Direction Monitoring of Secondary Microseismic Noise for Love and Rayleigh Waves with a Single 6-Degree-of-Freedom Station

Summary Knowing and monitoring the spatial distribution of ambient seismic noise sources is essential for correlation-based investigations in seismology. The omnipresent primary and secondary microseismic noise characterizes the ocean-generated noise of the seismic spectrum and has been studied through observations and modeling to better constrain the source mechanisms, the source regions, and their temporal variability. In this study, we demonstrate the potential of a single six-degree-of-freedom (6-DoF) station, observing co-located translational and rotational ground motions, to determine and continuously monitor the source directions of the secondary microseismic wavefield. We harness direct rotational ground motion data obtained with the ROMY ring laser array. The effective array-like capability of a single 6-DoF station to separate Love and Rayleigh waves, both constituents of the secondary microseismic wavefield, by their polarization enables the independent estimation of the dominant source directions and the monitoring of their seasonal evolution. An anticipated seasonality is evident, with a dominant source direction for Love (254○) and Rayleigh (277○) waves in winter, whereas in summer no clear dominant direction emerges due to an absence of strong nearby sources. We compare 6-DoF-based backazimuth estimates with those of seismic array beamforming and find close agreement concerning the dominant source directions, particularly in the case of migrating pelagic storms in the North Atlantic associated with strong ocean wave activity over several days. In winter, a systematic bias of about 25○ to 35○ for Rayleigh waves and 50○ for Love waves is identified between the 6-DoF-based backazimuth estimates and those inferred for significant wave heights obtained from satellite altimetry and model-based seismic noise source maps. Our results provide a proof-of-concept for the direction monitoring of the secondary microseismic noise for both Love and Rayleigh waves using direct rotational observations, thereby reducing reliance on array-derived estimates. The advent of portable high-sensitive rotation sensors will facilitate constraining spatial seismic noise source distributions and temporal variations therein with future 6-DoF station networks potentially replacing or complementing traditional seismic array deployments.