Ultrashallow seismic imaging of the causative fault of the 1980, M6.9, southern Italy earthquake by pre-stack depth migration of dense wide-aperture data

A two-step imaging procedure, including pre-stack depth migration (PSDM) and non-linear multiscale refraction tomography, was applied to dense wide-aperture data with the aim of imaging the causative fault of the 1980, M6.9, Irpinia normal faulting earthquake in a very complex geologic environment. PSDM is often ineffective for ultrashallow imaging (100 m of depth and less) of laterally heterogeneous media because of the difficulty in estimating a correct velocity model for migration. Dense wide-aperture profiling allowed us to build accurate velocity models across the fault zone by multiscale tomography and to record wide-angle reflections from steep reflectors. PSDM provided better imaging with respect to conventional post-stack depth migration, and improved definition of fault geometry and apparent cumulative displacement. Results indicate that this imaging strategy can be very effective for near-surface fault detection and characterization. Fault location and geometry are in agreement with paleoseismic data from two nearby trenches. The estimated vertical fault throw is only 29-38 m. This value, combined with the vertical slip rate determined by trench data, suggests a young age (97-127 kyr) of fault inception. © 2010 by the American Geophysical Union.