A review of recent developments concerning the structure, mechanics and fluid flow properties of fault zones

A review of about 300 published and unpublished rock friction experiments that reproduce seismic slip conditions suggests that a significant decrease in friction occurs at high slip rate. Extrapolating the experimental data to conditions that are typical of earthquake nucleation depths, the authors conclude that faults are lubricated during earthquakes, irrespective of the fault rock composition or specific weakening mechanism involved. This study reviews a large set of fault friction experiments and finds that a significant decrease in friction occurs at high slip rate. Extrapolating the experimental data to conditions typical of earthquake nucleation depths, it is concluded that faults are lubricated during earthquakes, irrespective of the fault rock composition or specific weakening mechanism involved. The determination of rock friction at seismic slip rates (about 1 m s−1) is of paramount importance in earthquake mechanics, as fault friction controls the stress drop, the mechanical work and the frictional heat generated during slip1. Given the difficulty in determining friction by seismological methods1, elucidating constraints are derived from experimental studies2,3,4,5,6,7,8,9. Here we review a large set of published and unpublished experiments (∼300) performed in rotary shear apparatus at slip rates of 0.1–2.6 m s−1. The experiments indicate a significant decrease in friction (of up to one order of magnitude), which we term fault lubrication, both for cohesive (silicate-built4,5,6, quartz-built3 and carbonate-built7,8) rocks and non-cohesive rocks (clay-rich9, anhydrite, gypsum and dolomite10 gouges) typical of crustal seismogenic sources. The available mechanical work and the associated temperature rise in the slipping zone trigger11,12 a number of physicochemical processes (gelification, decarbonation and dehydration reactions, melting and so on) whose products are responsible for fault lubrication. The similarity between (1) experimental and natural fault products and (2) mechanical work measures resulting from these laboratory experiments and seismological estimates13,14 suggests that it is reasonable to extrapolate experimental data to conditions typical of earthquake nucleation depths (7–15 km). It seems that faults are lubricated during earthquakes, irrespective of the fault rock composition and of the specific weakening mechanism involved.

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Fault lubrication during earthquakes

It is increasingly apparent that faults are typically not discrete planes but zones of deformed rock with a complex internal structure and three-dimensional geometry. In the last decade this has led to renewed interest in the consequences of this complexity for modelling the impact of fault zones on fluid flow and mechanical behaviour of the Earth's crust. A number of processes operate during the development of fault zones, both internally and in the sur- rounding host rock, which may encourage or inhibit continuing fault zone growth. The complexity of the evolution of a faulted system requires changes in the rheological properties of both the fault zone and the surrounding host rock volume, both of which impact on how the fault zone evolves with increasing displacement. Models of the permeability structure of fault zones emphasize the presence of two types of fault rock components: fractured conduits parallel to the fault and gran- ular core zone barriers to flow. New data presented in this paper on porosity- permeability relationships of fault rocks during laboratory deformation tests support recently advancing con- cepts which have extended these models to show that poro-mechanical approaches (e.g., critical state soil mechanics, fracture dilatancy) may be applied to predict the fluid flow behaviour of complex fault zones during the active life of the fault. Predicting the three-dimensional heterogen- eity of fault zone internal structure is important in the hydrocarbon industry for evaluating the retention capacity of faults in exploration contexts and the hydraulic behaviour in production contexts. Across-fault reservoir juxtaposition or non-juxtaposition, a key property in predicting retention or across-fault leakage, is strongly controlled by the three-dimensional complexity of the fault zone. Although algorithms such as shale gouge ratio greatly help predict capillary threshold pressures, quantification of the statistical variation in fault zone composition will allow estimations of uncertainty in fault retention capacity and hence prospect reserve estimations. Permeability structure in the fault zone is an important issue because bulk fluid flow rates through or along a fault zone are dependent on permeability variations, anisotropy and tortuosity of flow paths. A possible way forward is to compare numerical flow models using statistical variations of permeability in a complex fault zone in a given sandstone/shale context with field-scale estimates of fault zone permeability. Fault zone internal structure is equally important in understanding the seismogenic behaviour of faults. Both geometric and compositional complexities can control the nucleation, propagation and arrest of earthquakes. The presence and complex distribution of different fault zone materials of contrasting velocity-weakening and velocity-strengthening prop- erties is an important factor in controlling earthquake nucleation and whether a fault slips seismo- genically or creeps steadily, as illustrated by recent studies of the San Andreas Fault. A synthesis of laboratory experiments presented in this paper shows that fault zone materials which become stronger with increasing slip rate, typically then get weaker as slip rate continues to increase to seismogenic slip rates. Thus the probability that a nucleating rupture can propagate sufficiently to generate a large earthquake depends upon its success in propagating fast enough through these materials in order to give them the required velocity kick. This propagation success is hence controlled by the relative and absolute size distributions of velocity-weakening and vel- ocity-strengthening rocks within the fault zone. Statistical characterisation of the distribution of such contrasting properties within complex fault zones may allow for better predictive models of rupture propagation in the future and provide an additional approach to earthquake size fore- casting and early warnings.

