Field geological studies have revealed the heterogeneous structure of fault zones down to the sub-metric scale due to the juxtaposition of rocks presenting distinct deformation intensity and physical-transport properties. However, such internal variability is not generally resolved by most seismic tomography techniques due to spatial resolution limits. Quantifying the heterogeneous internal structure of fault zones is fundamental to understand their mechanical and hydrological characteristics. In this sense, determining seismic wave velocities and related physical properties (elastic moduli, porosity and fracture intensity) within fault zones, at different observational scales, is crucial.Here, the near-surface velocity structure of two active seismogenic fault zones located in the Central Apennines of Italy was quantified at different length scales, from laboratory measurements of ultrasonic velocities (rock samples of few centimeters, 1 MHz source) to high-resolution first-arrival seismic tomography (spatial resolution of few meters). Detailed structural mapping was conducted within the Vado di Corno and Monte Marine fault zones, two NW-SE trending structures with length of ~ 15 km and up to 1.5 km of extensional displacement. Distinct structural units separated by fault strands were recognized in the fault zone footwall blocks cutting Mesozoic dolomitic carbonates: (i) fault core cataclastic units, (ii) breccia unit, (iii) high-strain damage zone, (iv) low-strain damage zone. The single units were systematically sampled along transects orthogonal to the average strike of the faults and characterized in the laboratory in terms of directional P and S ultrasonic wave velocities, porosity and microstructures. The fault core cataclastic units were significantly “slower” (VP = 4.5±0.4 kms-1, VS = 2.7±0.2 kms-1) compared to the damage zone units (VP = 5.6±0.6 kms-1, VS = 3.2±0.3 kms-1) at short length scales (i.e. few centimeters). A general negative correlation between ultrasonic velocity and porosity was observed, with some variability within the fault core mostly related to the textural maturity (clast/matrix volume ratio) of the fault rocks and the degree of pore space sealing by calcite cements.Multiple P- and S-wave high-resolution seismic profiles (length 90-116 m, geophone spacing 1-1.5 m) were acquired across the two fault zones at different structural sites, moving from the principal fault surface into the outer damage zone. The derived first-arrival tomography models highlighted fault-bounded rock bodies with distinct velocities and characterized by geometries which well compared with those deduced from the structural mapping. At the larger length scale investigated by the active seismic survey, relatively “fast” fault core units (VP ≤ 3.0 kms-1, VS ≤ 1.8 kms-1) and very “slow” high-strain damage zones (VP < 1.6 kms-1, VS < 1 kms-1) were recognized. These velocity ranges were significantly different from those determined in the laboratory on small samples. This apparent discrepancy could be reconciled using an effective medium approach, considering the effect of mesoscale fractures density and size distributions affecting each structural unit.This combined study highlighted the high petrophysical variability of carbonate-hosted fault zones, with structural units characterized by sharp contacts and different velocity scaling. In particular, the persistence of compliant high-strain damage zones at shallow depth might strongly affect near-surface deformation.
Seismic signals propagating across a fault may yield information on the internal structure of the fault zone. Here we have assessed the amplification of seismic noise (i.e., ambient vibrations generated by natural or anthropogenic disturbances) across the Vado di Corno Fault (Campo Imperatore, central Italy). The fault zone is considered as an exhumed analogue of the normal faults activated during the L'Aquila 2009 earthquake sequence. Detailed structural geological survey of the footwall block revealed that the fault zone is highly anisotropic and is affected by a complex network of faults and fractures with dominant WNW–ESE strike. We measured seismic noise with portable seismometers along a ∼500 m long transect perpendicular to the average fault strike. Seismic signals were processed calculating the horizontal-to-vertical spectral ratios and performing wavefield polarization analyses. We found a predominant NE–SW to NNE–SSW (i.e., ca. perpendicular to the average strike of the fault-fracture network) amplification of the horizontal component of the seismic waves. Numerical simulations of earthquake-induced ground motions ruled out the role of topography in controlling the polarization and the amplitude of the waves. Therefore, the higher seismic noise amplitude observed in the fault-perpendicular direction was related to the measured fracture network and the resulting stiffness anisotropy of the rock mass. These observations open new perspectives in using measures of ambient seismic noise, which are fast and inexpensive, to estimate the dominant orientation of fracture networks within fault zones.
