Abstract
A centroid location catalog of P-wave microseisms is crucial for
understanding the origins of microseisms. Although a back-projection
method is feasible for locating the centroids, the computational cost is
still expensive for making a global catalog over ten years. Contrary,
although the computational cost of beamforming is low, it cannot
distinguish P from PP waves. To combine the advantages of both methods,
we develop the auto-focusing method as a natural extension of
beamforming. In the first step of this method, we estimated the slowness
vector based on conventional beamforming and the epicentral distance
inferred from the wavefront curvature by maximizing the beam power. In
the second step, we iteratively update the values based on the
perturbation theory. In the third step, based on the classified phase
according to the estimated epicentral distance and the slowness, we
infer the source location 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. Exceptionally, significant activities in the
Gulf of Carpentaria cannot be explained by the ocean wave model.