Jing Ci Neo, Wenyuan Fan, Yihe Huang, David R. Dowling

Back-projection has proven useful in imaging large earthquake rupture processes. The method is generally robust and does not require many assumptions about the fault geometry or the Earth velocity model. It can be applied in both the time and frequency domain. However, back-projection images are often obtained from records filtered in a narrow frequency range, limiting our ability to uncover the whole rupture process. Here we develop and apply a novel frequency-difference backprojection (FDBP) technique to image large earthquakes, which imitates frequencies below the bandwidth of the signal. The new approach originates from frequency-difference beamforming, which was initially designed to locate acoustic sources. The method stacks the phase-difference of frequency pairs, given by the autoproduct, and is less affected by multipathing and structural inhomogeneities. Additionally, it can potentially allow us to locate sources more accurately even in the presence of strong near-source scattering, albeit with lower resolution. In this study, we first develop the FDBP algorithm and then validate it by performing synthetic tests. We further compare two different stacking techniques of the FDBP method and their effects in the back-projection images. We then apply both the FDBP and conventional time-domain back-projection methods to the 2015 M 7.8 Gorkha earthquake as a case study. The back-projection results from the two methods agree well with each other, and we find that the peak radiation loci have standard error of less than 0.2° through a bootstrapping test. The FDBP method shows promise in resolving complex earthquake rupture processes in tectonically complex regions.