Quantifying the Mislocation of Back-Projection Imaging Due to Velocity Heterogeneities Beneath the Source Region

Abstract:

Accurately resolving the rupture evolution of large earthquakes is of great significance. This is especially important for studying elongated ruptures with variable rupture speeds, rupture jumping along complex faulting systems, and subduction megathrust earthquakes with down-dip overshooting. Back-Projection (BP) serves as a popular method in imaging rupture propagation. However, the method suffers from the travel-time errors when using a 1D reference velocity model to approximate the real 3D Earth structures. Previous studies attempted to correct such travel-time errors based on aftershock measurements by using empirical interpolations (Ishii et al., 2007) or by a parametric correction of slowness errors (Meng et al., 2016). In this study, we investigate the origin of the travel-time errors of BP by analyzing the contribution of traveltime errors from velocity perturbations along the teleseismic array path. We find that the errors are mainly caused by the velocity heterogeneity of the top hundreds-of-kilometer layers beneath the source region. We show that this travel-time error is directly responsible for the mislocation (spatial bias) of BP. We explored the potential to mitigate the spatial biases by solely incorporating a fine-scale near-source tomography model when calculating travel-time errors from the source region to the receiver array, such that the spatial accuracy of BP can be improved.