The fault activation （fault on） interrupts the enduring fault locking （fault off） and marks the end of a seismic cycle in which the brittle-ductile transition （BDT） acts as a sort of switch. We ...suggest that the fluid flow rates differ during the different periods of the seismic cycle （interseismic, pre-seismic, coseismic and post-seismic） and in particular as a function of the tectonic style. Regional examples indicate that tectonic-related fluids anomalies depend on the stage of the tectonic cycle and the tectonic style. Although it is difficult to model an increasing permeability with depth and several BDT transitions plus independent acquicludes may occur in the crust, we devised the simplest numerical model of a fault constantly shearing in the ductile deeper crust while being locked in the brittle shallow layer, with variable homogeneous permeabilities. The results indicate different behaviors in the three main tectonic settings. In tensional tectonics, a stretched band antithetic to the normal fault forms above the BDT during the interseismic period. Fractures close and fluids are expellecl during the coseismic stage. The mechanism reverses in compressional tectonics. During the interseismic stage, an over-compressed band forms above the BDT. The band dilates while rebounding in the coseismic stage and attracts fluids locally. At the tip lines along strike-slip faults, two couples of subvertical bancls show different behavior, one in dilationJcompression and one in compressionJdilation. This deformation pattern inverts during the coseismic stage. Sometimes a pre-seismic stage in which fluids start moving may be observed and could potentially become a precursor.
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.
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•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.
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.
Aftershocks number decay through time, depending on several parameters peculiar to each seismogenic regions, including mainshock magnitude, crustal rheology, and stress changes along the fault. ...However, the exact role of these parameters in controlling the duration of the aftershock sequence is still unknown. Here, using two methodologies, we show that the tectonic setting primarily controls the duration of aftershocks. On average and for a given mainshock magnitude (1) aftershock sequences are longer and (2) the number of earthquakes is greater in extensional tectonic settings than in contractional ones. We interpret this difference as related to the different type of energy dissipated during earthquakes. In detail, (1) a joint effect of gravitational forces and pure elastic stress release governs extensional earthquakes, whereas (2) pure elastic stress release controls contractional earthquakes. Accordingly, normal faults operate in favour of gravity, preserving inertia for a longer period and seismicity lasts until gravitational equilibrium is reached. Vice versa, thrusts act against gravity, exhaust their inertia faster and the elastic energy dissipation is buffered by the gravitational force. Hence, for seismic sequences of comparable magnitude and rheological parameters, aftershocks last longer in extensional settings because gravity favours the collapse of the hangingwall volumes.
Thrust fault ruptures during earthquakes do not often propagate down to the brittle‐ductile transition. Lithological variations control the behavior and depth of regional basal thrusts and ...decollement planes. Thrust fronts may be discontinuous along strike, limiting the dimension of single coseismic ruptures. These factors control the maximum expected magnitude in one region. This is the case of Italy where the convergence of few millimeter per year in the Apennines accretionary prism and along the retrobelt of the Alps generates compressional earthquakes with moderate to strong magnitudes. Here, using geological and geophysical data, we first compile a map of the undulated active basal thrust decollement for Italy that occurs from 1 to 17‐km depth. Then, we verify the relationship between the length of seismogenic ruptures in thrust faults (Lf) and the maximum depth of thrust faulting (zmax) of related earthquakes and find that their ratio (Lf/zmax) ranges between 2 and 4. Finally, we compute the potential seismogenic volume and estimate the maximum magnitude using an empirical relationship that multiplies the decollement depth and the Lf/zmax ratio. Maximum calculated magnitude is 6.7 ± 0.37 (depending on Lf/zmax and fault dip angle), consistent with the largest magnitude of thrust‐related earthquakes recorded in Italy (6.5–7.0). Lower magnitudes are predicted in the Ionian Seas at the external front of the Apennines where smaller crustal volumes are involved, whereas higher magnitudes are expected in the southern Po Basin, the western Adriatic Sea, Sicily offshore, and the Southern Alps where the decollement is deeper and the brittle volumes are far greater.
