Controls on Surface Wave Overtone Interference

Abstract:

Measurements of fundamental-mode (FM) surface waves along the minor arc are impacted by overtone interference. This is primarily due to major-arc overtones for Rayleigh waves and minor-arc overtones for Love waves. Interference contaminates measurements of phase and amplitude and can introduce bias in seismic images (Foster et al., 2014; Hariharan et al., 2020). Here, we explore how overtone interference depends on period, focal mechanism, source depth, and seismic velocity structure. By comparing measurements on synthetic seismograms that include all overtones to measurements on synthetic seismograms that include only the FM, we quantify the impact of interference.

We compare these measurements to calculations of the excitation of the overtones and FM, which explains the varying strength of interference for different source mechanisms and depths. Notably, these calculations illuminate source depths where Love wave overtone excitation is quite low and interference is unusually weak, and depths where Rayleigh wave FM excitation is low and interference is unusually strong. Our analysis also reinforces the dependence of the interference on the FM and overtone group velocities. For Love waves, this results in weak minor-arc overtone interference at long periods. For Rayleigh waves, the differing overtone and FM group velocities and the relative excitation of the overtones and FM explain variations in the strength of major-arc overtone interference as epicentral distance varies.

We show that real data are affected by the relative excitation of the FM and overtones. We find that errors in Rayleigh wave phase velocities across USArray are larger when overtone/FM excitation ratios are larger. We also show a dependence of phase velocity error on excitation ratio for Love waves, and identify the presence of major-arc overtone interference in Love wave measurements.

Our results highlight opportunities for more nuanced quality control of surface-wave measurements. The relative excitation ratio of overtones may be a better criterion for event selection than source depth, allowing deeper events that well excite FMs to be included and shallower events with large overtone excitation to be excluded. This would allow larger and cleaner datasets that will increase the precision of seismic images.