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Viti, Cecilia
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Viti, Cecilia
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- PublicationRestrictedMeso- to nano-scale evidence of fluid-assisted co-seismic slip along the normal Mt. Morrone Fault, Italy: Implications for earthquake hydrogeochemical precursors(2021)
; ; ; ; ; ; ; ; ; ; ; ; ;; ; ; ; ;; ; ; ; ;Fluids play an important role in seismic faulting both at hypocentral depths during earthquake nucleation and at shallower crustal levels during rupture propagation. Pre- to co-seismic anomalies of crustal fluid circulation have been identified by hydrogeochemical and seismological monitoring and interpreted as potential precursors of strong earthquakes. To shed light on the role of fluids in seismic and precursory mechanisms, the active carbonate-hosted principal slip zone (7-8 cm thick) of the exhumed (exhumation < 3 km) normal Mt. Morrone Fault (central Apennines) has been investigated with a multi-disciplinary approach from the macro- to the nano-scale. The distal slip zone consists of white cementitious calcite-rich bands and red cataclastic bands composed of dolomite and calcite clasts embedded in a clay-rich matrix. The proximal slip zone consists of subparallel ultracataclastic layers separated by sharp slip surfaces. The ultracataclastic layers mutually inject/overprint, bearing evidence of granular fluidization, dolomite thermal decomposition, and clay amorphization. Fluid inclusions and the distribution of both trace and major elements reveal the inflow of both shallow and deep external fluids into the slip zone. Presumably, the deep fluids originated from a magmatic-like source and ascended along the fault during pre-seismic dilation and seismic ruptures, interacting with shallow phyllosilicate-rich flysch deposits and the fluids hosted within them. In this context, vanadium-rich fluidized microlayers along the exhumed Mt. Morrone Fault are reminiscent of vanadium-rich potential hydrogeochemical precursors arose in the shallow aquifers of the study area since a few months before the 2016 Mw 6.0 Amatrice earthquake.240 17 - PublicationRestrictedFault lubrication and earthquake propagation in thermally unstable rocks(2011)
; ; ; ; ; ; ;De Paola, N.; Durham University, ;Hirose, T.; Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 200 Monobe-otsu, Kochi 783-8502, Japan ;Mitchell, T.; Experimental Geophysics Laboratory, Institute for Geology, Mineralogy, and Geophysics, Ruhr-University Bochum, D-44780 Bochum, Germany ;Di Toro, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Viti, C.; Dipartimento di Geoscienze, Università degli Studi di Padova, Via Giotto 1, Padua 35137, Italy ;Shimamoto, T.; Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, 1-3-1, Kagami-yama, Higashi-Hiroshima 739-8526, Japan; ; ; ; ; Experiments performed on dolomite or Mg-calcite gouges at seismic slip rates (v > 1 m/s) and displacements (d > 1 m) show that the frictional coeffi cient μ decays exponentially from peak values (mp ≈ 0.8, in the Byerlee’s range), to extremely low steady-state values (μss ≈ 0.1), attained over a weakening distance Dw. Microstructural observations show that discontinuous patches of nanoparticles of dolomite and its decomposition products (periclase and lime or portlandite) were produced in the slip zone during the transient stage (d < Dw). These observations, integrated with CO2 emissions data recorded during the experiments, suggest that particle interaction in the slip zone produces fl ash temperatures that are large enough to activate chemical and physical processes, e.g., decarbonation reactions (T = 550 C). During steady state (d ≥ Dw), shear strength is very low and not dependent upon normal stresses, suggesting that pressurized fl uids (CO2) may have been temporarily trapped within the slip zone. At this stage a continuous layer of nanoparticles is developed in the slip zone. For d >> Dw, a slight but abrupt increase in shear strength is observed and interpreted as due to fl uids escaping the slip zone. At this stage, dynamic weakening appears to be controlled by velocity dependent properties of nanoparticles developed in the slip zone. Experimentally derived seismic source parameter Wb (i.e., breakdown work, the energy that controls the dynamics of a propagating fracture) (1) matches Wb values obtained from seismological data of the A.D. 1997 M6 Colfi orito (Italy) earthquakes, which nucleated in the same type of rocks tested in this study, and (2) suggests similar earthquake-scaling relationships, as inferred from existing seismological data sets. We conclude that dynamic weakening of experimental faults is controlled by multiple slip weakening mechanisms, which are activated or inhibited by physicochemical reactions in the slip zone.152 22 - PublicationRestrictedDevelopment of interconnected talc networks and weakening of continental low-angle normal faults(2009-06)
