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A review of about 300 published and unpublished rock friction experiments that reproduce seismic slip conditions suggests that a significant decrease in friction occurs at high slip rate.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 Here we review a large set of published and unpublished experiments (∼300) performed in rotary shear apparatus at slip rates of 0.1–2.6 m s−1.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

https://hal.archives-ouvertes.fr/hal-01802145/document

Modeling and simulation in tribology across scales: An overview

Book Review: The techniques of modern structural geology. Volume 2: Folds and fractures. Ramsay, J. G. and Huber, M. I. 1987. Academic Press, London. 381 pp. Price: £17.50, $34.50

One of the ultimate goals in landslide hazard assessment is to predict maximum landslide extension and velocity. Despite much work, the physical processes governing energy dissipation during these natural granular flows remain uncertain. Field observations show that large landslides travel over unexpectedly long distances, suggesting low dissipation. Numerical simulations of landslides require a small friction coefficient to reproduce the extension of their deposits. Here, based on analytical and numerical solutions for granular flows constrained by remote-sensing observations, we develop a consistent method to estimate the effective friction coefficient of landslides. This method uses a constant basal friction coefficient that reproduces the first-order landslide properties. We show that friction decreases with increasing volume or, more fundamentally, with increasing sliding velocity. Inspired by frictional weakening mechanisms thought to operate during earthquakes, we propose an empirical velocity-weakening friction law under a unifying phenomenological framework applicable to small and large landslides observed on Earth and beyond.

/pdf/frictional-velocity-weakening-in-landslides-on-earth-and-on-578cr1d17g.pdf

Frictional velocity-weakening in landslides on Earth and on other planetary bodies

The development and behaviour of low-angle normal faults during Cenozoic asymmetric extension in the Northern Apennines, Italy

[1] Recently, in the Northern Apennines, geophysical data have identified the Triassic Evaporites (TE, anhydrites and dolomites) as the source region of the major extensional earthquakes of the area (M ∼ 6). In order to characterize fault zone architecture and deformation processes within the TE, we have studied exhumed evaporite-bearing normal faults within the upper crust. The structure of large displacement (>100 m) normal faults is given by 1) a zoned fault core with a wider portion of fault-parallel foliated Ca-sulphates (ductile deformation), overprinted by an inner fault core (IFC) of localized brittle deformation, and 2) wide (dolostones) to absent (Ca-sulphates) damage zones of fault fracture patterns. Fault rock assemblage within the IFC is characterized by fault breccia, gouge, and cataclasites of different grain size. Most of the deformation within the IFC is localized along thin and fault parallel principal slip surfaces (PSS) made of dolomite-rich fine-grained cataclasite. SEM analyses show an evolution from Ca- to St- to gypsum-rich mineralization, due to episodic fluid flow events channeled along the fault zones during different stages of fault exhumation. The development of the observed fault geometry can be explained by a mechanical fault evolution model where initial faulting occurs along broad and ductile shear zones within the anhydrites and causes fracturing within the dolostones. Progressive deformation within the fault core leads to the development of fault parallel dolomite-rich cataclastic layers. Their reactivation coupled with transient fluid overpressures can produce embrittlement and localization of brittle deformation within the IFC.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2007TC002230

N. De Paola

Papers

Fault lubrication during earthquakes

The development and behaviour of low-angle normal faults during Cenozoic asymmetric extension in the Northern Apennines, Italy

Fault zone architecture and deformation processes within evaporitic rocks in the upper crust

Fault Lubrication and Earthquake Propagation in Thermally Unstable Rocks

Partitioned transtension: an alternative to basin inversion models