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Green’s functions extraction and surface-wave tomography from microseisms in southern California

We use crosscorrelations of seismic noise data from 151 stations in southern California to extract the group velocities of surface waves between the station pairs for the purpose of determining the surface-wave velocity structure. We developed an automated procedure for estimating the Green’s functions and subsequent tomographic inversion from the 11,325 station pairs based on the characteristics of the noise field. We eliminate specific events by a procedure that does not introduce any spurious spectral distortion in the band of interest, 0.05–0.2 Hz. Further, we only used the emerging arrival structure above a threshold signal-to-noise ratio. The result is that mostly station pairs with their axes oriented toward the sea are used, consistent with the noise having a microseism origin. Finally, it is the time derivative of the correlation function that is actually related to the Green’s function. The emergence of the time-domain Green’s function is proportional to the square root of the recording time and inversely proportional to the square root of the distance between stations. The tomographic inversion yields a surface-wave velocity map that compares favorably with more conventional and elaborate experimental procedures.