Joint Interpretation of Geophysical Results and Geological Observations for Detecting Buried Active Faults: The Case of the “Il Lago” Plain (Pettoranello del Molise, Italy)

Abstract

We report a geophysical study across an active normal fault in the Southern Apennines. The surveyed area is the “Il Lago” Plain (Pettoranello del Molise), at the foot of Mt. Patalecchia (Molise Apennines, Southern Italy), a small tectonic basin filled by Holocene deposits located at the NW termination of the major Quaternary Bojano basin structure. This basin, on the NE flank of the Matese Massif, was the epicentral area of the very strong 26 July, 1805, Sant’Anna earthquake (I0 = X MCS, Mw = 6.7). The “Il Lago” Plain is bordered by a portion of the right-stepping normal fault system bounding the whole Bojano Quaternary basin (28 km long). The seismic source responsible for the 1805 earthquake is regarded as one of the most hazardous structures of the Apennines; however, the position of its NW boundary of this seismic source is debated. Geological, geomorphological and macroseismic data show that some coseismic surface faulting also occurred in correspondence with the border fault of the “Il Lago” Plain. The study of the “Il Lago” Plain subsurface might help to constrain the NW segment boundary of the 1805 seismogenic source, suggesting that it is possibly a capable fault, source for moderate (Mw < 5.5) to strong earthquakes (Mw ≥ 5.5). Therefore, we constrained the geometry of the fault beneath the plain using low-frequency Ground Penetrating Radar (GPR) data supported by seismic tomography. Seismic tomography yielded preliminary information on the subsurface structures and the dielectric permittivity of the subsoil. A set of GPR parallel profiles allowed a quick and high-resolution characterization of the lateral extension of the fault, and of its geometry at depth. The result of our study demonstrates the optimal potential of combined seismic and deep GPR surveys for investigating the geometry of buried active normal faults. Moreover, our study could be used for identifying suitable sites for paleoseismic analyses, where record of earthquake surface faulting might be preserved in Holocene lacustrine sedimentary deposits. The present case demonstrates the possibility to detect with high accuracy the complexity of a fault-zone within a basin, inferred by GPR data, not only in its shallower part, but also down to about 100 m depth.

An Integrated Geophysical and Unmanned Aerial Systems Surveys for Multi-Sensory, Multi-Scale and Multi-Resolution Cave Detection: The Gravaglione Site (Canale di Pirro Polje, Apulia)

“…Ground penetrating radar (GPR) is widely applied for geological issues thanks to the high resolution offered by the method despite the low depth of investigation, generally limited to a few meters. Indeed, GPR was successfully applied for characterizing and identifying geological faults [62][63][64] or for hydrogeological purposes [65][66][67][68]. Furthermore, the high sensitivity offered by GPR for the identification of voids and/or caves makes the method particularly efficient for detecting and identifying subsurface karst features [69][70][71][72][73][74][75].…”

“…The investigation depth varies from centimeters to a few to dozens of meters, depending on the antenna center frequency (2 GHz-10 MHz). For geological studies, the operating frequencies are generally lower than 200 MHz that provide the possibility to have submetric resolution at depths greater than 2-3 m. Considering the reduced depth obtainable with the GPR and the noise affecting the data when low frequency antennae, generally not shielded, are adopted [62][63][64], it is fundamental to use an integrated and compared approach which involves the use of other non-invasive (i.e., seismic analyses, electrical tomography) or invasive methodologies (borehole data) [76][77][78].…”

Reconstructing the Geometry of the Yushu Fault in the Tibetan Plateau Using TLS, GPR and Trenching

“…Among these methods, terrestrial laser scanning (TLS) and ground penetrating radar (GPR) have already proved valuable to offer sound and useful information for understanding fault behavior and seismic hazard. TLS was typically used to reconstruct a detailed 3D realistic model of micro geomorphology with fast, dense and accurate measurements [11][12][13][14][15][16][17]; while GPR was generally chosen to delineate the subsurface structures in a non-intrusive way [18][19][20][21][22][23][24][25][26]. However, buried structures of active faults were not obtained by visual or optical inspection, and subsurface data was valuable for assessing the fault's kinematics and characteristics with its corresponding superficial data.…”

Integration of Terrestrial Laser Scanner (TLS) and Ground Penetrating Radar (GPR) to Characterize the Three-Dimensional (3D) Geometry of the Maoyaba Segment of the Litang Fault, Southeastern Tibetan Plateau

“…In addition, the subsurface structures of active faults were commonly revealed by trenching or borehole rather than the visual or optical inspection. In order to reduce environmental disruption and economic losses, the near-surface geophysics methods, including the electrical resistivity imaging, GPR and microgravimetry, were chosen to depict the shallow geometry in the vicinity of active faults [27][28][29][30]. In these methods, GPR was the most prominent method for imaging the subsurface structures of active faults with a non-destructive and cost-effective fashion [27,[31][32][33][34][35][36].…”