Application of photogrammetric approaches to studying the 1971 dike-induced surface structures on Mt Etna, Italy

Mt Etna, located on the east coast of Sicily, Italy, is a basaltic stratovolcano with a volcanotectonic evolution of 500 ka, characterized by a wide horse-shoe-shaped depression on its eastern flank, called Valle del Bove. The study area is located near the northern escarpment of this depression, where it is possible to recognize the 1971 eruptive fissure system, generated by the lateral propagation of a feeder dike. The purpose of this research is to thoroughly examine the area affected by dike-induced surface deformation, which is marked by a textbook example of a graben structure produced by dike propagation. Due to the presence of meters-thick, recent pyroclastic deposits covering the study area and the difficult logistics, the main outcrops are inaccessible for classical field data collection. To overcome this limitation, we used the following methodology based on the analysis of photogrammetry-derived models.

We first designed a structural map related to the development of the 1971 dike-induced structures, using two sets of historical aerial photos characterized by a 2400 DPI resolution. Particularly, the 20 selected images, equally divided between 1954 and 1983, have been processed using the software Agisoft Metashape to produce two referenced orthomosaics with a resolution of 29 and 19.5 cm/pixel, respectively. By comparing the obtained orthomosaics, we identified and mapped all the normal faults associated with the 1971 dike intrusion. This structural map has been used to organize the subsequent drone surveys, performed by a DJI Phantom 4 Pro equipped with RTK high-precision technology, which allowed us to collect 656 pictures with an overlap and a side lap of 85% and 80% respectively. Afterward, we processed the drone-collected photos by using Structure-from-Motion photogrammetry techniques, so as to obtain a Digital Surface Model (DSM) and a 3D Tiled Model, with a resolution of 11 and 5.48 cm/pixel, respectively. Such models have been used to analyze in detail the graben faults, especially the ones along the Valle del Bove steep wall.

The analysis of photogrammetry-derived models over different time windows enabled us to individuate 14 lineaments within the study area, 2 eruptive fissures with a NE-SW strike, and 13 fault scarps associated with the dip-slip faults of the graben. Finally, thanks to the 3D Tiled Model obtained from drone-captured pictures, we were able to quantify the dip direction and dip angles of the graben faults, their vertical offsets, and the graben width related to the elevation.