Beyond -1 Geometric Spreading in the Near-field: Insights From Theory and Simulation
Description:
In many subfields of seismology, it is very common for researchers to assume that the rate of geometric spreading for peak shaking follows the canonical -1 slope in log space that follows from the spreading of a body wave in a halfspace. However, there is a large body of evidence from ground-motion modeling studies showing that in the real Earth, the true geometric spreading is much larger in the near-field (distances 10 < R < 50 km). This has implications for the modeling of small earthquakes that are well modeled as point sources, such as induced earthquakes. In this work, we seek to develop a theoretical framework to explain these observations. We simulate broadband waveforms from two end-member simple, plane-layered 1D velocity models and show that the rate of geometric spreading for peak shaking metrics, such as peak displacement and peak velocity, is stronger than -1, and varies with earthquake depth. We understand this phenomena in terms of the partitioning of seismic wave energy across layer boundaries that may represent the transition from deeper unweathered crust to shallow weathered rock or sedimentary layering. We also show significant variability in the rate of geometric spreading in our near-field range. Finally, our simulation results indicate that models with a more complicated near-surface structure tend to have steeper geometric spreading; we attribute this to a loss of energy arriving at the surface due to underside reflections from a greater number of layers. Our results strongly underscore the need for the rate of geometric spreading used for downstream applications such as stochastic-deterministic ground motion simulation, stress drop estimation, or the study of site effects, to be established from the data gathered for the region of interest, rather than to be assumed to be -1 a priori.
Session: Recent Advances in Modeling Near-source Ground Motions for Seismic Hazard Applications - II
Type: Oral
Date: 4/16/2025
Presentation Time: 10:30 AM (local time)
Presenting Author: Savvas
Student Presenter: Yes
Invited Presentation:
Poster Number:
Authors
Savvas Marcou Presenting Author Corresponding Author savvas.marcou@berkeley.edu University of California, Berkeley |
Douglas Dreger ddreger@berkeley.edu University of California, Berkeley |
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Beyond -1 Geometric Spreading in the Near-field: Insights From Theory and Simulation
Session
Recent Advances in Modeling Near-source Ground Motions for Seismic Hazard Applications