Some Earthquake Engineering Terminology

Response Spectrum

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

One thought on “Some Earthquake Engineering Terminology

  1. Pingback: Seismic Test & Analysis Webinars | Vibrationdata

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