High resolution seismic tomography across the 1980 (Ms 6.9) Southern Italy earthquake fault scarp

Abstract

A high-resolution multi-fold wide-angle seismic survey conducted across the Irpinia fault, Southern Italy, yields new information about the shallow structure of this normal fault that was reactivated in 1980. The fault zone was imaged to a depth of about 60 m by using a non-linear tomographic technique that is specially designed to image strongly heterogeneous media. The tomographic image confirms the location of the fault, as previously inferred by a trench excavated in warped soft soils, and clearly delineates a step in the bedrock, that is 30-35 m deeper in the hanging wall than in the footwall. This step is indicative of a narrow fault zone, which corresponds upward to the warped soils exposed in the trench, thus demonstrating that the near-surface warping is directly related to a brittle faulting in the bedrock. Assuming that the vertical slip rate yielded by paleoseismic data (0.25-0.35 mm/yr) has been constant since the fault’s inception, the latter should date back to about 100-140 kys ago. Such a young age, along with the small amount of cumulative slip imaged by tomography, may explain why the Irpinia fault is not associated with evident, large-scale geomorphic indicators of its activity.