Paleoseismology of the Roccapreturo Fault (Central Apennines): Insights from Antithetic Fault Trenching and Image-Enhanced Analysis

The Roccapreturo Fault (RF) is a 9 km-long NW-SE normal fault forming one of the major segments of the Middle Aterno Valley Fault system, located 20 km south of L’Aquila in the Central Apennines (Italy). Despite its clear seismogenic potential, no earthquakes have been documented in historical sources. Large earthquakes on such structures typically have time intervals of several millennia, making paleoseismology crucial for constraining their long-term seismic behavior. The RF exhibits 150–250 m-high triangular facets and a 10 m-high semi-vertical fault scarp that cuts the Cretaceous limestone sedimentary sequence and delineates the main fault trace. North of Roccapreturo village, Quaternary colluvial deposits and alluvial fans feed a small intermontane basin, bounded by a 25-m-high ridge most probably related to the cumulative displacement along an antithetic fault subparallel to the main fault. We excavated two paleoseismological trenches across this antithetic fault, where a refined sedimentary record enhances the preservation of coseismic deformation. An additional trench was excavated ~1 km south, at the base of the main fault scarp.

Trenches were logged using standard stratigraphic, structural, and event-identification criteria. Event ages were constrained through radiocarbon dating of 23 bulk-sediment and charcoal samples. To complement conventional trench analysis, we implemented an integrated workflow combining conventional paleoseismology with pixel-based image enhancement. This approach exploits multi-temporal orthophotography datasets acquired at different spatial resolutions and times. Photogrammetric products (orthomosaics, true- and false-color RGB composites, 3D textured point clouds, and raster derivatives) were integrated into a georeferenced multi-layer stack to support post-field interpretation and independent validation of trench observations.

In the trenches across the antithetic fault, the basal stratigraphy consists of fine-grained marsh deposits faulted and folded against fractured and brecciated limestone bedrock. These units are overlain by clast-supported colluvial sequences containing wedges that record cumulative vertical displacements of up to ~70 cm, defining multiple paleoearthquake horizons. Three to four surface-rupturing events were identified in the antithetic fault trenches, with clustered ages of 0–1.7 ka, 4–8 ka, 8–13 ka, and 15–21 ka. In contrast, the trench excavated at the base of the main scarp preserves only a single recent event within colluvial deposits, consistent with the youngest event recorded in the antithetic fault trenches.

Previous studies along the main RF focused on cosmogenic dating of the bedrock scarp, estimating Middle Pleistocene slip rates of 0.2–0.3 mm/yr, and on trenching at alluvial-fan intersections. Two Holocene surface-rupturing events (2–8 ka) were identified, indicating a recurrence of about 2 ka and magnitudes up to Mw 6.5. The earthquake events that yielded in our trenches correlate well with previous results, extending the seismic record of the RF into the Late Pleistocene. Together, these results are crucial for constraining the timing and recurrence of surface-rupturing events and for assessing the role of antithetic faults in accommodating distributed deformation within the fault system. In addition, integrating image-enhancement techniques improves the visualization of subtle deformation and stratigraphic relationships, reduces interpretative uncertainty, and provides a scalable, reproducible framework that effectively complements classical paleoseismological trenching.