Broken speleothems reveal Holocene and Late Pleistocene paleoearthquakes in Northern Calabria, Italy

Two underground cavities in the Northern Calabria region, the Romito and the San Paolo caves, have been selected for speleoseismic analysis in order to provide a timeframe for paleoearthquakes which have shocked the cave sites and the surrounding area. The caves are positioned in a seismogenically active region, which has been struck by medium to large earthquakes in historical and pre-historic times. Moreover, frequent instrumentally measured small to moderate seismicity occurs in the area (M ≤ 5.6). We document damaged carbonate cave deposits, including collapsed and broken stalactites, in-situ severed stalagmites and stalactites, and collapsed bedrock ceilings. Samples were sawed in order to expose breakages and contacts within the laminae. Sixteen laminae, eight pairs of pre and post event laminae, were drilled for dating by the UraniumThorium disequilibrium technique. Three discrete events were defined by six of the speleoseismites to (I) 7.4-2.9 ka, (II) 9.7-8.2 ka, and (III) 28.4-27.4 ka. Two additional speleoseismites gave wide age brackets, but are confined to 65.7-18.6 ka, which overlaps with event III, but might represent additional events. The present speleoseismic work depicts a scenario of a minimum of three well-constrained strong shaking events in a time span of the past ∼28 ky, yielding a maximum recurrence interval of ∼9 ky. In 2012 the area was shaken by a Mw5.2 earthquake that apparently did not produced damage within the study cavities, suggesting that this size is a lower magnitude limit for producing permanent effects in the caves deposits. The speleoseismites we studied collected must have been generated by larger magnitude events, and/or with different epicenter locations. This study adds medium or large earthquake events, previously lacking, to the historic and instrumental record. This may contribute to reduction of gaps in the earthquake record and consequently a more reliable recurrence interval estimate for damaging earthquakes in the region.