Imaging with Scattered Teleseismic Waves: Data, Method and Application to the Hellenic Subduction Zone
PEARCE, F.D., MIT, Cambridge, MA, USA, firstname.lastname@example.org; RONDENAY, S., MIT, Cambridge, MA, USA, email@example.com
Scattered teleseismic body waves are commonly used to image sharp changes in material properties within the lithosphere and mantle. When dense broadband arrays are available, high-resolution imaging can be achieved using teleseismic migration. Such methods involve the backprojection of scattered waves by stacking their signal along diffraction hyperbolae, with their relative arrival time and amplitude providing the position and magnitude of elastic discontinuities, respectively. Here, we use a 2D, elastic teleseismic migration method based on the Generalized Radon Transform. The method provides detailed images of perturbations in elastic properties with respect to a smooth reference model. We discuss the data requirements and assumptions necessary for optimal resolution. Quasi-linear, dense seismic arrays must be oriented perpendicular to the strike of geologic structures (e.g. arc-perpendicular in subduction zone). Station spacing, array length, and event distribution place important constraints on dip and depth resolution. Several preprocessing steps are necessary to isolate the scattered wavefield from raw data recorded at the surface. Deviations from the assumptions of isotropy and 2D symmetry result in degraded image quality. Synthetic data are used to test the resolving power of the method and to identify potential artifacts. The 2D GRT method is applied to two dense seismic arrays across the western Hellenic subduction zone. Results clearly image a ~10 km-thick low-velocity layer consistent with subducted oceanic crust within the southern segment, and a ~20 km-thick low-velocity layer within the northern segment interpreted as subducted continental crust. Comparing the two imaged subducted crusts shows evidence for rollback of the southern segment that may be partially accommodated by slab tearing subparallel to the Cephalonia transform fault.