No abstract

We provide a database of the coseismic geological surface effects following the Mw 6.5 Norcia earthquake that hit central Italy on 30 October 2016. This was one of the strongest seismic events to occur in Europe in the past thirty years, causing complex surface ruptures over an area of >400 km2. The database originated from the collaboration of several European teams (Open EMERGEO Working Group; about 130 researchers) coordinated by the Istituto Nazionale di Geofisica e Vulcanologia. The observations were collected by performing detailed field surveys in the epicentral region in order to describe the geometry and kinematics of surface faulting, and subsequently of landslides and other secondary coseismic effects. The resulting database consists of homogeneous georeferenced records identifying 7323 observation points, each of which contains 18 numeric and string fields of relevant information. This database will impact future earthquake studies focused on modelling of the seismic processes in active extensional settings, updating probabilistic estimates of slip distribution, and assessing the hazard of surface faulting.

In order to investigate the importance of including strike-variable geometry and the knowledge of historical and palaeoseismic earthquakes when modelling static Coulomb stress transfer and rupture propagation, we have examined the August–October 2016 A.D. and January 2017 A.D. central Apennines seismic sequence (Mw 6.0, 5.9, 6.5 in 2016 A.D. (INGV) and Mw 5.1, 5.5, 5.4, 5.0 in 2017 A.D. (INGV)). We model both the coseismic loading (from historical and palaeoseismic earthquakes) and interseismic loading (derived from Holocene fault slip-rates) using strike-variable fault geometries constrained by fieldwork. The inclusion of the elapsed times from available historical and palaeoseismological earthquakes and on faults enables us to calculate the stress on the faults prior to the beginning of the seismic sequence. We take account the 1316–4155 yr elapsed time on the Mt. Vettore fault (that ruptured during the 2016 A.D. seismic sequence) implied by palaeoseismology, and the 377 and 313 yr elapsed times on the neighbouring Laga and Norcia faults respectively, indicated by the historical record. The stress changes through time are summed to show the state of stress on the Mt. Vettore, Laga and surrounding faults prior to and during the 2016–2017 A.D. sequence. We show that the build up of stress prior to 2016 A.D. on strike-variable fault geometries generated stress heterogeneities that correlate with the limits of the main-shock ruptures. Hence, we suggest that stress barriers appear to have control on the propagation and therefore the magnitudes of the main-shock ruptures.

Fault bends, and associated changes in fault dip, play a key role in explaining the scatter in maximum offset versus surface rupture length fault scaling relationships. Detailed field measurements of the fault geometry and magnitude of slip in the 2016–2017 Central Italy earthquake sequence, alongside three examples from large historical normal‐faulting earthquakes in different tectonic settings, provide multiple examples in which coseismic throw increases across bends in fault strike where dip also increases beyond what is necessary to accommodate a uniform slip vector. Coseismic surface ruptures produced by two mainshocks of the 2016–2017 Central Italy earthquake sequence (24 August 2016 Mw 6.0 and 30 October 2016 Mw 6.5) cross a ~0.83‐km amplitude along‐strike bend, and the coseismic throws for both earthquakes increase by a factor of 2–3, where the strike of the fault changes by ~28o and the dip increases by 20–25o. We present similar examples from historical normal faulting earthquakes (1887, Sonora earthquake, Mw 7.5; 1981, Corinth earthquakes, Mw 6.7–6.4; and 1983, Borah Peak earthquake, Mw 7.3). We demonstrate that it is possible to estimate the expected change in throw across a bend by applying equations that relate strike, dip, and slip vector to horizontal strain conservation along a nonplanar fault for a single earthquake rupture. The calculated slip enhancement in bends can explain much of the scatter in maximum displacement (Dmax) versus surface rupture length scaling relationships. If fault bends are unrecognized, they can introduce variation in Dmax that may lead to erroneous inferences of stress drop variability for earthquakes, and exaggerate maximum earthquake magnitudes derived from vertical offsets in paleoseismic data sets.

The temporal evolution of slip on surface ruptures during an earthquake is important for assessing fault displacement, defining seismic hazard and for predicting ground motion. However, measurements of near-field surface displacement at high temporal resolution are elusive. We present a novel record of near-field co-seismic displacement, measured with 1-second temporal resolution during the 30th October 2016 Mw 6.6 Vettore earthquake (Central Italy), using low-cost Global Navigation Satellite System (GNSS) receivers located in the footwall and hangingwall of the Mt. Vettore - Mt. Bove fault system, close to new surface ruptures. We observe a clear temporal and spatial link between our near-field record and InSAR, far-field GPS data, regional measurements from the Italian Strong Motion and National Seismic networks, and field measurements of surface ruptures. Comparison of these datasets illustrates that the observed surface ruptures are the propagation of slip from depth on a surface rupturing (i.e. capable) fault array, as a direct and immediate response to the 30th October earthquake. Large near-field displacement ceased within 6–8 seconds of the origin time, implying that shaking induced gravitational processes were not the primary driving mechanism. We demonstrate that low-cost GNSS is an accurate monitoring tool when installed as custom-made, short-baseline networks.