Field image of a foreshock-mainshock pair: the Amatrice (Mw6.0)-Norcia (Mw6.5) 2016 earthquakes case (central Italy)

We analyze the pattern of coseismic surface faulting occurred along the Mt Vettore-Mt Bove fault (VBF, central Italy), repeatedly activated during the 2016 Central Italy seismic sequence climaxed at the 30 October 2016, Norcia mainshock (Mw 6.5). More than 3800 measurements of coseismic ruptures and 350 fault plane attitude were sampled along the VBF and stored in a comprehensive GIS-managed database reporting, for each survey site, the fracture type, attitude, kinematics (whenever possible), throw and opening. Subsequent elaborations led us to assess the coseismic surface-rupture length, rupture area, maximum and average throw and displacement and kinematics associated of the main events. Comparing the data collected along the Vettoretto-Redentore Fault segment (VRS) after the 24 August foreshock (Mw 6.0) with those collected after the 30 October mainshock (Mw 6.5), we found that throw values associated to the foreshock and the mainshock differ by ~ one order of magnitude (27 cm versus 217 cm maximum values). Nevertheless, the slip profiles display a similar multi-scale sinuosity controlled by the longterm fault segmentation pattern at different hierarchical orders. Additional statistical analysis of the fractures allowed us to calculate the percentage distribution of the different types of fractures with respect to the fault segmentation and the dispersion of the different fracture types from the long-term trend. Along the VRS slip profile, three well distinct wavelength orders of fluctuations are recognized that reflect the long-term hierarchic organization in fault segments, sections and sub-sections. The peaks of coseismic displacement, on fault sections and sub-sections activated by both events, have the same along-strike position. The 24 August ruptures on the VRS affected mainly two fault sections which exhibit distinctly different amounts of maximum and mean slip. A high slip gradient characterizes the lateral transition between them. The 30 October event, completely activated the VRS segment whose intermediate section, namely the Redentore section, shows the largest displacement and both its northern and southern continuations are characterized by sharp high lateral slip gradient. The 30 October mainshock also ruptured the northernmost VBF. The overall observed surface slip exhibits a roughly sinusoidal distribution, where wavelength, maximum displacement and mean slip progressively decrease northward. Finally, we calculated along the VRS, both the geologic and topographic long-term displacement and compared the obtained results with the diagram of coseismic displacement. The offset of the topographic slope across the fault scarp, was measured and interpreted as displacement cumulated by several slip events after the demise of the Last Glacial Maximum (LGM), that is after ~15±3 kyrs ago. The calculated vertical displacement was used to estimate the post-LGM throw rate.