; ; ; ; ;Collettini, C.; University of Perugia ;Viti, C.; University of Siena ;Smith, S .A. F; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Holdsworth, R. E.; University of Durham; ; ; Fault zones that slip when oriented at large angles to the maximum compressive stress, i.e., weak faults, represent a signifi cant mechanical problem. Here we document fault weakening induced by dissolution of dolomite and subsequent precipitation of calcite + abundant talc along a low-angle normal fault. Within the fault core, talc forms an interconnected foliated network that deforms by frictional sliding along 50–200-nm-thick talc lamellae. The low frictional strength of talc, combined with dissolution-precipitation creep, can explain slip on low-angle normal faults. In addition, the stable sliding behavior of talc is consistent with the absence of strong earthquakes along such structures. The development of phyllosilicates such as talc by fl uid-assisted processes within fault zones cutting Mg-rich carbonate sequences may be widespread, leading to profound and long-term fault weakness.170 26 - PublicationRestrictedThermal decomposition along natural carbonate faults during earthquakes(2013)
; ; ; ; ;Collettini, C.; Università Sapienza ;Viti, C.; Università Siena ;Tesei, T.; Università Perugia ;Mollo, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; ; ; Earthquake slip is facilitated by a number of thermally activated physicochemical processes that are triggered by temperature rise during fast fault motion, i.e., frictional heating. Most of our knowledge on these processes is derived from theoretical and experimental studies. However, additional information can be provided by direct observation of ancient faults exposed at the Earth’s surface. Although fault rock indicators of earthquake processes along ancient faults have been inferred, the only unambiguous and rare evidence of seismic sliding from natural faults is solidifi ed friction melts or pseudotachylytes. Here we document a gamut of natural fault rocks produced by thermally activated processes during earthquake slip. These processes occurred at 2–3 km depth, along a thin (0.3–1.0 mm) principal slip zone of a regional thrust fault that accommodated several kilometers of displacement. In the slip zone, composed of ultrafi ne-grained fault rocks made of calcite and minor clays, we observe the presence of relict calcite and clay, numerous vesicles, poorly crystalline/amorphous phases, and newly formed calcite skeletal crystals. These observations indicate that during earthquake rupture, frictional heating induced calcite decarbonation and phyllosilicate dehydration. These microstructures may be diagnostic for recognizing ancient earthquakes along exhumed faults.148 23 - PublicationOpen AccessThe Role of Fabric in Frictional Properties of Phyllosilicate-Rich Tectonic Faults(2021-11-06)
; ; ; ; ; ; ; ; ; ; ; ; ; ;; Many rock deformation experiments used to characterize the frictional properties of tectonic faults are performed on powdered fault rocks or on bare rock surfaces. These experiments have been fundamental to document the frictional properties of granular mineral phases and provide evidence for crustal faults characterized by high friction. However, they cannot entirely capture the frictional properties of faults rich in phyllosilicates. Numerous studies of natural faults have documented fluid-assisted reaction softening promoting the replacement of strong minerals with phyllosilicates that are distributed into continuous foliations. To study how these foliated fabrics influence the frictional properties of faults we have: 1) collected foliated phyllosilicate-rich rocks from natural faults; 2) cut the fault rock samples to obtain solid wafers 0.8-1.2 cm thick and 5 cm x 5 cm in area with the foliation parallel to the 5x5cm face of the wafer; 3) performed friction tests on both solid wafers sheared in their in situ geometry and powders, obtained by crushing and sieving and therefore disrupting the foliation of the same samples; 4) recovered the samples for microstructural studies from the post experiment rock samples; and 5) performed microstructural analyses via optical microscopy, scanning and transmission electron microscopy. Mechanical data show that the solid samples with well-developed foliation show significantly lower friction in comparison to their powdered equivalents. Micro- and nano-structural studies demonstrate that low friction results from sliding along the foliation surfaces composed of phyllosilicates. When the same rocks are powdered, frictional strength is high, because sliding is accommodated by fracturing, grain rotation, translation and associated dilation. Friction tests indicate that foliated fault rocks may have low friction even when phyllosilicates constitute only a small percentage of the total rock volume, implying that a significant number of crustal faults are weak.50 11 - PublicationOpen AccessFault structure and slip localization in carbonate-bearing normal faults: An example from the Northern Apennines of Italy(2014)
