Pousse-Beltran L., Benedetti L., Fleury J., Boncio P., Guillou V., Rizza M., Puliti I., Socquet A. & ASTER Team
The Central Apennines are among the most seismically active regions in Italy. This region is affected by an extension accommodated by active normal faults. Those faults with high seismogenic potential hosted Mw > 6 shallow damaging earthquakes, such as the Mw 6.5 event of Oct. 30, 2016. Although fault slip rates are crucial to seismic hazard assessments, in the Central Apennines, slip rate assessments encompassing several seismic cycles (> 10 ka) are sparse. This is particularly true for the Mt. Vettore-Mt. Bove Fault system that ruptured during the mainshocks of the 2016–2017 seismic sequence. In this study, we present new geochronological constraints of offset geomorphological markers along the northern portion of the fault system on the Mt. Porche Fault segment using in-situ produced 36Cl cosmogenic nuclides. Offset measurements were made using a 5-cm resolution DEM obtained through a drone survey and constrained a fault scarp height of 15.5 ± 1.4 m and a maximal cumulative offset estimated between 32 and 40.5 m. Samples were collected from the Valle Lunga terminal moraine at 1710 m asl and yielded 36Cl exposure ages of 12.7 + 2.2/−1.9 ka, while the abraded surface, located on top of the tectonic scarp yielded 36Cl exposure ages of 23.4 + 5.3/−4.3 ka. Using the fault scarp height and the exposure age of this abraded surface, we constrained a minimum vertical fault slip rate of 0.7 + 0.2/−0.1 mm/yr. Assuming the offset started to accumulate when climate conditions allowed the scarp preservation, we constrained a maximal vertical slip rate of 1.2 ± 0.2 mm/yr along the main fault of the Mt. Vettore-Mt. Bove Fault system. This rate is higher than those previously obtained from trenches along secondary branches of the Mt. Vettore-Mt. Bove Fault system. Besides, the yielded chronology for the last glacial maximum in that area at ~23 ka is in good agreement with the timing previously proposed for the LGM in the Apennines.
Tesson J., Benedetti L., Godard V., Novaes C., Fleury J., & ASTER Team
Facets are major topographic features built over several 100 k.y. above active normal faults. Their development integrates cumulative displacements over a longer time frame than many other geomorphological markers, and they are widespread in diverse extensional settings. We have determined the 36Cl cosmogenic nuclide concentration on limestone faceted spurs at four sites in the Central Apennines (Italy), representing variable facet height (100–400 m). The 36Cl concentration profiles show nearly constant values over the height of the facet, suggesting the facet slope has reached a steady-state equilibrium for 36Cl production. We model the 36Cl buildup on a facet based on a gradual exposure of the sample resulting from fault slip and denudation. Data inversion with this forward model yields accurate constraints on fault slip rates over the past 20–200 k.y., which are in agreement with the long-term rate independently determined on some of those faults over the past 1 m.y. 36Cl measurements on faceted spurs can therefore constrain fault slip rate over time spans as long as 200 k.y., a time period presently undersampled in most morphotectonic studies.
Scotti O., Visini F., Faure Walker J., Peruzza L., Pace B., Benedetti L., Boncio P., Roberts G.
The aim of the Fault2SHA European Seismological Commission Working Group Central Apennines laboratory is to enhance the use of geological data in fault-based seismic hazard and risk assessment and to promote synergies between data providers (earthquake geologists), end-users and decision-makers. Here we use the Fault2SHA Central Apennines Database where geologic data are provided in the form of characterized fault traces, grouped into faults and master faults, with individual slip rate estimates. The proposed methodology first derives slip rate profiles for each master fault. Master faults are then divided into distinct sections of length comparable to the seismogenic depth to allow consideration of variable slip rates along master faults and the exploration of multi-fault ruptures in the computations. The methodology further allows exploration of epistemic uncertainties documented in the database (e.g. master fault definition, slip rates) as well as additional parameters required to characterize the seismogenic potential of fault sources (e.g. 3D fault geometries). To illustrate the power of the methodology, in this paper we consider only one branch of the uncertainties affecting each step of the computation procedure. The resulting hazard and typological risk maps allow both data providers and end-users (1) to visualize the faults that threaten specific localities the most, (2) to appreciate the density of observations used for the computation of slip rate profiles, and (3) interrogate the degree of confidence on the fault parameters documented in the database (activity and location certainty). Finally, closing the loop, the methodology highlights priorities for future geological investigations in terms of where improvements in the density of data within the database would lead to the greatest decreases in epistemic uncertainties in the hazard and risk calculations. Key to this new generation of fault-based seismic hazard and risk methodology are the user-friendly open source codes provided with this publication, documenting, step-by-step, the link between the geological database and the relative contribution of each section to seismic hazard and risk at specific localities.