Plain Language Summary
Earthquakes are due to different kinds of crustal fault ruptures, and the observed magnitude depends on the rupture dimension, hence on the involved brittle volume. Those related to thrust faults generate magnitudes lower than expected if the decollement is shallow, whereas the magnitudes increase where the decollement is deeper. This is the case of Italy where many factors can explain the moderate to strong (M 5–7) observed magnitudes recorded so far in the contractional areas of the country: (i) the slow convergence rate (few millimeters per year) observed for the Apennines and Alps; (ii) the thrusts do not cut down the entire seismogenic crust; and that (iii) thrusts are undulated and segmented along strike, limiting the dimension of the fault rupture. We compile a map of the thrust decollement depth for Italy that is situated between 1–17 km. Then, we quantify the along‐strike discontinuity, verifying the ratio between thrusts length and faulting depth of related earthquakes. Finally, we estimate the maximum expected magnitude for the study area. Calculated magnitudes are consistent with those occurred for thrust‐related earthquakes in Italy, pointing to the importance of knowing the depth and the lateral continuity of faults to correctly assess the seismic potential of a region.
Key Points
The basal active thrust decollement is mapped on the Italian accretionary prisms
We define the relationship between the length of thrusts ruptures and maximum faulting depth (Lf/zmax) for Italian earthquakes
Seismic volumes calculated from faulting depth and Lf/zmax ratio provide a good estimation of expected magnitudes
Post‐orogenic back‐arc magmatism is accompanied by hydrothermal ore deposits and mineralizations derived from mantle and crustal sources. We investigate Zannone Island (ZI), back‐arc Tyrrhenian ...basin, Italy, to define the source(s) of mineralizing hydrothermal fluids and their relationships with the regional petrological‐tectonic setting. On ZI, early Miocene thrusting was overprinted by late Miocene post‐orogenic extension and related hydrothermal alteration. Since active submarine hydrothermal outflow is reported close to the island, Zannone provides an ideal site to determine the P‐T‐X evolution of the long‐lived hydrothermal system. We combined field work with microstructural analyses on syn‐tectonic quartz veins and carbonate mineralizations, X‐ray diffraction analysis, microthermometry and element mapping of fluid inclusions (FIs), C, O, and clumped isotopes, and analyses of noble gases (He‐Ne‐Ar) and CO2 content in FIs. Our results document the evolution of a fluid system of magmatic origin with increasing mixing of meteoric fluids. Magmatic fluids were responsible for quartz veins precipitation at ∼125 to 150 MPa and ∼300°C–350°C. With the onset of extensional faulting, magmatic fluids progressively interacted with carbonate rocks and mixed with meteoric fluids, leading to (a) host rock alteration with associated carbonate and minor ore mineral precipitation, (b) progressive fluid neutralization, (c) cooling of the hydrothermal system (from ∼320°C to ∼86°C), and (d) embrittlement and fracturing of the host rocks. Both quartz and carbonate mineralizations show noble gases values lower than those from the adjacent active volcanic areas and submarine hydrothermal systems, indicating that the fossil‐to‐active hydrothermal history is associated with the emplacement of multiple magmatic intrusions.
Key Points
Deciphering the fossil‐to‐active hydrothermal system on Zannone Island in which magmatic and meteoric fluids mixed
Polyphase and long‐lived hydrothermal activity associated with mantle‐ and crustal‐derived magmas
Fluid mixing and fluid‐rock interaction led to fluid neutralization, cooling, embrittlements, alteration, and minor ore minerals
Abstract
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 (M
w
5.5) and M
L
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.
The Oman Mountains expose Permo‐Mesozoic shelf rocks of Arabia overridden by continental slope/basinal sediments and Semail Ophiolites during Late Cretaceous. A major syntaxis is represented by the ...Musandam Peninsula and Dibba Zone. The overthrusting of allochthonous units onto the Musandam shelf carbonates initiated during the Cenomanian. Structural analyses in the Musandam Peninsula constrained top‐to‐the‐west thrusting that took place 74–60 Ma ago (U‐Pb datings of synkinematic calcites), about 15–30 Ma after the obduction of the Semail Ophiolite. The Dibba faults exhibit a first interval of thrusting (top‐to‐the‐west) followed by dextral slip. We propose that SW vergent thrusts, initially parallel to those of the Central and Southern Oman Mountains, were subsequently rotated to their present‐day NE‐SW strike during the development of the syntaxis and then reactivated by dextral slip. Mixed layers illite‐smectite (I‐S) constrain the thermal evolution of the passive margin sequence and of the allochthonous deep‐water sediments. In particular, the proximal Hawasina unit (Hamrat Duru) and Sumeini groups of the Dibba Zone are characterized by long‐range ordered mixed layers I‐S with an illite content of 90–95%, whereas Musandam carbonate units show mixed layers I‐S with illite layers ranging from 80–90%, indicating deep diagenetic conditions. Such levels of thermal maturity were acquired during the Late Cretaceous emplacement of a 3.5‐km‐thick pile of allochthonous units, which were removed by erosion and denudation since the Campanian. U‐Pb dating of synkinematic calcite vein highlights reactivation of thrusts at 13.2 Ma, likely due to the involvement of the Musandam area in the Arabia‐Eurasia collision.
