Cristiano Collettini

Aftershocks driven by a high-pressure CO2 source at depth

In northern Italy in 1997, two earthquakes of magnitudes 5.7 and 6 (separated by nine hours) marked the beginning of a sequence that lasted more than 30 days, with thousands of aftershocks including four additional events with magnitudes between 5 and 6. This normal-faulting sequence is not well explained with models of elastic stress transfer, particularly the persistence of hanging-wall seismicity that included two events with magnitudes greater than 5. Here we show that this sequence may have been driven by a fluid pressure pulse generated from the coseismic release of a known deep source of trapped high-pressure carbon dioxide (CO2). We find a strong correlation between the high-pressure front and the aftershock hypocentres over a two-week period, using precise hypocentre locations and a simple model of nonlinear diffusion. The triggering amplitude (10–20 MPa) of the pressure pulse overwhelms the typical (0.1–0.2 MPa) range from stress changes in the usual stress triggering models. We propose that aftershocks of large earthquakes in such geologic environments may be driven by the coseismic release of trapped, high-pressure fluids propagating through damaged zones created by the mainshock. This may provide a link between earthquakes, aftershocks, crust/mantle degassing and earthquake-triggered large-scale fluid flow.

Fault zone fabric and fault weakness

Geological and geophysical evidence suggests that some crustal faults are weak compared to laboratory measurements of frictional strength. Explanations for fault weakness include the presence of weak minerals, high fluid pressures within the fault core and dynamic processes such as normal stress reduction, acoustic fluidization or extreme weakening at high slip velocity. Dynamic weakening mechanisms can explain some observations; however, creep and aseismic slip are thought to occur on weak faults, and quasi-static weakening mechanisms are required to initiate frictional slip on mis-oriented faults, at high angles to the tectonic stress field. Moreover, the maintenance of high fluid pressures requires specialized conditions and weak mineral phases are not present in sufficient abundance to satisfy weak fault models, so weak faults remain largely unexplained. Here we provide laboratory evidence for a brittle, frictional weakening mechanism based on common fault zone fabrics. We report on the frictional strength of intact fault rocks sheared in their in situ geometry. Samples with well-developed foliation are extremely weak compared to their powdered equivalents. Micro- and nano-structural studies show that frictional sliding occurs along very fine-grained foliations composed of phyllosilicates (talc and smectite). When the same rocks are powdered, frictional strength is high, consistent with cataclastic processes. Our data show that fault weakness can occur in cases where weak mineral phases constitute only a small percentage of the total fault rock and that low friction results from slip on a network of weak phyllosilicate-rich surfaces that define the rock fabric. The widespread documentation of foliated fault rocks along mature faults in different tectonic settings and from many different protoliths suggests that this mechanism could be a viable explanation for fault weakening in the brittle crust.

Normal faults, normal friction?

Debate continues as to whether normal faults may be seismically active at very low dips (d , 308) in the upper continental crust. An updated compilation of dip estimates (n 5 25) has been prepared from focal mechanisms of shallow, intracontinental, normal-slip earthquakes (M . 5.5; slip vector raking 90 86 308 in the fault plane) where the rupture plane is unambiguously discriminated. The dip distribution for these moderate-to-large normal fault ruptures extends from 658 . d . 308, corresponding to a range, 258 , ur , 608, for the reactivation angle between the fault and inferred vertical s1. In a comparable data set previously obtained for reverse fault ruptures (n 5 33), the active dip distribution is 108 , d5u r , 608. For vertical and horizontal s1 trajectories within extensional and compressional tectonic regimes, respectively, dip-slip reactivation is thus restricted to faults oriented at ur # 608 to inferred s1. Apparent lockup at ur 608 in each dip distribution and a dominant 30 86 58 peak in the reverse fault dip distribution, are both consistent with a friction coefficient ms 0.6, toward the bottom of Byerlee’s experimental range, though localized fluid overpressuring may be needed for reactivation of less favorably oriented faults.

Fault zone weakening and character of slip along low-angle normal faults: insights from the Zuccale fault, Elba, Italy

A seismically active low-angle normal fault is recognized at depth in the Northern Apennines, Italy, where recent exhumation has also exposed ancient examples at the surface, notably the Zuccale fault on Elba. Field-based and microstructural studies of the Zuccale fault reveal that an initial phase of pervasive cataclasis increased fault zone permeability, promoting influx of CO2-rich hydrous fluids. This triggered low-grade alteration and the onset of stress-induced dissolution–precipitation processes (e.g. pressure solution) as the dominant grain-scale deformation process in the pre-existing cataclasites leading to shear localization and the formation of a narrow foliated fault core dominated by fine-grained hydrous mineral phases. These rocks exhibit ductile deformation textures very similar to those formed during pressure-solution-accommodated ‘frictional–viscous’ creep in experimental fault rock analogues. The presence of multiple hydrofracture sets also points to the local attainment of fluid overpressures following development of the foliated fault core, which significantly enhanced the sealing capacity of the fault zone. A slip model for low-angle normal faults in the Apennines is proposed in which aseismic frictional–viscous creep occurs on a weak, slow-moving (slip rate <1 mm a−1) fault, interspersed with small seismic ruptures caused by cyclic hydrofracturing events. Our findings are potentially applicable to other examples of low-angle normal faults in many tectonic settings.

