High-Resolution Spatiotemporal Monitoring of Secondary Microseisms via Multi-Array Analysis

This study presents a workflow to monitor spatiotemporal variations of the secondary microseisms using multi-array analysis. We employ ambient-noise cross-correlation beamforming (CC beamforming) across three dense seismic networks with different instrument responses: ANTICS in Albania (nodal-geophone and broadband), Hi-net in Japan (short-period) and SCSN (broadband) in Southern California. Independent of their instrumentation, these networks enable us to track the spatial and temporal evolution of secondary microseism sources in the Northern Hemisphere from autumn 2022 to spring 2023. The workflow involves continuous data pre-processing for different instrumented sensors, ambient-noise cross-correlation, beamforming and beam-power back-projection into a global map. We also propose sliding-window raw-data beamforming (RA beamforming) for the continuous broad-band data in this workflow to record the absolute amplitudes of secondary microseisms recorded by ANTICS. Joint CC beamforming analysis across the three different networks improves the resolution of ambient-noise source localization and displays high consistency with the equivalent vertical force at the ocean floor. The results indicate that secondary microseism sources in the Northern Hemisphere are predominantly driven by winter storms in the northern Atlantic and northern Pacific. The relative and absolute amplitudes of the beam-power for the northern Atlantic are also extracted from CC beamforming based on geophone sensors and RA beamforming based on broad-band instruments from ANTICS, respectively. Both approaches provide robust estimates of microseism strength in the northern Atlantic, with CC beamforming displaying a higher correlation with the modelled ocean floor equivalent forces. This study confirms the feasibility of using cost-effective nodal seismic arrays for detailed monitoring of secondary microseisms and highlights the potential for integrating multi-array seismic data with oceanographic models for an improved understanding of seismic noise generation and propagation.