Observed Ground Motions That Exceeded Design Response Spectra in the Western United States

Seismic design values are intended to be rarely exceeded, yet their reasonableness is often debated. This paper uses ground motion recordings to document exceedances of four contemporary seismic design spectra in the Western U.S.: the risk‐targeted maximum considered earthquake (MCE R ) spectrum, its probabilistic component without the deterministic cap, and the design earthquake spectrum (all from ASCE/SEI 7‐22), as well as the 2475‐year uniform‐hazard spectrum from the 2023 U.S. Geological Survey National Seismic Hazard Model. We compare each recording's RotD100 spectral acceleration to its site‐specific target spectra and interpret the results through seismic hazard disaggregation at selected locations. The survey reveals that all target spectra are exceeded by multiple recordings. MCE R exceedances concentrate near active California faults at short source‐to‐site distances and short periods, primarily driven by moderate‐magnitude events, with the deterministic cap increasing exceedance rates relative to the underlying probabilistic spectrum. Long‐period exceedances ( T > 1 s), caused by infrequent large‐magnitude earthquakes, are relatively sparse. Frequent short‐distance and small‐magnitude exceedances explain the observed high total residuals compared to disaggregation‐based epsilons. These observations demonstrate that high‐amplitude code spectra are physically attainable and encourage shifting the debate from the spectra's reasonableness toward balancing safety and construction costs.