A low-angle normal fault in the Umbria region (Central Italy): a mechanical model for the related microseismicity

Abstract In the Northern Apennines, the upper crust is thinned by a set of east-dipping low-angle normal faults (LANFs): the easternmost and more recent of these LANFs is the Altotiberina Fault (ATF) located in Northern Umbria. The geometry of the ATF has been reconstructed matching surface geology with seismic reflection profiles and borehole data. The fault, whose average dip is ∼20°, borders the Upper Pliocene–Quaternary Tiber basin and has a displacement of about 8 km. The deeper portion of the ATF is located below the axial zone of the Northern Apennines where the strongest instrumental and historical seismicity is recorded; the microseismicity of the region ( M σ 1 , to evaluate the boundary conditions for the brittle activity of the ATF. The fault can be reactivated for low values of differential stress ( σ 1 − σ 3 T ∼10 MPa), and tensile fluid overpressure P f > σ 3 (e.g. λ v >0.93). In the peculiar situation of the Northern Apennines, the deep emissions of large amounts of CO 2 documented in the area can be entrapped in their ascent by structural seals (e.g. ATF) favouring localised fluid overpressures. The impossibility of sustaining P f > σ 3 for wide fault portions, counteracted by hydraulic fracturing, increased permeability under low effective stress and load weakening behaviour for normal faulting, would prevent the nucleation of moderate ruptures along the fault. The short-lived attainment of P f > σ 3 along small fault portions can account for the microseismic activity located along the ATF, which occurs on rupture surfaces in the range of 10 −1 –10 −3 km 2 .

Development of interconnected talc networks and weakening of continental low-angle normal faults

Fault zones that slip when oriented at large angles to the maximum compressive stress, i.e., weak faults, represent a significant 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 fluid-assisted processes within fault zones cutting Mg-rich carbonate sequences may be widespread, leading to profound and long-term fault weakness.

Thermal decomposition along natural carbonate faults during earthquakes

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 solidified 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 ultrafine-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.

Frictional strength and healing behavior of phyllosilicate‐rich faults

[1] We study the mechanisms of frictional strength recovery for tectonic faults with particular focus on fault gouge that contains phyllosilicate minerals. We report laboratory and microstructural work from fault rocks associated with a regional, low-angle normal fault in Central Italy. Experiments were conducted in a biaxial deformation apparatus at room temperature and humidity, nominally dry, under constant normal stresses of 20 and 50 MPa, and at a sliding velocity of 10 mm/s. Our results for nominally dry conditions show good agreement with previous work conducted under controlled pore fluid pressure. The phyllosilicate contents of our samples, which include clay, talc and chlorite range from 0 to 52 weight %. We study both intact rock samples, sheared in their in situ geometry, and powders made from the same rocks to address the role of fabric in fault healing. We measured frictional healing, Dm, using slide-hold-slide tests with hold periods ranging from 3 to 3000 s. Phyllosilicate-free materials show friction values of m ≈ 0.6 and healing rates that are larger in powdered samples, b ≈ 0.006 (Dm per decade in time, s) compared to intact wafers of fault rock, b ≈ 0.004. For phyllosilicate-bearing materials, healing rates are low, b < 0.002, and independent of fabric, phyllosilicate content and normal stress. We observe that frictional strength decreases systematically with increasing phyllosilicate content. Intact, phyllosilicate-bearing fault rock is consistently weaker than its powdered equivalent (0.2 < m < 0.3 versus 0.4 < m < 0.5, respectively). We compare our data to results from experiments conducted on a wide range of materials and conditions. Deformation microstructures show localized slipping along sub-parallel shear planes. We suggest that low values of frictional strength and near zero healing rates will combine to exacerbate the weakness of phyllosilicate-bearing faults and promote stable, aseismic creep.

The Internal Structure of Fault Zones: Implications for Mechanical and Fluid-Flow Properties

Faults are primary focuses of both fluid migration and deformation in the upper crust. The recognition that faults are typically heterogeneous zones of deformed material, not simple discrete fractures, has fundamental implications for the way geoscientists predict fluid migration in fault zones, as well as leading to new concepts in understanding seismic/aseismic strain accommodation. This book captures current research into understanding the complexities of fault-zone internal structure, and their control on mechanical and fluid-flow properties of the upper crust. A wide variety of approaches are presented, from geological field studies and laboratory analyses of fault-zone and fault-rock properties to numerical fluid-flow modelling, and from seismological data analyses to coupled hydraulic and rheological modelling. The publication aims to illustrate the importance of understanding fault-zone complexity by integrating such diverse approaches, and its impact on the rheological and fluid-flow behaviour of fault zones in different contexts.

Seismic expression of active extensional faults in northern Umbria (Central Italy)

Abstract This paper deals with the geometry and kinematics of the active normal faults in northern Umbria, and their relationship with the seismicity observed in the area. In particular, we illustrate the contribution of seismic reflection data (a network of seismic profiles, NNW–SSE and WSW–ENE trending) in constraining at depth the geometry of the different active fault systems and their reciprocal spatial relationships. The main normal fault in the area is the Alto Tiberina fault, NNW trending and ENE dipping, producing a displacement of about 5 km, and generating a continental basin (Val Tiberina basin), infilled by up to 1500 m with Upper Pliocene–Quaternary deposits. The fault has a staircase trajectory, and can be traced on the seismic profiles to a depth of about 13 km. A set of WSW-dipping, antithetic faults can be recognised on the profiles, the most important of which is the Gubbio fault, bordering an extensional Quaternary basin and interpreted as an active fault based on geological, geomorphologic and seismological evidence. The epicentral distribution of the main historical earthquakes is strictly parallel to the general trend of the normal faults. The focal mechanisms of the major earthquakes show a strong similarity with the attitude of the extensional faults, mapped at the surface and recognised on the seismic profiles. These observations demonstrate the connection between seismicity in the area and the activity of the normal faults. Moreover, the distribution of the instrumental seismicity suggests the activity of the Alto Tiberina fault as the basal detachment for the extensional tectonics of the area. Finally, the action of the Alto Tiberina fault was simulated using two dimensional finite element modelling: a close correspondence between the concentration of shear stresses in the model and the distribution of the present earthquakes was obtained.