A Global Centroid Single Force Catalog of P‐Wave Microseisms

Abstract

AbstractP‐wave microseisms are excited by ocean swell activities. The recent development of an ocean wave model allows us to compare them quantitatively. For the comparison, a single force at a centroid location can characterize the seismic excitation. A beamforming method is feasible for locating the centroids efficiently. However, the required plane wave assumption reduces the accuracy for a large aperture array. The lack of information on the wavefront curvature also makes P and PP phases indistinguishable. To utilize information on the curvature, we developed the auto‐focusing method as a natural extension of beamforming: (a) We estimated the slowness vector and the wavefront curvature by a grid‐search method, which maximizes the beam power. (b) We iteratively update the values based on the perturbation theory. (c) Based on the classified phase according to the wavefront curvature and the slowness, we locate the centroid from the slowness vector with corrections for a global 3‐D P‐wave velocity structure. We also infer the centroid‐single‐force (CSF) from the beam power. We applied this method to the vertical components of seismic records at approximately 780 Hi‐net stations in Japan from 2004 to 2020. We also compare the CSF catalog with a synthetic CSF catalog based on a numerical ocean wave model: WAVEWATCH III. Both catalogs generally show similar temporal‐spatial patterns of centroids. The amplitudes of CSF are consistent with each other, although the seismic signal‐to‐noise ratio limits the detected events. The ocean wave model cannot explain significant activities in the Gulf of Carpentaria.