Validation of Deterministic Broadband Ground Motion and Variability with Ground Motion

Kyle B. Withers

Published August 3, 2016, SCEC Contribution #6481, 2016 SCEC Annual Meeting Poster #178

The decay of energy at high frequencies, decay as a function of distance, source model, and complementary relations between scattering and apparent attenuation are all important features that simulations need to accurately capture to be used for engineering purposes. With the recent addition of realistic fault topography in 3D simulations of earthquake source models, ground motion can be deterministically calculated more realistically up to higher frequencies. Here, we model dynamic rupture propagation for both a generic strike-slip event and blind thrust scenario earthquakes matching the fault geometry of the 1994 Mw 6.7 Northridge earthquake along rough faults up to 7.5 Hz. We include frequency-dependent attenuation via a power law above a reference frequency in the form Q0fn and include nonlinear effects via Drucker-Prager plasticity. We model the region surrounding the fault with and without small-scale medium complexity in both a 1D layered model characteristic of southern California rock and a 3D medium extracted from the SCEC CVM-4.26 model including a near-surface geotechnical layer. However, before the broadband deterministic simulations can be used for engineering applications, validation is required to demonstrate that the energy decay in the synthetics as a function of distance is similar to that for observed ground motions. Here, in addition to comparing with GMPEs, we compare with simple proxy metrics to evaluate the performance of our deterministic models and to determine the importance of the different complexities within our model. We find that 3D media heterogeneity, at both the long and short scale, is necessary to agree with data, and should be included in future simulations to best model the earthquake ground motion and variability. We also measure the decay of energy at high frequencies, known as κ, and find realistic results across a narrow bandwidth. We introduce a shallow layer with frequency-independent Q layer in the near surface that serves to simulate the site-effect of κ; this technique may become necessary as simulations continue to extend to higher frequencies.

Citation
Withers, K. B. (2016, 08). Validation of Deterministic Broadband Ground Motion and Variability with Ground Motion . Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology