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

D. Di Bucci^,1, B. Massa^,2 and A. Zuppetta^,2

¹ Dipartimento della Protezione Civile, Servizio Sismico Nazionale, Via Vitorchiano n. 4, 00189 Rome, Italy
² Dipartimento di Studi Geologici ed Ambientali, Università del Sannio, Via Port'arsa n. 11, 82100 Benevento, 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 geomorphic 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 seismogenic 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 implications on the seismic potential of two seismogenic structures linked by a relay ramp.

Key Words: transfer zone • fault interaction • seismogenic fault system • Benevento • southern Apennines