Ferrara, Giuseppe (2024) High-resolution (pre-stack) seismic imaging of the Pantano-Ripa Rossa segment of the Irpinia Fault (Southern Italy). [Tesi di dottorato]

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Abstract

The Irpinia Fault, located in the Southern Apennines (Italy), is one of the most seismically active structures in the Mediterranean region. It was the source of the 1980 Ms 6.9 earthquake, a multi-segment rupture that caused significant damage and more than 3,000 casualties. Previous shallow seismic profiles across the southern splay of the Irpinia Fault, specifically the Pantano Ripa Rossa segment in the Pantano San Gregorio Magno (SA) basin, revealed the complex fault-zone geometry and its high spatial variation within the associated basin. These findings highlighted that subsurface structures cannot be fully captured by 2D surveys alone. Therefore, comprehensive three dimensional imaging represents a crucial advancement in understanding the shallowest portion of the fault, as well as investigating the near-surface basin structure, its age, and its kinematics. For this reason, one of the key objectives of this PhD thesis was to conduct the first 3D high resolution active-source seismic pilot study on a representative outcrop of the Irpinia Fault in Southern Italy, which significantly enhanced the knowledge of the near surface. The project, using active seismic surveys (e.g., 2D and 3D seismic data), studied an area where the fault splay has repeatedly ruptured co-seismically over the last 20,000 years. The study aimed to better understand the interaction between the basin's shallow structures and fault geometry, shedding light on the fault zones, sedimentation processes, and basin evolution, as well as the relationship between the Pantano Ripa-Rossa segment and the Pantano San Gregorio Magno fault system. This work demonstrates the effectiveness of velocity model-building techniques combined with pre-stack depth migration for near-surface seismic imaging in fault zones and intramontane basins, especially in areas lacking a-priori velocity information (e.g., deep wells, check shots, or borehole velocity logs). In conclusion, this thesis suggests that the Pantano-Ripa Rossa fault segment weakens towards the NW, becoming a distinct intra-basin fault as it moves away from the carbonate ridge. Evidence of fault activity diminishes in the NW direction, likely due to slip tapering near the fault tip, as indicated by the disappearance of the 1980 coseismic scarp. Additionally, the deepening of the carbonate basement in the footwall towards NW suggests that the Pantano-Ripa Rossa fault displaces a pre-existing, complex substratum.

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