A geophysical approach to the paleoseismology using 3D Ground Penetrating Radar (GPR) data.

The faults responsible of strong earthquakes are usually studied with quantitative geomorphologic and paleoseismological techniques investigating shallow Quaternary sediments with trenches to find geological evidences of ancient seismic events. Recently, the improvements of applied geophysical techniques as the 3D Ground Penetrating Radar (GPR), have been successfully applied on studies in different geological environments. Some 3D-GPR applications on paleoseismological studies are available in literature, showing unambiguous geophysical evidences of shallow faulting compared to 2D data. The Mt. Vettore fault (Italy, Central Appennines) has been extensively investigated by various paleoseismological studies. This structure shows both geomorphological and trench-derived evidences of Late-Quaternary activity and a recent new seismic catalogue includes events possible related with the activity of this fault. In our study, we employed an high-resolution 3D-GPR survey across a branch of the Mt. Vettore fault tied to trench-derived stratigraphy data in order to cross validate the two independent data. We used a 300 MHz GPR system above a small scarp rising on a Late Pleistocene-Holocene alluvial fan, recording, at first, some 2D profiles using Common Offset configuration to precisely locale the fault zone. After detecting the fault signature on 2D profiles, a 20 x 20 m high-resolution 3D-GPR grid was acquired across the fault. The processed 3D GPR volume shows a clear imaging of the fault zone and faulted units due to the simultaneous display of profiles and time-slices. We used an open source geophysical interpretation software to interactively manage the entire 3D dataset and to detect and interpolate interesting horizons that allowed to build a detailed geometrical model of the subsurface up to 4m below the topographic surface. Our experience demonstrates that GPR constitutes a geophysical technique particularly useful to paleoseismology, not only to optimize the possible sites for trench digging but also to obtain data along and across the strike. Moreover, the promising results make the GPR an interesting alternative to other studies in sites where topographical evidences or geological contest are poorly defined. The study here presented provides qualitative and quantitative geological information in a completely non-invasive way, essential especially in nature reserves like the Sibillini Mountain's National Park.