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The seismic potential of active low-angle normal faults (LANFs, <30° dip) remains enigmatic under Andersonian faulting theory, which predicts that normal faults dipping less than 30° should be inactive. The Alto Tiberina fault (ATF) in the northern Apennines, a partly creeping 17°-dipping LANF, has not been associated with any historical earthquakes but could potentially generate earthquakes up to Mw~7. We investigate the mechanical preconditions and dynamic plausibility of large ATF earthquakes using 3D dynamic rupture and seismic wave propagation simulations constrained by multidisciplinary data from the Alto Tiberina Near Fault Observatory (TABOO-NFO). Our models incorporate the complex non-planar ATF fault geometry, including hanging wall secondary faults and a recent geodetic coupling model. We show that potential large earthquakes (up to Mw~7.4) are mechanically viable under Andersonian extensional stress conditions if the ATF is statically relatively weak (μs=0.37). Large earthquakes only nucleate on favorably oriented, steeper fault sections (dip ≥30°), and remain confined to the coupled portion, limiting earthquake magnitude. These ruptures may dynamically trigger an intersecting synthetic branch but are unlikely to affect more distant antithetic faults. Jointly integrating fault geometry and geodetic coupling is crucial for forecasting dynamic rupture nucleation and propagation. 

Abstract. Fault slip is a complex natural phenomenon involving multiple spatiotemporal scales from seconds to days to weeks. To understand the physical and chemical processes responsible for the full fault slip spectrum, a multidisciplinary approach is highly recommended. The Near Fault Observatories (NFOs) aim at providing high-precision and spatiotemporally dense multidisciplinary near-fault data, enabling the generation of new original observations and innovative scientific products. The Alto Tiberina Near Fault Observatory is a permanent monitoring infrastructure established around the Alto Tiberina fault (ATF), a 60 km long low-angle normal fault (mean dip 20°), located along a sector of the Northern Apennines (central Italy) undergoing an extension at a rate of about 3 mm yr−1. The presence of repeating earthquakes on the ATF and a steep gradient in crustal velocities measured across the ATF by GNSS stations suggest large and deep (5–12 km) portions of the ATF undergoing aseismic creep. Both laboratory and theoretical studies indicate that any given patch of a fault can creep, nucleate slow earthquakes, and host large earthquakes, as also documented in nature for certain ruptures (e.g., Iquique in 2014, Tōhoku in 2011, and Parkfield in 2004). Nonetheless, how a fault patch switches from one mode of slip to another, as well as the interaction between creep, slow slip, and regular earthquakes, is still poorly documented by near-field observation. With the strainmeter array along the Alto Tiberina fault system (STAR) project, we build a series of six geophysical observatory sites consisting of 80–160 m deep vertical boreholes instrumented with strainmeters and seismometers as well as meteorological and GNSS antennas and additional seismometers at the surface. By covering the portions of the ATF that exhibits repeated earthquakes at shallow depth (above 4 km) with these new observatory sites, we aim to collect unique open-access data to answer fundamental questions about the relationship between creep, slow slip, dynamic earthquake rupture, and tectonic faulting. 

Abstract The protracted nature of the 2016-2017 central Italy seismic sequence, with multiple damaging earthquakes spaced over months, presented serious challenges for the duty seismologists and emergency managers as they assimilated the growing sequence to advise the local population. Uncertainty concerning where and when it was safe to occupy vulnerable structures highlighted the need for timely delivery of scientifically based understanding of the evolving hazard and risk. Seismic hazard assessment during complex sequences depends critically on up-to-date earthquake catalogues—i.e., data on locations, magnitudes, and activity of earthquakes—to characterize the ongoing seismicity and fuel earthquake forecasting models. Here we document six earthquake catalogues of this sequence that were developed using a variety of methods. The catalogues possess different levels of resolution and completeness resulting from progressive enhancements in the data availability, detection sensitivity, and hypocentral location accuracy. The catalogues range from real-time to advanced machine-learning procedures and highlight both the promises as well as the challenges of implementing advanced workflows in an operational environment. 

The earthquake catalog includes high-precision hypocenter relocations for 390,334 earthquakes recorded during the 2016-2017 Amatrice (Central Italy)  earthquake sequence. The relative locations were computed by double-difference inversion of a  combination of INGV phase picks and cross-correlation differential  times measured from correlated seismograms with correlation coefficients > 0.7. Planes of normal faults (idx=1-5) are derived from PCA analysis of 2 months of aftershock  locations in the CAT4 catalog following large events. Surfaces of detachment faults (idx=7-10) are derived from mapping out the location of correlated earthquakes.  Citation: Waldhauser, F., Michele, M., Chiaraluce, L., Di Stefano, R., & Schaff, D. P. (2021). Fault planes, fault zone structure and detachment fragmentation resolved with highprecision aftershock locations of the 2016-2017 central Italy sequence. Geophysical Research Letters, 48, e2021GL092918. https://doi.org/10.1029/2021GL092918 

Abstract The 2016–2017 central Italy seismic sequence occurred on an 80 km long normal-fault system. The sequence initiated with the Mw 6.0 Amatrice event on 24 August 2016, followed by the Mw 5.9 Visso event on 26 October and the Mw 6.5 Norcia event on 30 October. We analyze continuous data from a dense network of 139 seismic stations to build a high-precision catalog of ∼900,000 earthquakes spanning a 1 yr period, based on arrival times derived using a deep-neural-network-based picker. Our catalog contains an order of magnitude more events than the catalog routinely produced by the local earthquake monitoring agency. Aftershock activity reveals the geometry of complex fault structures activated during the earthquake sequence and provides additional insights into the potential factors controlling the development of the largest events. Activated fault structures in the northern and southern regions appear complementary to faults activated during the 1997 Colfiorito and 2009 L’Aquila sequences, suggesting that earthquake triggering primarily occurs on critically stressed faults. Delineated major fault zones are relatively thick compared to estimated earthquake location uncertainties, and a large number of kilometer-long faults and diffuse seismicity were activated during the sequence. These properties might be related to fault age, roughness, and the complexity of inherited structures. The rich details resolvable in this catalog will facilitate continued investigation of this energetic and well-recorded earthquake sequence. 

A set of six large catalogues documenting the seismic sequence that occurred in central Italy between 2016 and 2017, characterized by a cascade of four MW5.5–6.5 events. The earthquake catalogues possess different levels of resolution and completeness that result from progressive enhancements in both detection sensitivity and hypocentral location determination. These quality differences reflect the subsequent application of advanced methods. 

Abstract Three devastating earthquakes ofM_W ≥ 5.9 activated a complex system of high‐angle normal, antithetic, and sub‐horizontal detachment faults during the 2016–2017 central Italy seismic sequence. Waveform cross‐correlation based double‐difference location of nearly 400,000 aftershocks illuminate complex, fine‐scale structures of interacting fault zones. The Mt. Vettore–Mt. Bove (VB) normal fault exhibits wide and complex damage zones, including a system of bookshelf faults that intersects the detachment zone. In the Laga domain, a comparatively narrow, shallow dipping segment of the deep Mt. Gorzano fault progressively ruptures through the detachment zone in four subsequentM_W∼ 5.4 events. Reconstructed fault planes show that the detachment zone is fragmented in four sub‐horizontal, partly overlaying shear planes that correlated with the extent of the mainshock ruptures. We find a new, deep reaching seismic barrier that coincides with a bend in the VB fault and may play a role in controlling rupture evolution.