Key Points
Musandam faults display multiple kinematics acquired between 74 and 13 Ma
Mixed layers illite‐smectite paleothermometers indicate deep diagenetic conditions
Levels of thermal maturity were acquired during the Late Cretaceous obduction
The evolution of the Apennine wedge has seen the time‐space migration of the forebulge, foredeep, thrust wedge, and back‐arc extension phases in the wake of the Eastward rollback of the subducting ...Adria slab. In this framework, thrusting and post‐orogenic extensional faulting have occurred in two parallel forelandward‐migrating ribbons, with extensional deformation overprinting or partly exploiting anisotropies of the inherited thrust system. Here, we explore the tectonic framework and the timing of thrusting and subsequent negative inversion of the Circeo thrust, one of the major thrusts in the inner portion of the central Apennines, with the main aim to constrain the timing and mode of the compression to extension switch. Structural analysis, carbonate C and O and clumped isotopes analysis, X‐ray diffraction of clay minerals, and U‐Pb dating of calcite slickenfibers have been integrated with seismic interpretation, cross‐section balancing, and 1D burial and thermal modeling. We show that the Circeo thrust developed during Langhian‐Serravallian time. Its extensional reactivation is dated at the Serravallian, during the stacking of an underlying thrust slice, before the onset of Pliocene back‐arc extension in the area. Combination of our data with the age of thrusts, extensional basins, and base of the foredeep infill of the central Apennines, demonstrates that forelandward migration of the foredeep‐thrust system occurred at variable velocities. Accelerations and decelerations are synchronous, respectively, with the opening of the Liguro‐Provençal and Tyrrhenian back‐arc basins and with the interluding quiescent period.
Key Points
We build a balanced cross section across the Circeo thin‐skinned thrust, one of the innermost structures of the central Apennines
The section is validated by means of carbonate C and O and clumped isotopes, U‐Pb dating, seismic interpretation and burial modeling
A regional synthesis of the age of foredeep infill, thrusts, and extensional basins for the central Apennines is presented
The Tyrrhenian back‐arc basin developed at the rear of the E‐ward migrating Apennine fold‐and‐thrust belt, with northward decreasing rollback of the subducting Adria slab leading to northward fading ...of back‐arc extension. The northern portion of the Tyrrhenian basin is made of thinned continental crust, whereas in the central/southern portion extension eventually evolved to oceanic crust production. In this framework, a long‐lasting debate concerns the existence of a >200 km long transform zone along the 41st parallel, which should separate the two portions of the Tyrrhenian basin. At its eastern termination, a branch of the presumed transform zone enters the Tyrrhenian margin of the Apennine belt and occurs as an accommodation zone made of a ribbon of extensional faults and related basins. This accommodation zone, which separates areas of mutually perpendicular extension directions, is here introduced, described, and named the Ponza‐Alife accommodation zone. Interpretation of seismic lines and new structural and stratigraphic data from this accommodation zone have been used to constrain the pre‐orogenic and syn‐orogenic architecture of the subducting plate and the Plio‐Quaternary back‐arc extensional stage. Our data indicate that the studied zone retraces a deep‐seated transform fault system located in the subducting plate and inherited from an Early Jurassic rifting episode, which caused the lateral juxtaposition of different rift domains in the subducting plate. We propose that during collision and trench retreat, this lateral juxtaposition has controlled differential retreat of the subducting plate across the studied zone, forcing the development of the Ponza‐Alife accommodation zone in the overlying back‐arc basin's margin.
Key Points
The Ponza‐Alife accommodation zone segmenting the Apenninic margin of the Tyrrhenian back‐arc basin is introduced
Structural and stratigraphic data, and seismic interpretation allowed us to unravel the evolution of this accommodation zone
The accommodation zone originated from tearing in the downgoing plate reworking Jurassic faults that segmented the associated rift system
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