SCEC2024 Poster #181: Apparent geometric decay in near field for a 1D layered earth

Group A, Poster #181, Ground Motions (GM)

Apparent geometric decay in near field for a 1D layered earth

Poster Presentation

2024 SCEC Annual Meeting, Poster #181, SCEC Contribution #14013

Most earthquake damage resulting from high frequency strong ground motion occurs within a close proximity to the fault, for example the distance to source R <50 km. Recent studies have revealed that the high frequency ground motion decay much more rapidly within this distance range than the conventional assumption of R^-1 suggests. This indicates either significant inelastic attenuation or a steep geometric decay of R^-b, where b ranges from 1.3 to 1.6 (Atkinson, 2024). Distinguishing these two scenarios with observations alone is challenging (Atkinson, 2024; Ji et al., 2024). Here we investigate it using synthetic simulations, aiming to explore whether a 1D layered structure can produce ...such a steep geometric decay and evaluate how well the conventional assumption—that S-waves dominate near-field observations—holds. To simulate conditions of ground motion prediction and stress drop estimation, we consider a series of Mw 4-5 point sources with varying focal mechanisms, source depths, and Brune’s time functions, located beneath the center of a circular area. For each point source, we calculate 3-component 1D synthetic seismograms at a set of uniformly distributed stations within this circle. We then estimate the Fourier spectral amplitudes, ranging from 0.2 Hz to 25 Hz, and regress their expected amplitude decay rates with distance and corresponding uncertainties.
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