Between-event variance for large repeating earthquakes

We estimate the variance in ground motions related to repeated large earthquakes occurring on the same fault segment with similar magnitudes. We find eight earthquake pairs for which suitable strong-motion records exist. Two are crustal strike-slip earthquakes from California and six are subduction zone earthquakes from Japan. We consider only large earthquakes and deal with frequencies greater than the earthquake corner frequency, so the variability that is considered here is related to smaller scale differences in the rupture process, particularly on the part of the fault nearest the station. We find that the variance of the 5% damped spectral accelerations of these pairs, termed τ²_F, averages to about 45% and 80% of τ² for the crustal and subduction zone earthquakes, respectively, in which τ ² is the contribution of source variability to the total variability of ground motion estimated by some recent groundmotion prediction equations. We suggest that τ²_F is lower than τ ², for the frequencies at which τ ²_F is estimated, because it depends primarily on only local physical properties of a fault that are the same in repeated earthquakes. We therefore suggest that at sites where the hazard is controlled by a single rerupturing source, one could potentially use a between-event variance that is smaller than τ ² in seismic-hazard calculations. Thus, these results may help to resolve the inconsistencies that are now present between the national hazard maps and some precariously balanced rocks in southern California.