Recent advances in the understanding of fault zone internal structure: a review

Field emission scanning electron microscopy (FESEM), two-dimensional X-ray diffraction (2D-XRD), and transmission electron microscopy coupled with selected area electron diffraction (TEM-SAED) analyses of the reactant/product textural relationship show that the thermal decomposition of Iceland spar single crystals according to the reaction CaCO3(s) → CaO(s) + CO2(g) is pseudomorphic and topotactic. This reaction begins with the formation of a mesoporous structure made up of up to four sets of oriented rod-shaped CaO nanocrystals on each rhombohedral cleavage face of the calcite pseudomorph. The four sets formed on (1014)calcite display the following topotactic relationships: (1) (1210)calcite//(110)CaO; (2) (1104)calcite⊥ (110)CaO; (3) (1018)calcite//(110)CaO; and (4) (0114)calcite⊥(110)CaO; with [841]calcite//[110]CaO in all four cases. At this stage, the reaction mechanism is independent of P CO2 (i.e., air or high vacuum). Strain accumulation leads to the collapse of the mesoporous structure, resulting in the oriented aggregation of metastable CaO nanocrystals (~5 nm in thickness) that form crystal bundles up to ~1 μm in cross-section. Finally, sintering progresses up to the maximum T reached (1150 °C). Oriented aggregation and sintering (plus associated shrinking) reduce surface area and porosity (from 79.2 to 0.6 m2/g and from 53 to 47%, respectively) by loss of mesopores and growth of micrometer-sized pores. An isoconversional kinetic analysis of non-isothermal thermogravimetric data of the decomposition of calcite in air yields an overall effective activation energy E α = 176 ± 9 kJ/mol (for α > 0.2), a value which approaches the equilibrium enthalpy for calcite thermal decomposition (177.8 kJ/mol). The overall good kinetic fit with the F1 model (chemical reaction, first order) is in agreement with a homogeneous transformation. These analytical and kinetic results enable us to propose a novel model for the thermal decomposition of calcite that explains how decarbonation occurs at the atomic scale via a topotactic mechanism, which is independent of the experimental conditions. This new mechanistic model may help reinterpret previous results on the calcite/CaO transformation, having important geological and technological implications.

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Thermal decomposition of calcite: Mechanisms of formation and textural evolution of CaO nanocrystals

Textbooks and popular science books that describe the broad field of space plasma physics are increasingly important as the space physics community seeks to broaden its base of support among both scientists and the public. Introduction to Space Physics is such a text; it provides a survey of space physics observation and theory meant for the upper division or graduate level student who is well-versed in standard classical mechanics and electromagnetism but has not been exposed to the variety of phenomena that occur in space. The text is based on a series of lectures given at UCLA in 1990 by experts in areas ranging from the magnetohydrodynamic (MHD) processes on the Sun and in the solar wind to the solar wind interactions with both magnetized and unmagnetized planets, ULF and plasma waves, and the collisional and chemical processes in planetary ionospheres.

Introduction to Space Physics

High-velocity weakening of faults may drive fault motion during large earthquakes. Experiments on simulated faults in Carrara marble at slip rates up to 1.3 meters per second demonstrate that thermal decomposition of calcite due to frictional heating induces pronounced fault weakening with steady-state friction coefficients as low as 0.06. Decomposition produces particles of tens of nanometers in size, and the ultralow friction appears to be associated with the flash heating on an ultrafine decomposition product. Thus, thermal decomposition may be an important process for the dynamic weakening of faults.

https://science.sciencemag.org/content/sci/316/5826/878.full.pdf

Ultralow Friction of Carbonate Faults Caused by Thermal Decomposition

Supporting Online Material for Ultralow Friction of Carbonate Faults Caused by Thermal Decomposition

Dynamic weakening of fault gouge affected by thermal conductivity of host specimen: implications for the high-velocity weakening mechanisms

How rapidly fault strength recovers after an earthquake is an importantquestion for understanding the earthquake generation mechanism in seismic cycles.Here we show in laboratory experiments where a fault weakened dynamically at sub-seismic slip rates (∼85 mm=sec) recovers its frictional strength logarithmically withtime and the healing rate of 0.2–0.3, one order of magnitude greater than those inprevious studies. The fault can completely recover its frictional strength to presliplevel within one day. We suggest that immediately after an earthquake a slipped faultsurface can regain its potential to trigger the next earthquake, which might haveimportant implications for forecasting future large earthquakes.Introduction After an earthquake,a fault starts to recover its frictionalstrength and to store tectonic stress for the next earthquake.The healing of faults is crucial for a better understanding ofthe seismic cycle and the physics of earthquake faulting,especiallyforevaluatingwhetherafaulthasenoughpotentialfor the next earthquake. Previous friction experiments onbare rocks and gouge materials at slow slip rates have shownthat friction increases logarithmically with time and theincrease rate depends on temperature, slip rate, and normalstress (Dieterich, 1972; Beeler et al., 1994; Karner et al.,1997; Karner and Marone, 1998; Marone, 1998). This in-crease in friction is interpreted as a result of time-dependentgrowth of real asperity contact area under load between twoopposite rock walls and between grain particles comprisinggouge materials. These laboratory data are consistent withseismological estimates of fault healing rates obtained fromthe increase in stress drop with time observed for repeatingmicroearthquakes (Marone et al., 1995). However, it hasremained an open question whether data on the healing rateobtained from slow-slip-rate experiments (<10

Toshihiko Shimamoto

Papers

Ultralow Friction of Carbonate Faults Caused by Thermal Decomposition

Supporting Online Material for Ultralow Friction of Carbonate Faults Caused by Thermal Decomposition

Dynamic weakening of fault gouge affected by thermal conductivity of host specimen: implications for the high-velocity weakening mechanisms

Short Note Fault Heals Rapidly after Dynamic Weakening