GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione A matrice 19 The processing of high resolution satellite radar images (Alos 2, Sentinel and COSMO-Sky- Med) have allowed the drawing of the InSAR coseismic deformation maps shown in Fig. 2. Two major rupture patches have been modeled to have occurred along the same fault or along two distinct faults (ref. INGV CNR-IREA- (http://www.irea.cnr.it/index.php?option=com_ k2&view=item&id=755:terremoto-di-amatrice). A secondary deformation along the western slope of the Mt. Vettore (indicated by an arrow) has been interpreted as a minor gravity-driven mass movement. This is the extreme end of a broad span of interpretations on the significance of the “Cordone del Vettore” surface rupture, that goes from primary surface faulting, i.e., directly related to the seismogenic fault, to pure sliding of the slope deposits. Here, we support the idea that the movement is basically due to deep-seated slip on a fault that is currently a splay of the main seismogenic fault, very likely locally emphasized by a non- trivial gravitational component. In fact, the analysis of the fractures surveyed in the field, very continuous and consistent as regard offsets and opening extent, together with the observation on the overall western slope of Mt Vettore, bring to exclude a pure slide of slope debris cover, because it would have been associated to some bulging at its base to be recognizable on the InSAR images. Instead, a “collapse of the whole slope” along a deep fault plane with a close relation with the seismogenic fault (see Fig. 3 below) is in good agreement with the analysis of the coseismic ground deformation from InSAR analysis of COSMO-Sky-Med images (Fig. 2 right), showing the deformation of the Mt. Vettore western slope in substantial continuity with the main coseismic ground deformation pattern shown by the InSAR image (cfr, COMET report- http://comet.nerc.ac.uk/) ). About the Mt. Vettoretto ruptures, the gravity-driven component is larger and more evident. According to the official Geological Map of Italy (1:100,000, Scarsella, 1941), the Mt. Vettoretto fault has an arc shaped, bending from the NNW-SSE trend in its southern part, to an about E-W direction, parallel to the “Valle Santa” (Fig. 1a). Actually, the 2016 surface fracturing followed the NNW-SSE trending section and the bending of the fault until the upper part of the “Valle Fig. 2 – Left: Surface ruptures, possibly linked to surface faulting, and InSAR-derived vertical coseismic surface deformation (Sentinel-1; Marinkovic and Larsen, 2016). The two mainshocks location and focal mechanisms (source http://cnt.rm.ingv.it/tdmt) are also shown; capable faults (after ITHACA database - https://goo.gl/55FwDH) are mapped for comparison. Right: Coseismic ground deformation map obtained from InSAR analysis of COSMO-Sky- Med images (ASI) acquired along a descending orbit on August 20, 2016 (before the event) and on August 28, 2016 (after the event). The arrow indicates the deformation along the slope of Vettore Mt, suggested to be due to slope instability (http://www.irea.cnr.it/index.php?option=com_k2&view=item&id=755 :terremoto-di-amatrice).