Validation Of Deterministic Broadband Ground Motion And Variability From Dynamic Rupture Simulations Of Buried Thrust Earthquakes

We numerically model broadband ground motion (up to 5-7: 5 Hz) from blind-thrust scenario earthquakes matching the fault geometry of the 1994 M-w 6.7 Northridge earthquake. Several realizations are modeled (by varying the hypocenter location in the dynamic rupture simulation) in a 1D-layered velocity profile. In addition, we include Q(f), nonlinear effects from Drucker-Prager plasticity, and superimpose small-scale medium complexity in both a 1D-layered and 3D velocity model within the subsequent wave propagation. We investigate characteristics of the ground motion and its variability up to 50 km from the fault by comparing them with ground-motion prediction equations (GMPEs), simple proxy metrics, as well as strong ground motion records from the Northridge event. We find that median ground motion closely follows the trend predicted by GMPEs and that the intraevent standard deviation, although varying with hypocenter location, lies near that of GMPE models. Plasticity affects ground-motion amplitudes in regions near the source, reducing intraevent variability above similar to 0.5 Hz. Heterogeneity in the velocity structure on both the regional and small scales is needed for the simulated data to match two proxy metrics: the period-to-period correlation of spectral acceleration (SA) and the ratio of maximum-to-median SA. Although small-scale heterogeneity has a negligible effect on median SA for this style of rupture, it serves to significantly increase the cumulative absolute velocity, better agreeing with observations. When compared with strong-motion data, we find that long-wavelength velocity structure within our deterministic simulations reduces bias at both short and long periods. Finally, synthetic ground motion at both footwall and hanging-wall sites has no clear dependence on the distance to rupture (at both short and long periods); directivity is likely overpowering any hanging-wall effect.