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The fast individuation and modeling of faults responsible for large earthquakes are fundamental for understanding the evolution of potentially destructive seismic sequences. This is even more challenging in case of buried thrusts located in offshore areas, like those hosting the 9 November 2022 Ml 5.7 (Mw 5.5) and ML 5.2 earthquakes that nucleated along the Apennines compressional front, offshore the northern Adriatic Sea. Available on- and offshore (from hydrocarbon platforms) geodetic observations and seismological data provide robust constraints on the rupture of a 15 km long, ca. 24° SSW-dipping fault patch, consistent with seismic reflection data. Stress increase along unruptured portion of the activated thrust front suggests the potential activation of longer portions of the thrust with higher magnitude earthquake and larger surface faulting. This unpleasant scenario needs to be further investigated, also considering their tsunamigenic potential and possible impact on onshore and offshore human communities and infrastructures. [ABSTRACT FROM AUTHOR]

One major critical issue in seismic hazard analysis deals with the computation of the maximum earthquake magnitude expected for a given region. Its estimation is usually based on the analysis of past seismicity that is incomplete by definition, or derived from the dimension of faults through empirical relationships with the intrinsic uncertainty in source characterization. Here, we propose a workflow aimed at providing a time-independent estimate for the maximum possible magnitude based on geological and geophysical evidence. Our estimate is also source unrelated as it is constrained by the seismic brittle volume of the crust that scales with the effective seismic energy. The seismic brittle volume is calculated considering fault kinematics and rock rheology (i.e., the brittle-ductile transition depth) over a grid that covers the entire study area. The maximum earthquake magnitude is calculated at each point of the grid based on a volume/magnitude empirical relationship. We apply this model to Italy for which we propose a map of the maximum possible magnitudes. Maximum predicted magnitudes are 7.3 ± 0.25 for thrust faulting, 7.6 ± 0.77 for normal faulting and 7.6 ± 0.37 for strike-slip faulting (± deviation from the mean value calculated at each node). These magnitudes are locally higher than the historical record. This could be due to an overestimation of the involved volumes; smaller volumes and lower magnitudes may occur where faults are detached at decollements shallower than the brittle ductile transition or where they behave aseismically. Alternatively, strong or major earthquakes could be possible, but they have longer recurrence time and they have never been recorded yet in Italy. Regardless these values are fully reliable or not, the recurrence of earthquakes with the predicted magnitude is related to current strain rates. We conclude that a large part of the Italian territory is prone to trigger Mw > 5 earthquakes. [Display omitted] • A map of the maximum earthquake magnitude for Italy is compiled based on potential brittle volume. • Expected magnitudes correlate with the calculated brittle volumes. • Areas of relative low geodetic strain rate are more prone to nucleate earthquakes with respect to the surroundings. • Maximum magnitudes calculated on volumetric constraints are time independent. [ABSTRACT FROM AUTHOR]

In this work, is analysed the control exerted by the stress axes orientation on the evolution of seismic sequences developing in compressive and extensional regime. According to the Anderson fault theory, the vertical stress is the minimum principal stress in compressional tectonic regimes, whereas it is the maximum principal stress in extensional regimes. Using Mohr diagrams and discussing the present knowledge about the distribution of vertical and horizontal stress with depth we show that, in absence of localised fluid overpressure, such changes imply that thrust and normal faults become more unstable at shallower and greater depths, respectively. These opposite mechanical behaviours predict, in a rather isotropic body, easier rupture at shallower level in compressional regimes later propagating downward. On the contrary, a first deep rupture propagating upward is expected in extensional regimes. This is consistent with observations from major earthquakes from different areas in the world. We show that the exceptions to downward migration along thrusts occur along steeply inclined faults and probably imply localised supra-hydrostatic fluid pressures. Moreover, we show that the inversion of the meaning of the lithostatic load has consequences also for the role of topography. High topography, increasing the vertical load, should inhibit earthquake development in compressional environments and should favour it in extensional settings. Although several factors, such as geodynamic processes, local tectonic features and rock rheology, are likely to control earthquake locations, stress distribution and tectonic regime, these model predictions are consistent with seismicity distribution in Italy, central Andes and Himalaya. In these areas, large to medium compressional earthquakes occur at the low elevation borders of compressional mountain belts, whereas large extensional earthquakes occur in correspondence to maximum elevations. [Copyright &y& Elsevier]

