Following the Stribeck model proposed by Canudas de Wit et al. The second and the third right-hand terms of Eq. In fact, this relation holds for other silicate-bearing rocks gabbro, peridotite, etc. As was the case for the experiments performed on Westerly Granite, the sheared samples had a surface roughness very similar to the initial nonsheared one. We now briefly discuss to what degree the lubrication processes measured here may promote rupture propagation and earthquake slip in a faulting scenario.
Therefore, lubrication affects two competing mechanisms and does not necessarily promote dynamic earthquake rupture propagation. Fracture energy and D c vs. Experiments were performed at acceleration of 6. As seen in the left-hand term, the effectiveness of lubrication increases with the amount of fault slip U. A large value of G c may prevent the dynamic propagation of smaller earthquakes, but as U increases the lubrication effect will become dominant. As a result, the viscosity of fluids in the upper crust is estimated to range from 0. The L parameter in S under EHD conditions corresponds to the longest and dominant wavelength in the mismatch between two rough sliding surfaces.
In natural faults, L corresponds to the asperity wavelength, the longest of which is proportional to slip 4 and the magnitude during a given earthquake.
Mechanical behaviour of fluid-lubricated faults | Nature Communications
This geological evidence support the hypothesis that EHD can operate in some natural earthquakes. In particular, highly viscous fluids 1 slightly reduce the static friction coefficient fostering fault reactivation and 2 trigger EHD lubrication during seismic slip. In contrast, the presence of increasingly viscous fluids decreases the fracture energy dissipated for large events, making the fault weaker.
Therefore, our results suggest that in the presence of highly viscous fluids, rupture is expected to grow quasi-statically on the fault until the slip of the order of tens of cm is attained until the distance D c is overcome. While the effect of fluid pressure in earthquake rupture has been previously explored mostly in terms of thermal pressurisation and effective stress, we argue that the role of fluid viscosity is also important in understanding the dynamics of a lubricated fault system, with implications for rupture energy budget and dynamic weakening of both natural and man-induced earthquakes.
The experiments were performed with SHIVA slow to high velocity apparatus 21 , a rotary shear machines installed in Rome, on full cylinder samples of Westerly granite and Carrara marble. Slip, slip-rate and shear stress were determined using the method outlined in Niemeijer et al. In the figure, the apparent friction coefficient is fitted following the exponential decay function proposed by Mizoguchi et al. When the Sommerfeld number S was higher than 1, the RMS of the samples at the end of the experiments was of the same order of magnitude of the initial RMS.
In the experiments performed at high slip-rate the presence of high-viscous lubricants prevented the formation of melt and debris on the surfaces of Westerly Granite and Carrara Marble samples. The LuGre dynamic friction model was proposed by Canudas de Wit et al. The friction generated from the bending of the bristles is. According to the model, in mixed and hydrodynamic regimes, the friction force for constant slip-rate at steady state is given by:. The characteristic slip-rate of the Stribeck function v s determines how quickly g v approaches F c and depends on fluid viscosity and loading conditions.
The increase of temperature in the slip zone and wall rocks of the sheared bulk samples during frictional sliding weas estimated using a 2D FEA time dependent model for heat diffusion. The model reproduced the sizes of the experimental sample, i. Two different materials were used to simulate the slip zone and the bulk material. A Neumann boundary condition was applied to the bottom external edge of the model i. At the inner boundary between the slip zone and the wall-rock, the continuity of the solution was granted.
The thermal diffusivity of the slip zone:.
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Therefore we obtain:. Correspondence and request for additional material should be addressed to chiara.
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Influence of fault strength on precurosy processes during laboratory earthquakes.
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