Walker J. F., Boncio P., Pace B., Roberts, G., Benedetti L., Scotti O., ... & Peruzza L.
We present a database of field data for active faults in the central Apennines, Italy, including trace, fault and main fault locations with activity and location certainties, and slip-rate, slip-vector and surface geometry data. As advances occur in our capability to create more detailed fault-based hazard models, depending on the availability of primary data and observations, it is desirable that such data can be organized in a way that is easily understood and incorporated into present and future models. The database structure presented herein aims to assist this process. We recommend stating what observations have led to different location and activity certainty and presenting slip-rate data with point location coordinates of where the data were collected with the time periods over which they were calculated. Such data reporting allows more complete uncertainty analyses in hazard and risk modelling. The data and maps are available as kmz, kml, and geopackage files with the data presented in spreadsheet files and the map coordinates as txt files. The files are available at: https://doi.pangaea.de/10.1594/PANGAEA.922582
Puliti I., Pizzi A., Benedetti L., Di Domenica A., & Fleury J.
In 2016, the Mt. Vettore-Mt. Bove normal fault system (VBFS) broke during three earthquakes (Mw 6.0, Mw 5.9, and Mw 6.5), associated with clear coseismic ruptures. Based on high-resolution topography and geological field data, we determined the displacements of the VBFS. The distributions of the coseismic and postglacial displacements exhibit similar asymmetric shapes, suggesting self-similar slip profiles over the last 18 kyr. The highest displacement during the 2016 events is localized on the southern Mt. Vettore segments, the southern tip of the VBFS, which also showed the maximum throw of 32 m over the last 18 kyr yielding to a very fast throw rate of 1.6 ± 0.5 mm/yr. Assuming a constant throw rate, the geological displacement we determined suggests an inception age of 200–250 kyr for the surface rupturing faults that ruptured in 2016 on the Mt. Vettore sector. This value can be interpreted as the minimum age for the onset of those faults in the Mt. Vettore and might result from a southward shift of VBFS activity with a change of the fault system pattern. We infer that the observed slip distribution, maximum at Mt. Vettore and tapering-off southward, might be due to the VBFS lengthening processes towards the Laga Fault and its interaction with pre-existing structure such as Olevano-Antrodoco-Sibillini thrust that might play a key role in controlling the VBFS's evolution.
Pousse‐Beltran L., Socquet A., Benedetti L., Doin M. P., Rizza M., & d'Agostino N.
The Mw 6.5 Norcia earthquake occurred on 30 October 2016, along the Mt Vettore fault (Central Apennines, Italy), it was the largest earthquake of the 2016-2017 seismic sequence that started 2 months earlier with the Mw 6.0 Amatrice earthquake (24 August). To detect potential slow slip during the sequence, we produced Interferometric Synthetic Aperture Radar (InSAR) time series using 12- to 6-day repeat cycles of Sentinel-1A/1B images. Time series indicates that centimeter-scale surface displacements took place during the 10 weeks following the Norcia earthquake. Two areas of subsidence are detected: one in the Castelluccio basin (hanging wall of the Mt Vettore fault) and one in the southern extent of the Norcia earthquake surface rupture, near an inherited thrust. Poroelastic and viscoelastic models are unable to explain these displacements. In the Castelluccio basin, the displacement reaches 13.2 +/- 1.4 mm in the ascending line of sight on 6 January 2017. South of the Norcia earthquake surface rupture (a zone between the Norcia and Amatrice earthquakes), the postseismic surface displacements affect a smaller area but reach 35.5 +/- 1.7 mm in ascending line of sight by January 2017 and follow a logarithmic temporal decay consistent with postseismic afterslip. Our analysis suggests that the structurally complex area located south of the Norcia rupture (30 October) is characterized by a conditionally stable frictional regime. This geometrical and frictional barrier likely halted rupture propagation during the Amatrice (24 August) and Norcia (30 October) earthquakes at shallow depth (<3-4 km).