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

摘要

Pre-Stack Depth Migration (PSDM) is an effective tool to improve seismic reflection imaging of laterally heterogeneous media. However, migration performance strongly depends on the accuracy of the velocity model, therefore, PSDM is often ineffective for ultrashallow imaging (100 m and less) of complex structures. In this study, we applied non-linear multiscale refraction tomography and PSDM on dense wide-aperture data to image the causative fault of the 1980, M6.9, Irpinia normal faulting earthquake in a very complex geologic environment. Dense wide-aperture profiling allowed us to build accurate velocity models by multi-scale non linear refraction tomography and to record wide-angle reflections from steep reflectors. PSDM provided superior imaging with respect to conventional post-stack depth migration, and a better definition of fault geometry and apparent cumulative displacement. The estimated throw of Irpinia fault is only 29-38 m. This value, combined with the vertical slip rate estimated by nearby trenches, suggests a young age (97-127 kyr) of fault inception. Our outcomes are in agreement with paleoseismic data and indicate that this imaging strategy can be very effective for near-surface fault detection and characterization.