A Near-source Saturation Model for EAS Based on the NGA-west3 Database
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. This is a geometrical effect: at near-fault stations, the relative distance between the closest and farthest portions of the rupture is larger. 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. Here we use the closest distance to the surface projection of the fault, the Joyner Boore distance, RJB. However, 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-fault data for site response and Q using fits to far-fault data. We parameterize a 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 parameters that minimize the L1 norm for records with RJB ≤ 10 km. Our model predicts h values for large M, where RJB = RRup, that are consistent with previous models for response spectra (~15-30 km). However, we find that saturation distances for smaller M depend on f and zTOR, where shallow earthquakes and high frequencies have shorter saturation distances, a previously unmodeled behavior. Overall, our new model fits the data better compared to existing models without these added input parameters.
Session: Recent Advances in Modeling Near-source Ground Motions for Seismic Hazard Applications [Poster]
Type: Poster
Room: Exhibit Hall
Date: 4/16/2025
Presentation Time: 08:00 AM (local time)
Presenting Author: Grace Parker
Student Presenter: No
Invited Presentation:
Poster Number: 55
Additional Authors
Grace Parker Presenting Author Corresponding Author gparker@usgs.gov U.S. Geological Survey |
Annemarie Baltay abaltay@usgs.gov U.S. Geological Survey |
Gail Atkinson gmatkinson@aol.com Western University |
David Boore dboore@yahoo.com U.S. Geological Survey |
Tristan Buckreis tristanbuckreis@ucla.edu University of California, Los Angeles |
Jonathan Stewart jstewart@seas.ucla.edu University of California, Los Angeles |
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A Near-source Saturation Model for EAS Based on the NGA-west3 Database
Category
Recent Advances in Modeling Near-source Ground Motions for Seismic Hazard Applications
Description