Modeling of ground-motion amplitude saturation at large magnitudes and short distances

Camilo Ignacio Pinilla Ramos, Norman A. Abrahamson, Yehuda Ben-Zion, Chih-Hsuan Sung, Jeff Bayless, & Robert W. Graves

Published September 8, 2024, SCEC Contribution #13750, 2024 SCEC Annual Meeting Poster #166

As part of the NGA-W3 project, we are developing ground-motion models that include constraints on near field behavior from numerical modeling of ground motions. Current observational datasets of ground motions (e.g., NGA-W2, NGA-W3) are sparsely populated at large magnitudes and short distances, a range that usually governs the seismic hazard in sites close to active faults. To better characterize near-fault shaking effects within ground-motion models, we need to combine our sparse observations with other approaches. To support the NGA-W3 project, we have performed a series of simulations using the Graves and Pitarka (GP) method as implemented in the Broadband Platform (BBP) of SCEC. This method computes waveforms deterministically up to a specified frequency (“merging” frequency, typically 1 Hz), and adopts a stochastic scheme for higher frequencies. A match filter process is adopted to combine the low- and high-frequency portions of the simulated wave field. The deterministic GP simulations scale the rise time with the subfault displacement, leading to partial saturation of the ground motion, but with some magnitude scaling at short distances and large magnitudes. In contrast, the high-frequency scheme, which models shorter wavelengths and rupture scales, produces a saturation of the simulated ground-motion amplitude at magnitudes larger than about 6.8: i.e., at a fixed short distance, the high frequency ground-motion amplitude on average does not increase as the magnitude increases for magnitudes greater than 6.8. The strong saturation in the stochastic part of the simulation implies that the subfaults of the stochastic scheme radiate statistically similar seismic energy independently of the size of the earthquake. To reconcile the differences in the magnitude-distance saturation across the merging frequency, we are extending the deterministic part of the GP rupture generator to higher frequencies and are considering additional source effects that may improve the short-distance saturation modeling, as for example including non-double couple components in the shear dislocations. Intensity data sets, which include more observations at short distances from large magnitudes than the ground-motion data sets, also provide useful information to evaluate the short-distance saturation. Thus, the results will be compared not only with the sparse available observations at these magnitude and distance ranges, but also using intensity prediction equations (IPE).

Key Words
Ground-motion Saturation at large magnitudes and short distances

Citation
Pinilla Ramos, C., Abrahamson, N. A., Ben-Zion, Y., Sung, C., Bayless, J., & Graves, R. W. (2024, 09). Modeling of ground-motion amplitude saturation at large magnitudes and short distances. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions (GM)