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

Abstract

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-14 C, and bulk-14 C) dating of before and after 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-to 1.0-g peak ground acceleration 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 speleoseismic shaking has occurred in the last 5.5 ka, the probability of occurrence of an M > 6 event in the area in the next few centuries is quite high.Plain Language Summary Seismic shaking can cause collapse or tilt of cave speleothems (stalactites and stalagmites) and dating the calcite layers immediately after and before the deformation provides the age of unknown prehistorical earthquakes. By dating collapsed and tilted speleothems from the Pollino Range (Calabria, southern Italy), we built a catalog of six speleoseismic events that occurred in the last~42 ka in the area. We defined a seismic hazard model for the cave sites using known or newly proposed seismic sources. The model was tested against an estimate of the vulnerability to seismic shaking for several unbroken and currently forming stalactites based on their geometry (slender speleothems are more vulnerable) and their mechanical properties measured in laboratory. Results indicate that a peak ground acceleration threshold of~1 g for speleothem collapse because of strong (M > 6) and close (distance <12 km from the main seismogenic sources) is reached every 5.6 ka and is in agreement with the age of unbroken speleothems (<10 ka, mostly <5.6 ka). Considering a mean speleoseismic event recurrence of 5.6 ka and that no speleoseismic shaking has occurred in the last 5.5 ka, an M > 6 event in the area is pending in the next few centuries.

Damaged Speleothems and Collapsed Karst Chambers Indicate Paleoseismicity of the NE Bohemian Massif (Niedźwiedzia Cave, Poland)

“…In addition, we calculated the aspect ratio (AR = H / D = height/diameter) for the studied speleothems following the method of Ferranti et al. (2019). The average AR reached a value of 2.9 (Figure 6c, solid line).…”

“…The distributions of the measured heights and diameters are shown in Figures 6a and 6b, respectively. In addition, we calculated the aspect ratio (AR = H/D = height/diameter) for the studied speleothems following the method of Ferranti et al (2019). The average AR reached a value of 2.9 (Figure 6c, solid line).…”

“…Therefore, only places where the destruction is significant have been marked on the plan, that is, localities that can be described as "speleothem cemeteries," where broken and fallen speleothems cover the entire floor or places where there are at least a few damaged speleothems in one location and are a single but sizable feature. Furthermore, for 50 broken speleothems, we acquired geometric measurements, including length (minimum) and failure plane diameter, to investigate the relationship between speleothem dimensions and the horizontal ground acceleration required to damage speleothems (Ferranti et al, 2019;Gribovszki et al, 2018; To date a speleothem damage event, sampling sites have been chosen based on the methodology developed by Forti and Postpischl (1984), Postpischl et al (1991), and Kagan et al (2005). If possible, pre and postevent samples were taken from broken or fallen and regrown or enveloped speleothems, respectively.…”

A Large Paleoearthquake in the Central Apennines, Italy, Recorded by the Collapse of a Cave Speleothem

“…Sample COLA‐3 shows a high detrital contamination, as indicated by 232 Th (Table 1), and its corrected age strongly depends on the chosen initial 230 Th/ 232 Th activity ratio. For example, if a value of 2.96 ± 0.1 were applied, as in Ferranti et al (2019), the 230 Th‐corrected age would be 4.3 ka. This is not the case for the other speleothem samples that show a very small detrital thorium component.…”

“…Center of Applied Physics, Dating and Diagnostics (CEDAD) Laboratory of the University of Salento, Italy (three samples) and at Beta Analytic, Inc. (one sample). Following Ferranti et al (2019), the radiocarbon ages were first corrected for the "dead carbon portion" (DCP) and then converted to calendar years (cal. yr BP, BP = AD 1950) using the IntCal13 calibration curve (Reimer et al, 2013) and the Calib 7.10 program (Stuiver et al, 2020).…”