Extracting Fault-Zone Structures Using Virtual Source Method:A Theoretical And Synthetic Test

The reciprocity of earthquake sources allows for recovery of Greens functions using finly sampled spatial observations. One popular application of the source reciprocity is to extract Greens functions between seismic stations using ambient noise. Another application of such philosophy is to extract Greens functions between two earthquakes (virtual source), when dense spatial seismic observations of both events are available. This strategy maybe particularly helpful to extract fault zone velocity structures at depth, which is typically not well resolved by tomography method, while related theoretical and obsevational studies are rare. We conduct theoretical study on virtual Greens function extraction using surface observation of waveforms of earthquake pairs. We demonstrate the upper- and lower bound of available frequency band are determined by the spatial interval and array apparature of seismic stations. Virtual greens function can be properly recovered using 1D and 2D station distribution if the weighting and geometry spreading effect are properly addressed. Our 3D modeling technique also reveals earthquake pairs occurred within or outside fault zones produce distinct Greens function, which clearly reveals reverberation phases. Thus virtual Greens functions serve as a promising method to discriminate fault zone structures at depths.