Speleoseismological Constraints on Ground Shaking Threshold and Seismogenic Sources in the Pollino Range (Calabria, Southern 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.