Observations of comet nuclei indicate that the main constituent is a mix of ice and refractory materials characterized by high porosity (70–75%) and low bulk strength (10 −4 –10 −6 MPa); however, the nature and physical properties of these materials remain largely unknown. By combining surface inspection of comet 67P/Churyumov–Gerasimenko and three-dimensional (3D) modeling of the independent concentric sets of layers that make up the structure of its two lobes, we provide clues about the large-scale rheological behavior of the nucleus and the kinematics of the impact that originated it. Large folds in the layered structure indicate that the merging of the two cometesimals involved reciprocal motion with dextral strike–slip kinematics that bent the layers in the contact area without obliterating them. Widespread long cracks and the evidence of relevant mass loss in absence of large density variations within the comet’s body testify that large-scale deformation occurred in a brittle-plastic regime and was accommodated through folding and fracturing. Comparison of refined 3D geologic models of the lobes with triaxial ellipsoids that suitably represent the overall layers arrangement reveals characteristics that are consistent with an impact between two roughly ellipsoidal cometesimals that produced large-scale axial compression and transversal elongation. The observed features imply global transfer of impact-related shortening into transversal strain. These elements delineate a model for the global rheology of cometesimals that could be possible evoking a prominent bonding action of ice and, to a minor extent, organics.
How major crustal-scale seismogenic faults nucleate and evolve in crystalline basements represents a long-standing, but poorly understood, issue in structural geology and fault mechanics. Here, we address the spatio-temporal evolution of the Bolfin Fault Zone (BFZ), a >40-km-long exhumed seismogenic splay fault of the 1000-km-long strike-slip Atacama Fault System. The BFZ has a sinuous fault trace across the Mesozoic magmatic arc of the Coastal Cordillera (Northern Chile) and formed during the oblique subduction of the Aluk plate beneath the South American plate. Seismic faulting occurred at 5-7 km depth and ≤ 300°C in a fluid-rich environment as recorded by extensive propylitic alteration and epidote-chlorite veining. Ancient (125-118 Ma) seismicity is attested by the widespread occurrence of pseudotachylytes. Field geologic surveys indicate nucleation of the BFZ on precursory geometrical anisotropies represented by magmatic foliation of plutons (northern and central segments) and andesitic dyke swarms (southern segment) within the heterogeneous crystalline basement. Seismic faulting exploited the segments of precursory anisotropies that were optimal to favorably oriented with respect to the long-term far-stress field associated with the oblique ancient subduction. The large-scale sinuous geometry of the BFZ resulted from the hard linkage of these anisotropy-pinned segments during fault growth.
A major part of the seismicity striking the Mediterranean area and other regions worldwide is hosted in carbonate rocks. Recent examples are the destructive earthquakes of L'Aquila Mw 6.1 2009 and Norcia Mw 6.5 2016 in Central Italy. Surprisingly, within this region, fast (\approx 3km/s) and destructive seismic ruptures coexist with slow (maximum 10 m/s) and non-destructive rupture phenomena. Despite of its relevance for seismic hazard studies, the transitions from fault creep to slow and fast seismic rupture propagation are still poorly constrained by seismological and laboratory observations. Here, we reproduced in the laboratory the complete spectrum of natural faulting on samples of dolostones representative of the seismogenic layer in the region. The transitions from fault creep to slow ruptures and from slow to fast ruptures, are obtained by increasing both confining pressure (P) and temperature (T) up to conditions encountered at 3-5 km depth (i.e., P = 100 MPa and T = 100 $^{o}$C), which corresponds to the hypocentral location of slow earthquake swarms and the onset of regular seismicity in Central Italy. The transition from slow to fast rupture is explained by the increase of the ambient temperature, which enhances the elastic loading stiffness of the fault and consequently the slip velocity during the nucleation stage, allowing flash weakening. The activation of such weakening induces the propagation of fast ruptures radiating intense high frequency seismic waves.
Abstract. Earthquake swarms commonly occur in upper-crustal hydrothermal-magmatic systems and activate mesh-like fault networks. How these networks develop through space and time along seismic faults is poorly constrained in the geological record. Here, we describe a spatially dense array of small-displacement (< 1.5 m) epidote-rich fault veins (i.e., hybrid extensional–shear veins) within granitoids, occurring at the intersections of subsidiary faults with the exhumed seismogenic Bolfin Fault Zone (Atacama Fault System, northern Chile). Epidote hybrid extensional–shear veining occurred at 3–7 km depth and 200–300 °C ambient temperature. At a distance of ≤ 1 cm to fault veins, the magmatic quartz of the wall rock shows (i) thin (< 10 µm thick) interlaced deformation lamellae and (ii) systematically crosscutting veinlets healed by quartz and feldspars, and it appears shattered at the vein contact. Clasts of deformed magmatic quartz, with deformation lamellae and healed veinlets, are included in the epidote-rich fault veins. Deformation of the wall-rock quartz is interpreted to record the transient large stress perturbation associated with the propagation of small earthquakes preceding conspicuous epidote mineralization. Conversely, the epidote-rich fault veins record cyclic events of extensional-to-hybrid veining and either aseismic or seismic shearing. The dilation and shearing behavior of the epidote-rich fault veins are interpreted to record the later development of a mature and hydraulically connected fault–fracture system. In this latter stage, the fault–fracture system cyclically ruptured due to fluid pressure fluctuations, possibly correlated with swarm-like earthquake sequences.
