Spatially continuous models of aleatory variability in seismic site response for southern California

Abstract

We develop an empirical, spatially continuous model for the single-station within-event ( ϕ SS ) component of earthquake ground motion variability in the Los Angeles area. ϕ SS represents event-to-event variability in site response or remaining variability due to path effects not captured by ground motion models. Site-specific values of ϕ SS at permanent seismic network stations were estimated during our previous work [1]. Here we first fit a model to ϕ SS conditioned on the time-averaged shear wave velocity in the upper 30 m (V S30 ). We observe that stations on soft soil have larger average variability in site response than those on rock, especially at short periods. We use regression kriging to spatially interpolate ϕ SS to an even grid spacing, using the V S30 -scaling as the background model [2]. This improves on previous work that used ordinary kriging for interpolation [1]. We find that ϕ SS ranges from about 0.1 to 0.4 natural log units in our study area, representing variations in site response at single locations of a factor of 1.1 up to a factor of 1.5. There is both greater variability and more coherency in the variability for short-period site response than for long periods. We recommend using these ϕ SS models with the site response models of [1] for applications where quantification of variability is needed, such as probabilistic seismic hazard analyses.