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How S/P Amplitude Ratio Data Can Bias Focal Mechanism Estimates

Description: 

Earthquake focal mechanisms provide crucial information about fault geometry and stress orientations in the Earth’s crust. For small earthquakes, focal mechanisms are typically inferred through analysis of seismic radiation patterns, for example by using P-wave first-motion polarities to identify the mechanism’s nodal planes. In recent years, it has become common to incorporate S/P amplitude ratio data alongside P-wave polarities to further refine the mechanism solution. The motivation for this procedure is well-founded, as P- and S-wave radiation patterns depend systematically on the fault orientation, and thus S/P amplitude ratios provide potentially useful observations for the inference of focal mechanisms. However, in practice, S/P amplitude ratio measurements can be noisy and strongly influenced by factors that are unrelated to the source mechanism. In this study, I characterize several underappreciated and systematic issues with S/P amplitude ratio data that are relevant to the focal mechanism inversion problem. The analysis combines synthetic tests with new measurements of waveform amplitudes from tens of thousands of well-recorded M_L 1.0 and greater earthquakes in Nevada and eastern California. Key findings include that (1) the statistical distribution of S/P amplitude ratio data differs markedly in shape and width from the theoretical expectation, (2) S/P amplitude ratios decay systematically with distance beyond ~ 60 km or so, (3) this distance effect is more severe for small earthquakes than for large ones, and (4) modifying the frequency band in which amplitudes are measured can shift the observed amplitude ratio distribution but does not significantly mitigate issues (1) through (3). Taken together, these findings indicate that S/P amplitude ratios measurements are significantly influenced by differential path attenuation and signal-to-noise effects that are not easily accounted for with existing workflows. Unless handled with care, S/P amplitude ratio data may hinder rather than facilitate robust mechanism solutions.

Session: Advances in Reliable Earthquake Source Parameter Estimation - II

Type: Oral

Date: 4/16/2025

Presentation Time: 11:30 AM (local time)

Presenting Author: Daniel

Student Presenter: No

Invited Presentation: Yes

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