A New Near-Source Saturation Model for EAS based on the NGA-West3 Database

Grace A. Parker, Annemarie S. Baltay, Gail M. Atkinson, David M. Boore, Tristan E. Buckreis, & Jonathan P. Stewart

Published September 8, 2024, SCEC Contribution #13891, 2024 SCEC Annual Meeting Poster #173

We develop a model for the near-source saturation of ground motions for shallow crustal earthquakes using the NGA-West3 database of effective amplitude spectra (EAS, the smoothed quadratic mean of two horizontal-component Fourier amplitude spectra). Observations of ground motions show amplitude saturation at the closest source-to-site distances; in other words, the amplitude does not continue to increase as distance decreases. This is a geometrical effect: at near-fault stations, there is a larger relative distance between the closest and farthest portions of the rupture. Large-magnitude earthquakes have larger fault ruptures, and a larger proportion of those ruptures are also farther away from any given near-fault location, causing stronger saturation. Ground-motion models account for this by using a magnitude-dependent modification to the distance metric (here termed h), sometimes referred to as a pseudo-depth term. Most work on this topic has been performed using the distance to the closest point on the rupture surface (RRup) (e.g., Atkinson and Silva, 2000; Yenier and Atkinson, 2014; Boore and Thompson ,2015). Here, consistent with previous NGA models (Boore and Atkinson, 2008; Boore et al., 2014), we use the closest distance to the surface projection of the fault, the Joyner Boore distance (RJB). RJB has many benefits, including ease of calculation, but no magnitude-dependent models for h compatible with RJB exist. We develop an empirical h model that depends on moment magnitude (M), depth to top of rupture (ZTOR), and frequency (f), assuming a geometrical spreading of 1/R and adjusting near-field data for site response and Q using fits to far-field data. We parameterize a piecewise function of h with M that has a f-dependent breakpoint, with a constant h value for large M above the breakpoint, where ZTOR is 0. Below the breakpoint for small magnitudes there is a f- and ZTOR-dependent slope. Using a grid-search approach we find model parameters that minimize the L1 norm for records with RJB ≤ 10 km. We estimate h values for large M, where RJB approaches RRup, that are consistent with previous models for response spectra (~15-30 km). However, we find that saturation distances for small-to-moderate M depend on f and ZTOR, where shallow earthquakes and high frequencies have shorter saturation distances, a previously unmodeled behavior. Overall, our new model improves the fit to the data compared to existing models without these added input parameters.

Citation
Parker, G. A., Baltay, A. S., Atkinson, G. M., Boore, D. M., Buckreis, T. E., & Stewart, J. P. (2024, 09). A New Near-Source Saturation Model for EAS based on the NGA-West3 Database. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions (GM)