High-resolution geophysical investigation for the seismotectonic characterization of the Raša Fault, SW Slovenia

Slovenia is located on an active convergent boundary between the Adriatic microplate and the Eurasian plate, characterized by compressive active fault systems and moderate seismicity. Relatively high seismic hazard in W Slovenia is related to the Dinaric Fault System of dextral strike slip faults. After the Idrija fault, second most important is more than 100 km long Raša Fault. Its activity has been estimated in previous geomorphological studies and the slip rate is estimated at about 0.7 mm/year. Due to its length, relatively large earthquake magnitudes are expected in the albeit less likely scenario of the entire fault trace activation.

Raša Fault runs through carbonate and siliciclastic rocks. The geomorphological trace of the Raša fault is well expressed through prevailing carbonate rocks, while difficult to follow through siliciclastic rocks. Quaternary deposits are generally thin and mainly present along the river and some streams crossing the fault.  The broader area is characterized by a complex regional geologic setting, overprint of various tectonic phases, low levels of deformations, high level of erosion and the influence of both karstic and slope mass processes, making it difficult to identify and characterize the fault solely by means of structural geologic and tectonic geomorphological mapping. Therefore, we employed an extensive GPR and ERT survey to support the seismotectonic characterization at several locations along the fault with supposed favorable characteristics of Quaternary sediments for further paleoseismological investigations. Both methods were consistent in delineating lateral and vertical changes in sediment composition, along with strike-slip fault related level of bedrock and sediment deformation. Overall, high amplitude GPR reflections resulted from bedrock and coarse alluvial sediments, and attenuated with the increasing clay/water content in the sediments limiting the depth of investigations, which was resolved with the ERT. The attenuated GPR signal along with an abrupt termination of reflectors was useful to delimit potential zones of highly fractured media, which in term result in a decrease of ERT determined resistivity. The Raša fault core zone is nicely visible with both methods, and is characterized by a low resistivity anomaly in ERT and by attenuation and abrupt termination of GPR reflections, extending from the overlaying alluvial sediment deposits to greater depths in the bedrock. Combining both methods we got a better insight related to the fault zone location and its extension bellow the Quaternary cover, as well as in some cases within the deposits. Moreover, general information about the sedimentological and hydrogeological characteristics  was obtained, contributing to characterization of candidate sites for consecutive paleoseismological investigations. Our study contributes valuable new data on the near-surface deformation along the active Raša Fault and demonstrates the successful integration of geophysical techniques into the study of active faults within this complex environment.