0-5 Hz Deterministic 3D Ground Motion Simulations for the 2014 La Habra, CA, Earthquake

Zhifeng Hu

Under Review January 26, 2022, SCEC Contribution #11781

We have simulated 0-5 Hz deterministic wave propagation for a suite of 17 models with the 2014 Mw 5.1 La Habra, CA, earthquake in a mesh from the Southern California Earthquake Center Community Velocity Model Version S4.26-M01 with a finite-fault source. Strong motion data at 259 sites within a 148 km by 140 km area are used to validate our simulations. Our simulations quantify the effects of statistical distributions of small-scale crustal heterogeneities (SSHs), frequency-dependent attenuation Q(f), surface topography, and near-surface low velocity material (via a 1D approximation) on the resulting ground motion synthetics. The shear wave quality factor QS(f) is parameterized as QS,0 and QS,0 f^γ for frequencies less than and higher than 1 Hz, respectively. We find the most favorable fit to data for models using ratios of QS,0 to shear-wave velocity VS of 0.075-1.0 and γ values of 0.0-0.4. Models including topography and a realistic near-surface weathering layer tend to increase peak velocities at mountain peaks and ridges, with a corresponding decrease behind the peaks and ridges in the direction of wave propagation. We find a clear negative correlation between the effects on peak ground velocity amplification and duration lengthening, suggesting that topography redistributes seismic energy from the large-amplitude first arrivals to the adjacent coda waves. A weathering layer with realistic near-surface low velocities is found to enhance the amplification at mountain peaks and ridges, and may partly explain the underprediction of the effects of topography on ground motions found in models. Our models including topography tend to improve the fit to data, as compared to models with a flat free surface, while distributions of SSHs with constraints from borehole tend to impose second-order effects on the ground motion synthetics. Accuracy of the velocity model, particularly the near-surface low velocities, controls the resolution with which the anelastic attenuation can be determined. Our results demonstrate that it is feasible to use fully deterministic physics-based simulations to estimate ground motions for seismic hazard analysis up to 5 Hz. Here, the effects of, and trade-offs with, near-surface low-velocity material, topography, SSHs and Q(f) become increasingly important as frequencies increase toward 5 Hz, and should be included in the calculations. Future improvement in community velocity models, wider access to computational resources, more efficient numerical codes and guidance from this study are bound to further constrain the ground motion models, leading to more accurate seismic hazard analysis.

Hu, Z. (2022). 0-5 Hz Deterministic 3D Ground Motion Simulations for the 2014 La Habra, CA, Earthquake. Geophysical Journal International, (under review).

Related Projects & Working Groups
High-f, Ground Motion