How can we retrieve body waves between two sources?

Intersource Interferometry

Although seismic interferometry to retrieve the Green’s function between receivers is widely applied, the technique is limited to locations with seismic arrays. A reciprocal method of interferometry called “inter-source interferometry” or “virtual seismometer method (VRM)”, introduced by Curtis et al. (2009), addresses this limitation by sampling the seismic wavefield at the location of seismic sources (e.g., earthquakes and explosions). Our work is to study the requirements of using inter-source interferometry to retrieve a body wavefield between two sources and apply the technique to the real earthquakes. The study of Patipan Saengduean shows that one can retrieve direct body waves, albeit with a slight phase shift, between two seismic sources using inter-source interferometry if the sampling criteria of 4 samples/wavelength is satisfied and samples are from the stationary phase location. Figure 1 shows the retrieved direct body waves between two sources using 4 samples/wavelength and the receivers sampled inside stationary phase zone of the direct waves (Z1). Patipan also applies the inter-source method to real seismograms recorded near San Andreas fault. His study shows that the retrieved P waves arrive approximately at the expected time deduced from the earthquake catalog and exhibit similar wave trains to numerical models of the low-velocity layer of the San Andreas fault. Figure 2 shows the interferometric P waves between two earthquakes (A) and the three numerical waveforms between two sources with different thicknesses of the slow-velocity layer (B-D).

Velocity analysis