE)
OBE is a ground motion with 10% probability of exceedance within 50 year period (475 years return period). The facility is expected to remain operational after the OBE event without damage. Reference: NFPA 59A.
Safe Shutdown Earthquake (SSE)
SSE is a Maximum Considered Earthquake (MCE) ground motion with 2% probability of exceedance within 50 year period. Plastic behavior and significant finite movements and deformations are permissible. The facility is not required to remain operational after the SSE event. Reference: NFPA 59A.
SSE & OBE Relationship
SSE is the maximum potential earthquake of the selected site, and the OBE will be decided accordingly.
The OBE acceleration is one-half of the SSE value in some regulatory standards.
The OBE acceleration may be less than one-third the SSE value in other references.
The U.S. Nuclear Regulatory Commission, Regulatory Guide 1.61 gives different damping values for SSE & OBE analysis. The damping values are higher for the SSE case.
Note that damping is non-linear for seismic response due to joint slipping and other factors. Higher damping is expected for higher input excitation.
Further information is at: Vibrationdata Earthquake Engineering Page
– Tom Irvine