Seismic Hazard Deaggregation

Seismic hazard deaggregation (also spelled disaggregation) is the post-processing step that opens up a probabilistic seismic hazard result to reveal which earthquakes actually drive it. A probabilistic seismic hazard analysis (PSHA) integrates over all magnitudes, distances, and ground-motion variability to return a single mean rate at which a ground-motion level is exceeded, but in doing so it loses sight of the individual scenarios. Bazzurro and Cornell's 1999 paper formalized how to invert this aggregation, expressing the contribution to the exceedance rate as a probability distribution over magnitude, distance, and epsilon — the number of standard deviations a target motion sits above the median prediction. The result identifies the controlling earthquake: the magnitude-distance-epsilon combination most responsible for the hazard at a chosen return period. This deaggregation is what lets engineers select realistic scenario earthquakes and ground-motion records for design and analysis. It bridges the probabilistic and deterministic worlds by naming the events hidden inside the integral.