Seismic source mapping by surface wave time reversal: application to the great 2004 Sumatra earthquake


 Different approaches to map seismic rupture in space and time often lead to incoherent results for the same event. Building on earlier work by our team, we “time-reverse” and “back-propagate” seismic surface-wave recordings to study the focusing of the time-reversed field at the seismic source. Currently used source-imaging methods relying on seismic recordings neglect the information carried by surface waves, and mostly focus on the P-wave arrival alone. Our new method combines seismic time reversal approach with a surface wave ray tracing algorithm based on a generalized spherical-harmonic parameterization of surface-wave phase velocity, accounting for azimuthal anisotropy. It is applied to surface-wave signal filtered within narrow frequency bands, so that the inherently three-dimensional problem of simulating surface-wave propagation is separated into a suite of two-dimensional problems, each of relatively limited computational cost. We validate our method through a number of synthetic tests, then apply it to the great 2004 Sumatra-Andaman earthquake, characterized by the extremely large extent of the ruptured fault. Many studies have estimated its rupture characteristics from seismological data (e.g., Lomax, 2005; Ni et al., 2005; Guilbert et al., 2005; Ishii et al., 2005; Kr¨uger and Ohrnberger, 2005; Jaffe et al., 2006) and geodetic data (e.g., Banerjee et al., 2005; Bletery et al., 2016; Catherine et al., 2005; Vigny et al., 2005; Hashimoto et al., 2006). Applying our technique to recordings from only 89 stations of the Global Seismographic Network (GSN) and band-pass filtering the corresponding surface-wave signal around 80s-to-120s, 50s-to-110s and 40s-to-90s, we reproduce the findings of earlier studies, including in particular the northward direction of rupture propagation, its approximate spatial extent and duration, and the locations of the areas where most energy appears to be released.