Earthquakes are dissipation of energy throughout elastic waves. Canonically is the elastic energy accumulated during the interseismic period. However, in crustal extensional settings, gravity is the main energy source for hangingwall fault collapsing. Gravitational potential is about 100 times larger than the observed magnitude, far more than enough to explain the earthquake. Therefore, normal faults have a different mechanism of energy accumulation and dissipation (graviquakes) with respect to other tectonic settings (strike-slip and contractional), where elastic energy allows motion even against gravity. The bigger the involved volume, the larger is their magnitude. The steeper the normal fault, the larger is the vertical displacement and the larger is the seismic energy released. Normal faults activate preferentially at about 60° but they can be shallower in low friction rocks. In low static friction rocks, the fault may partly creep dissipating gravitational energy without releasing great amount of seismic energy. The maximum volume involved by graviquakes is smaller than the other tectonic settings, being the activated fault at most about three times the hypocentre depth, explaining their higher b-value and the lower magnitude of the largest recorded events. Having different phenomenology, graviquakes show peculiar precursors. [ABSTRACT FROM AUTHOR]

Abstract: We model a fault cross-cutting the brittle upper crust and the ductile lower crust. In the brittle layer the fault is assumed to have stick–slip behaviour, whereas the lower ductile crust is inferred to deform in a steady-state shear. Therefore, the brittle–ductile transition (BDT) separates two layers with different strain rates and structural styles. This contrasting behaviour determines a stress gradient at the BDT that is eventually dissipated during the earthquake. During the interseismic period, along a normal fault it should form a dilated hinge at and above the BDT. Conversely, an over-compressed volume should rather develop above a thrust plane at the BDT. On a normal fault the earthquake is associated with the coseismic closure of the dilated fractures generated in the stretched hangingwall during the interseismic period. In addition to the shear stress overcoming the friction of the fault, the brittle fault moves when the weight of the hangingwall exceeds the strength of the dilated band above the BDT. On a thrust fault, the seismic event is instead associated with the sudden dilation of the previously over-compressed volume in the hangingwall above the BDT, a mechanism requiring much more energy because it acts against gravity. In both cases, the deeper the BDT, the larger the involved volume, and the bigger the related magnitude. We tested two scenarios with two examples from L’Aquila 2009 (Italy) and Chi-Chi 1999 (Taiwan) events. GPS data, energy dissipation and strain rate analysis support these contrasting evolutions. Our model also predicts, consistently with data, that the interseismic strain rate is lower along the fault segment more prone to seismic activation. [Copyright &y& Elsevier]

• Large seismic sequences in Italy are confined at depth by basement rocks. • Basement with shear along networks of phyllosilicates surrounding quartz lenses. • Low friction and rate-strengthening of phyllosilicates dictate shear zone rheology. • Aseismic slip along phyllosilicates limits the depth of the seismogenic zone. Crustal seismicity is in general confined within the seismogenic layer, which is bounded at depth by processes related to the brittle-ductile transition (BDT) and in the shallow region by fault zone consolidation state and mineralogy. In the past 10-15 years, high resolution seismological and geodetic data have shown that faulting within and around the traditional seismogenic zone occurs in a large variety of slip modes. Frictional and structural heterogeneities have been invoked to explain such differences in fault slip mode and behaviour. However, an integrated and comprehensive picture remains extremely challenging because of difficulties to properly characterize fault rocks at seismogenic depths. Thus, the central-northern Apennines provide a unique opportunity because of the integration of deep-borehole stratigraphy and seismic reflection profiles with high resolution seismological data and outcrop studies. These works show that seismic sequences are limited within the sedimentary cover (depth < 9-10 km), suggesting that the underlaying basement plays a key-role in dictating the lower boundary of the seismogenic zone. Here we integrate structural data on exhumed outcrops of basement rocks with laboratory friction data to shed light on the mechanics of the Apenninic basement. Structural data highlight heterogeneous and pervasive deformation where foliated and phyllosilicate-rich rocks surround more competent quartz-rich lenses up to hundreds of meters in thickness. Phyllosilicate horizons deform predominantly by folding and foliation-parallel frictional sliding whereas quartz-rich lenses are characterized by brittle signatures represented by extensive fracturing and minor faulting. Laboratory experiments revealed that quartz-rich lithologies have relatively high friction, μ ≈ 0.51 , velocity-strengthening to neutral behaviour, and elevated healing rates. On the contrary, phyllosilicate-rich (muscovite and chlorite) lithologies show low friction, 0.23 < μ < 0.31, a marked velocity strengthening behaviour that increases with increasing sliding velocity and negligible rates of frictional healing. Our integrated approach suggests that in the Apenninic basement deformation occurs along shear zones distributed on thickness up-to several kilometres, where the frictionally stable, foliated, and phyllosilicate-rich horizons favour aseismic deformation and therefore confine the depth of major earthquake ruptures and the seismogenic zone. [ABSTRACT FROM AUTHOR]

