Marie Violay, EPFL
Frederica Paglialunga, EPFL

Natural and human Induced Fluid Earthquakes (FIEs) have been observed and recorded for decades. These events can be responsible for significant human, economical and infrastructure damage. FIEs result from the interaction between fluid pressure perturbations, in-situ stresses, frictional and rupture processes at micro to macro scales, and the geometric complexity of the fault zone. Methods for risk assessment and forecasting (in terms of time, location and magnitude) of FIEs require a sound physical basis. However, much of the primary parameters controlling FIE dynamics cannot be measured by geophysical methods. Thus, to establish new general constitutive physical FIE laws, the temporal- and spatial-scale dependence of FIEs should first be properly investigated in the laboratory. here we studied the influence of viscous lubricant in the nucleation and propagation of spontaneous frictional ruptures. We adopted a multi-scale experimental approach.

• cm-scale friction experiments on rock sample to study the effect of fluid viscosity on earthquake nucleation and propagation under crustal deformation conditions (pressure, temperature, presence of fluids, stress perturbations, etc.). Experiments were performed (on Granite at normal effective stress up to 20 MPa (drained conditions), at sliding velocity ranging between 10 μm·s^(-1) and 1 m·s^(-1) and under both dry and viscous fluid conditions. Four different fluid viscosities were tested: distilled water (η ~1 mPa⋅s) and three mixtures of water and glycerol with concentrations of 40/60 wt%, 15/85 wt% and 1/99 wt% (i.e. η=10.9,108.0 and 1226.6 mPa·s respectively).  We show that both static and dynamic friction coefficients decrease with viscosity and that dynamic friction depends on the dimensionless Sommerfeld number (S) as predicted by the elastohydrodynamic-lubrication theory (EHD). Under favourable conditions (depending on the fluid viscosity, co-seismic slip-rate, fault geometry and earthquake nucleation depth) EHD might be an effective weakening mechanism during natural and induced earthquakes.
• m-scale dynamic rupture experiments on an experimental fault filled by viscous fluid to provide new insight into the influence of multi-scale dynamic weakening processes on the nucleation and propagation of FIE ruptures. Spontaneous frictional ruptures were reproduced along artificial interfaces by putting into contact two polymethyl methacrylate (PMMA) samples in a biaxial apparatus. The normal load was kept constant during the whole duration of the experiment while shear load was applied until the fault exhibited instabilities. The nucleation and the propagation of dynamic rupture phenomena were recorded using a high-speed camera coupled with a photo-elasticity set-up. To capture the details of dynamic ruptures, the fault was equipped with an array of 16 one-direction strain gauges. This system allows a maximum bandwidth frequency of 500 kHz, allowing the complete capture of the dynamic of the rupture front. We show that the rupture velocity decreases with viscosity while the nucleation length increases with viscosity. We also show that rupture transitions from crack like rupture to pulse like rupture with increasing fluid viscosity.

Our results can provide fundamental insight into both natural earthquakes and tectonic processes as well as further aid scientists and engineers to better understand, and one day manage, induced seismicity, an increasingly relevant topic in geoengineering both globally and in Switzerland.

Short biography
Marie Violay completed her PhD at the Geoscience faculty of the Montpellier University in 2011. She then was a research assistant at National Institute of Geophysics and Volcanology in Rome and at ETH Zurich. In 2015 she was appointed Assistant Professor and head of the Laboratory of Experimental Rock Mechanics (LEMR) at EPFL and was awarded one of the seven Energy grant of the SNSF. In 2017, Violay was awarded the ERC Starting Grant in the area of Earth System Science. She got promoted to associate professor at EPFL in 2022.

The focus of Violay’s research is to better understand the mechanical and physical processes in the first 15 kilometers of the earth’s crust. She brings better understanding on how fluids and rocks interact at these depths, which is crucial for the development of deep geothermal energy production. Understanding earthquake nucleation and propagation are other focuses of her work. She has developed new approaches combining experimental deformation, microstructural studies of the micro-scale processes, and modelling of these processes for the study of earthquakes and geological reservoirs.