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Luigi Ferranti1, Bruno Pace2, Alessandro Valentini2, Paolo Montagna3,
Edwige Pons-Branchu4, Nadine Tisnérat-Laborde4, and Laura Maschio5
Journal of Geophysical Research: Solid Earth, 124.
1CRUST-DiSTAR, Università degli studi Federico II, Napoli, Italy.
2CRUST-DISPUTER, Università degli studi Gabriele d’Annunzio, Chieti, Italy.
3Institute of Marine Sciences, ISMAR-CNR, Via Gobetti, 101, 40129 Bologna, Italy.
4Laboratoire des Sciences du Climat et de l’Environnement, LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France.
5THOLOS GROUP s.r.l., Via Stadio a Monte, 35, 89129, Reggio Calabria, Italy.


Speleoseismological research carried out in the Pollino Range (Calabria, Southern Italy), an area of alleged seismic gap in the active extensional belt running along the Southern Apennines, has placed constraints on the recurrence of M>6 earthquakes, on the expected ground shaking threshold, and on definition of seismogenic sources in the region. Radiometric (U-Th, AMS-14C and bulk-14C) dating of pre- and post-deformation layers from collapsed or tilted cave speleothems indicates that six speleoseismic events have occurred in the area during the last ~42 ka, with a mean recurrence of ~5.6 ka.
Based on the in-situ measured geometry and laboratory determined mechanical properties of speleothems and using an ad hoc seismogenic source model for northern Calabria which involved both normal and strike-slip faults, we evaluate the seismic hazard at the cave sites. The numerical models to compute the ground horizontal acceleration threshold for speleothem failure was tested against intact and currently growing stalactites. The inferred age of these stalactites calibrated using established average speleothem growth rates of 0.3-1.2 cm/ka, ranges from ~0.7 to ~10 ka, with most of them younger than ~5.6 ka.
Results show that the ~0.8-1.0 g PGA threshold estimated for collapsed speleothems every 5.6 ka was achieved during strong (M>6) and close (epicentral distance<12 km) earthquakes. Considering a mean speleoseismic event recurrence of 5.6 ka and that no speleo-seismic shaking has occurred in the last 5.5 ka, the probability of occurrence of a M>6 event in the area in the next few centuries is quite high.
Keywords: Speleoseismic events, stalactite vulnerability, fault-based seismic hazard model, normal and strike-slip faults, Pollino range, Southern Italy
Figura 1:
Speleothems from the Pollino caves: a) measuring the geometry of active stalactites in the conduit beneath the P9 (Branch A, Serra del Gufo); b) collapsed stalagmite SG2A, which was regrown by stalagmite SG2B (Branch C, Serra del Gufo); c) striated fault plane in bedrock (Damale Cave); d) stalagmite SA7 with tilted growth axes (S. Angelo Cave); e) offset and fractured columns with cemented fractures (S. Angelo Cave); f) low-angle shear planes within small columns (S. Angelo Cave); g) high-angle open fracture within concretions (Morano Cave, S. Paolo branch). DC, deformation class number.