Classification of Main Shocks and Aftershocks in the NGA‐West2 Database

Previous studies have found a systematic difference between short‐period ground motions from aftershocks and main shocks, but have not used a consistent methodology for classifying earthquakes in strong motion data sets. A method for unambiguously classifying earthquakes in strong motion data sets is developed. The classification is based on the Gardner and Knopoff time window, but with a distance window based on a new distance metric, CR JB , defined as the shortest horizontal distance between the centroid of the surface projection of the potential aftershock rupture plane and the surface projection of the main shock rupture plane. Class 2 earthquakes are earthquakes that have a CR JB distance less than a selected limit and within a time window appropriate for aftershocks. All other earthquakes are classified as Class 1. For maximum CR JB of 0 km and 40 km, 11% and 36% of the earthquakes in the NGA‐West2 database are Class 2 events, respectively.