; ; ; ; ; ; ; ; ; ;Collettini, C.; Dipartimento di Scienze della, Terra Università La Sapienza di Roma, ;Carpenter, B. M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Viti, C.; Dipartimento di Scienze della Terra Università degli Studi di Siena, ;Cruciani, F.; Dipartimento di Fisica e Geologia, Università degli Studi di Perugia, ;Mollo, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Tesei, T.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Trippetta, F.; Uni Sapienza ;Valoroso, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione CNT, Roma, Italia ;Chiaraluce, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione CNT, Roma, Italia; ; ; ; ; ; ; ; Carbonate-bearing normal faults are important structures for controlling fluid flow and seismogenesis within the brittle upper crust. Numerous studies have tried to characterize fault zone structure and earthquake slip processes along carbonate-bearing faults. However, due to the different scales of investigation, these studies are not often integrated to provide a comprehensive fault image. Here we present a multi-scale investigation of a normal fault exhumed from seismogenic depths. The fault extends for a length of 10 km with a maximum width of about 1.5 km and consists of 5 sub-parallel and interacting segments. The maximum displacement (370e650 m) of each fault segment is partitioned along sub-parallel slipping zones extending for a total width of about 50 m. Each slipping zone is characterized by slipping surfaces exhibiting different slip plane phenomena. Fault rock development is controlled by the protolith lithology. In massive limestone, moving away from the slip surface, we observe a thin layer (<2 cm) of ultracataclasite, cataclasite (2e10 cm) and fault breccia. In marly limestone, the fault rock consists of a cataclasite with hydrofractures and smectite-rich pressure solution seams. At the micro-nanoscale, the slip surface consists of a continuous and thin (<300 mm) layer composed of coarse calcite grains (~5e20 mm in size) associated with sub-micrometer grains showing fading grain boundaries, voids and/or vesicles, and suggesting thermal decomposition processes. Micrometer-sized calcite crystals show nanoscale polysynthetic twinning affected by the occurrence of subgrain boundaries and polygonalized nanostructures. Investigations at the kilometres-tens of meter scale provide fault images that can be directly compared with high-resolution seismological data and when combined can be used to develop a comprehensive characterization of seismically active fault structures in carbonate lithologies. Micro and nanoscale investigations along the principal slipping zone suggest that different deformation processes, including plastic deformation and thermal decomposition, were active during seismic slip.392 602 - PublicationRestrictedEvolution of shear fabric in granular fault gouge from stable sliding to stick slip and implications for fault slip mode(2017-08)
; ; ; ; ; ; ; ; ; Laboratory and theoretical studies provide insight into the mechanisms that control earthquake nucleation, when fault slip velocity is slow (<0.001 cm/s), and dynamic rupture when fault slip rates exceed centimeters per second. The application of these results to tectonic faults requires information about fabric evolution with shear and its impact on the mode of faulting. Here we report on laboratory experiments that illuminate the evolution of shear fabric and its role in controlling the transition from stable sliding (v ∼0.001 cm/s) to dynamic stick slip (v > 1 cm/s). The full range of fault slip modes was achieved by controlling the ratio K = k/kc, where k is the elastic loading stiffness and kc is the fault zone critical rheologic stiffness. We show that K controls the transition from slow-and-silent slip (K > 0.9) to fast-and-audible (K < 0.7, v = 3 cm/s, slip duration 0.003 s) slip events. Microstructural observations show that with accumulated strain, deformation concentrates in shear zones containing sharp shear planes made of nanoscale grains, which favor the development of frictional instabilities. Once this fabric is established, fault fabric does not change significantly with slip velocity, and fault slip behavior is mainly controlled by the interplay between the rheological properties of the slipping planes and fault zone stiffness.116 5