Relay ramps in active normal fault zones: A clue to the identification of seismogenic sources (1688 Sannio earthquake, Italy)

Relay ramps commonly occur in areas of active extension. They have relevance in terms of fault segmentation, fault linkage, and related seismic potential. We propose consideration of field evidence of an active relay ramp in a seismic area as a clue to the definition of a seismogenic normal fault. We have chosen the Calore River valley as our study area. It is located within the core of the Apennine chain of the Italian Peninsula and is characterized by active extension and associated severe seismicity. In particular, a destructive earthquake struck the study area in 1688. We integrated data deriving from a geological field survey with original geo- morphic and mesostructural analyses and new radiometric dating (40Ar/39Ar). Results indicate that brittle deformation detected in the northern part of Mt. Camposauro could represent the surface expression of the NE- dipping main fault of an active extensional system we refer to as the Calore River fault system. Its structural connection with the neighboring Boiano Basin extensional fault system is marked by an area of complex deformation, which links them and can be described as a stage 3 relay ramp. Given that the Boiano Basin fault system is a seis- mogenic source, the identification of a relay ramp implies a seismogenic role also for the Calore River fault system. In turn, these results allow us to suggest a new hypothesis for the seismogenic source of the 1688 Sannio earthquake. Finally, we discuss the implica- tions on the seismic potential of two seismo- genic structures linked by a relay ramp.