Understanding the behavior of fluids in seismically active faults and their chemical-physical (dis)equilibrium with the host rock is important to understand the role of fluids upon seismicity and their possible potential for forecasting earthquakes. The small number of case studies where seismic and geochemical data are available and the lack of accessibility to fault zones at seismogenic depth for recent earthquakes limit our understanding of fluid circulation and its relationship to seismicity. The study of fault-fluid relationships in exhumed faults can broaden the number of case histories and improve our understanding of the role of fluids in the seismic cycle in different tectonic settings. Here we use new geochemical and thermal data and a review of published studies from the Apennines fold-and-thrust belt (Italy) to provide a model of fluid circulation during the seismic cycle related to either the local orogenic compressional or post-orogenic extensional tectonics. We also suggest a workflow based upon different methods to identify tectonic-related chemical-physical (isotopic and thermal) (dis)equilibria in fluid-rock systems during the seismic cycle. The proposed workflow involves multiscale structural and isotope geochemical analyses, radiometric dating, and burial-thermal modeling. It is applied to carbonate-hosted faults exhumed from a depth shallower than 4 km (temperature ≤ ∼130 °C and pressure ≤ ∼130 MPa). We show that in the Apennines, during syn-orogenic shortening, thrusting is mostly assisted by fluid circulation in an effectively closed system where fluid and host rock remain close to chemical and thermal equilibrium. In contrast, post-orogenic normal faulting occurs in association with upward and/or downward open fluid circulation systems leading to chemical-physical disequilibria between the host rock and the circulating fluids. Isotopic and thermal fluid-rock disequilibria are particularly evident during pre- and co-seismic extensional deformation. Mineralizing fluids, whose temperature can vary between 30 °C warmer and 16 °C colder than the host rock, result from the mixing of fluids derived from both the deforming host rock and external sources (meteoric or deep crustal). The proposed workflow offers the potential to track past seismic cycles and provide indications on actual fluid-earthquake relationships including the identification of potential seismic precursors and modes of triggered seismicity that might be different in extensional and compressional tectonic settings. • Fluid-rock chemical-physical (dis)equilibrium during faulting in the Apennines • Structural, isotopic, and thermal proxies for tectonic mineralizations • Fluid-rock systems and relationships to the seismic cycle [ABSTRACT FROM AUTHOR]

The Mt. Gorzano Fault (MGF) is a major seismically active extensional fault of the central Apennines, responsible for the destructive Mw 6.0 Amatrice earthquake in 2016. The MGF developed during post-orogenic extensional tectonics, generating a continental basin in the hanging wall. The age of the onset of the MGF and the relationship among faulting, fluid circulation, and the seismic cycles are unknown. We investigate these issues by studying the footwall damage zone of the MGF (exhumed from ~2-3 km depth), where extensional structures and related mineralizations cut and partially overprint the pre-existing compressional orogenic fabric. Structural and geochemical analyses (REE, O, C and clumped isotopes) combined with U-Pb dating of calcite mineralizations show that extensional deformation along the MGF began at least ~2.5 Myr ago. Its activity has continued to the present-day with a mean slip rate of ~0.9 mm/yr, consistent with other seismically active extensional faults of the central Apennines. During Apennine contractional tectonics, orogenic structures and tectonic overburden hindered the ascent of deep fluids, which, therefore, may have been progressively overpressured. We postulate that such overpressure occasionally or cyclically triggered impulsive deformation (earthquakes). During the subsequent extensional phase (younger ~2.5 Ma), pre- or co-seismic crustal dilation opened pathways for the ascent of deep mineralizing fluids, which impregnated the fault damage zone and may have triggered earthquakes. The crustal reservoir of carbonate-rich fluids nowadays is the source of water for the Acquasanta thermal springs close to the Mt. Gorzano seismic area. • The activity of the Mt. Gorzano Fault began at least ~2.5 Ma. • Inter-seismic slickenfibers. • Pre−/co-seismic veins with a meshed geometry. • Upward squeezing of deep fluids. • Radiometric dating of active fault. [ABSTRACT FROM AUTHOR]