Surface faulting of the 30 October 2016 Mw 6.5 central Italy earthquake: Detailed analysis of a complex coseismic rupture

Abstract

he study of coseismic surface ruptures provides insights into earthquakes dynamics and fault growth processes. We analyze the surface faulting related to the seismic sequence that hit central Italy in 2016–2017, focusing on the ruptures caused by 30 October 2016 Mw 6.5 Norcia earthquake. They are located on the NW trending normal fault splays of the Mount Vettore-Mount Bove fault system (VBFS), forming a fracture network made of hundreds of strands striking N135–160°. The surface rupture length for this event is ~22 km, with average surface slip of ~0.44 m and peak of ~2.10 m. The collected coseismic slip vectors yield an average N233° trending extension, consistent with the local structural setting and seismological data. Surface slip displays cumulative frequency-size distributions of rupture length and offset that follow power law and exponential scaling over 2 orders of magnitude, respectively. We observe strain localization on a few major fault splays of the VBFS, causing a markedly asymmetric along-strike slip profile, with a high gradient to the southeast. The ~5-km-long Cordone del Vettore fault accounts for 40% of the overall coseismic surface slip. We infer that the heterogeneous slip at depth, coupled with the highly segmented nature of the VBFS and its interference with thrusts and adjacent active normal faults, has control over the pattern of surface faulting. For the Norcia earthquake, a robust scaling of surface slip area with rupture length accounts for extreme slip peaks over relatively short ruptures, which we envisage may be typical of the VBFS long- term growth.