Unraveling overtone interferences in Love-wave phase velocity measurements by array-based radon transform

Abstract:

Surface waves contain fundamental mode and higher modes, which could interfere with each other. Different modes of surface waves have completely different sensitivities to earth structures. If they are not properly separated, the inverted Earth structures using surface waves could be biased. Especially, for Love waves propagating over oceanic paths, the group velocities of fundamental mod and first/second overtoneS are overlapped with each at periods shorter than 100 sec, resulting in strong overtone interferences in seismograms. Most surface wave tomography studies use dispersion curves of fundamental modes in imaging. One challenge in surface wave tomography is to accurately measure the fundamental-mode phase velocities and avoid the contamination by overtones.

In this study, we develop an effective way by applying Linear Radon Transform (LRT) to a seismic array to separate fundamental-mode surface waves from higher modes. We apply this method to both synthetic data and real surface waves from USArray. Analysis on synthetic seismograms shows that two-station measurements on reconstructed data obtained after mode separation can completely retrieve the fundamental-mode Love-wave phase velocities. Results on USArray data show that higher mode contamination effects reach up to ∼10-15 percent for two-station and array-based measurements of Love waves, while two-station measurements on mode-separated data obtained by LRT are very close to the predicted values from a global dispersion model of GDM52, demonstrating that the contamination of overtones on fundamental-mode Love wave phase velocity measurements is effectively mitigated by the LRT method and accurate fundamental-mode Love-wave phase velocities can be measured.