On the Vulnerability of Teleseismic Surface Wave Group Measurements to Overtone Interference

SUMMARY Measurements of the propagation of teleseismic fundamental-mode surface waves are essential for studies of Earth structure and earthquake source processes. Understanding sources of noise and error in these measurements can help improve the accuracy and precision of analyses that use these measurements. One prominent source of noise is interference of overtones with the fundamental mode, which is well-studied in the context of surface wave phase observations. In this work, we show that overtone interference also has a substantial impact on group measurements and has uniquely different characteristics when compared with the analogous interference in phase measurements. We illustrate these characteristics using measurements on both synthetic and real data. Importantly, our experiments suggest that group measurements are more vulnerable than phase measurements to interference from overtones; both synthetic data and published data sets show larger and more variable interference in group measurements than in phase measurements. This interference leads to significant errors in group velocity estimates made using regional or array-based approaches. We show that some quality control measures designed to eliminate overtone interference in phase measurements may not be applicable for group measurements. Our results emphasize the need for careful monitoring of group velocity overtone interference in tomographic imaging, as well as the need for accurate uncertainty quantification when group velocity maps are used in further studies.