GPR studies in the piano di pezza area of the ovindoli-pezza fault, Central Apennines, Italy: Extending palacoselsmic trench investigations with high resolution GPR profiling

A high-resolution 200 MHz frequency GPR profile with 0.1 m station spacing was collected using a Pulse EKKO 100 system, adjacent to a trench site previously excavated and interpreted by Pantosti et al., 1996. The profile intersected the fault scarp on one of the alluvial fans on the northern slope of the Piano di Pezza; this fault scarp was thought to be associated with repe:ated earthquake activity along the Ovindoli-Pezza fault. The trench excavation exposed a 4 metre sedimentary section formed principally of palaeosols and alluvial fan gravel dipping gently toward the south west, into the Piano di Pezza basin. The deposits exposed in the trench are strongly deformed in a 7 metre complex fault zone, located beneath the main topographic scarp. The scarp is 6.8 metres high at this location. Two palaeo-earthquake events were recognised in the trench, the most recent (circa. 1300 A.D.?) with a vertical displacement of 2.8 to 3.0 metres, and thoucght to be responsible for the current scarp formation, surface rupture, and deformation of the sedimentary section exposed in the upper part of the trench. The second event (circa. 1900 B.C.?) is thought to be represented in the lower part of the trench, and estimated to have a minimum vertical displacement of 2.5 metres. The GPR data were processed using Win EKKO radar data processing software and included the application of SEC gain and time-depth conversion utilising a field derived CMP analysis which provided a velocity of 0.06 dns. In the upper part of the GPR section a strong continuous radar reflector, thought to represent the top of the carbonate scarp, is clearly imaged beneath 1.5 m of surficial cover, and is abruptly down faulted vertically by approximately 2.8 metres, to the south west. The complex, 7 - metre wide deformation zone, recognised in the trench and associated with the most recent catastrophic event, is clearly imaged in the hanging wall of the fault and exhibits a diminishing deformation intensity towa- ds the south west, terminating with a vertical sequence of antithetic faults with a local cumulative offset of 1.5 metres, thus forming a low angle, south west tilted graben structure. In the lower part of the GPR section, the down thrown carbonate surface forms the north eastern margin of a rotated basement block, above which an asymmetric graben has developed within the sedimentary section, with maximum antithetic fault offset of the order of 0.6 metres. This graben is imaged beneath the sedimentary units exposed by the trenching operations, and is interpreted as representing the base of an approximately 12 metres cumulative vertical offset from the original carbonate surface. The GPR data has thus provided a framework for estimating the extent of extensional and episodic catastrophic activity along the fault, and elucidated the geometric relationship between the faulting in response to recent tectonic events.