From slow to fast faulting: recent challenges in earthquake fault mechanics

Faults—thin zones of highly localized shear deformation in the Earth—accommodate strain on a momentous range of dimensions (millimetres to hundreds of kilometres for major plate boundaries) and of time intervals (from fractions of seconds during earthquake slip, to years of slow, aseismic slip and millions of years of intermittent activity). Traditionally, brittle faults have been distinguished from shear zones which deform by crystal plasticity (e.g. mylonites). However such brittle/plastic distinction becomes blurred when considering (i) deep earthquakes that happen under conditions of pressure and temperature where minerals are clearly in the plastic deformation regime (a clue for seismologists over several decades) and (ii) the extreme dynamic stress drop occurring during seismic slip acceleration on faults, requiring efficient weakening mechanisms. High strain rates (more than 104 s−1) are accommodated within paper-thin layers (principal slip zone), where co-seismic frictional heating triggers non-brittle weakening mechanisms. In addition, (iii) pervasive off-fault damage is observed, introducing energy sinks which are not accounted for by traditional frictional models. These observations challenge our traditional understanding of friction (rate-and-state laws), anelastic deformation (creep and flow of crystalline materials) and the scientific consensus on fault operation. This article is part of the themed issue ‘Faulting, friction and weakening: from slow to fast motion’

Rupture by damage accumulation in rocks

The deformation of rocks is associated with microcracks nucleation and propagation, i.e. damage. The accumulation of damage and its spatial localization lead to the creation of a macroscale discontinuity, so-called "fault" in geological terms, and to the failure of the material, i.e. a dramatic decrease of the mechanical properties as strength and modulus. The damage process can be studied both statically by direct observation of thin sections and dynamically by recording acoustic waves emitted by crack propagation (acoustic emission). Here we first review such observations concerning geological objects over scales ranging from the laboratory sample scale (dm) to seismically active faults (km), including cliffs and rock masses (Dm, hm). These observations reveal complex patterns in both space (fractal properties of damage structures as roughness and gouge), time (clustering, particular trends when the failure approaches) and energy domains (power-law distributions of energy release bursts). We use a numerical model based on progressive damage within an elastic interaction framework which allows us to simulate these observations. This study shows that the failure in rocks can be the result of damage accumulation

Comment on "Alpine thermal and structural evolution of the highest external crystalline massif: The Mont Blanc'' by P. H. Leloup, N. Arnaud, E. R. Sobel, and R. Lacassin

In this comment we discuss the approach used and the significance of Ar-Ar dating of synkinematic phengite within low-grade Alpine shear zones, and we comment the geodynamic models that can be derived from this method. The paper by Leloup et al. [2005] is a good step forward in the tectonic comprehension of the Mont Blanc area and provides a good synthesis of preexisting data. Leloup et al. [2005] have proposed a polyphase Alpine history for the Mont Blanc Massif (west Alps) based on a multidisciplinary approach: Ar-Ar on biotite for the higher pressure-temperature events of the Mont Blanc, and Ar-Ar on K-feldspar, fission tracks (FT) on zircon and apatite for its later exhumation stages. However, at this point of our knowledge of Alpine deformation in the Mont Blanc Range, the polyphased tectonic evolution, in particular the timing of thrust and back thrust events are not in agreement with recently obtained Ar-Ar data

Flash vaporization during earthquakes evidenced by gold deposits

Much of the world's known gold has been derived from arrays of quartz veins. The veins formed during periods of mountain building that occurred as long as 3 billion years ago, and were deposited by very large volumes of water that flowed along deep, seismically active faults. The veins formed under fluctuating pressures during earthquakes, but the magnitude of the pressure fluctuations and their influence on mineral deposition is not known. Here we use a simple thermo-mechanical piston model to calculate the drop in fluid pressure experienced by a fluid-filled fault cavity during an earthquake. The geometry of the model is constrained using measurements of typical fault jogs, such as those preserved in the Revenge gold deposit in Western Australia, and other gold deposits around the world. We find that cavity expansion generates extreme reductions in pressure that cause the fluid that is trapped in the jog to expand to a very low-density vapour. Such flash vaporization of the fluid results in the rapid co-deposition of silica with a range of trace elements to form gold-enriched quartz veins. Flash vaporization continues as more fluid flows towards the newly expanded cavity, until the pressure in the cavity eventually recovers to ambient conditions. Multiple earthquakes progressively build economic-grade gold deposits