The project REALISM (Reproducing Earthquakes in the Laboratory: Imaging, Speed and Mineralogy) proposes a simple idea: to reproduce earthquakes in the laboratory. Indeed, earthquakes being spectacular examples of uncontrollable catastrophes, the opportunity to study them under controlled conditions in the laboratory is a unique opportunity to further our understanding of earthquake source physics.
The aim of the project is interdisciplinary, at the frontiers between Rock Fracture Mechanics, Seismology, and Mineralogy. Its ultimate goal is to improve, on the basis of integrated experimental data, our understanding of the earthquake source physics. During laboratory earthquakes, by measuring all of the physical quantities related to the rupturing process, we are unravelling what controls the rupture speed, rupture arrest, the earthquake rupture energy budget, as well as the common role played by mineralogy. Our work aims at constraining seismological statistical laws (Omori law for foreshocks and aftershocks, Gutenberg-Richter of earthquake magnitude statistics) and producing an unprecedented data set on rock fracture dynamics at in-situ conditions. In the future, our work may also provide insights for earthquake hazard mitigation or opportunities to test seismic slip inversion and dynamic rupture modelling techniques.
The new experimental infrastructure we have installed at the Laboratoire de Géologie of ENS Paris is unique in the world, in such a way it can reproduce on relatively large rock samples the pressure, temperature and stress conditions of depths where earthquakes occur in the crust as well as in the upper mantle of the Earth, with never achieved spatio-temporal imaging resolution to this day. This will eventually open the door to a better understanding of all the processes happening under stress within the first hundreds of kilometres of the Earth.
The research group involves three PhD. students, three post docs, a research engineer, and three permanent researchers as well as associated researchers and visiting scientists.
Dr. Julien Gasc, initially hired for two years on 01/02/2017 and now on a permanent position, is in charge of the developments in the laboratory and in particular of the new generation Paterson apparatus. Julien also works on the mechanics of deep earthquakes and mantle minerals, sometimes using fancy equipment at the synchrotron.
Over the last two years and a half, a number of visiting scientists were hosted at ENS. They have constituted a pool of international experts from which the project has greatly benefitted. Amongst these: Timm John (Freie Universitat Berlin), Pamela Burnley (University of Nevada in Las Vegas), Eiichi Fukuyama (National Insitute for Earthquake Disasters Prevention), Paul Johnson (Los Alamos National Laboratories), Takahiro Hatano (Earthquake Research Institute, University of Tokyo), Yehuda Ben-Zion (University of South California), Shamita Das (University of Oxford).
A form of new-generation Paterson press, a gas-medium tri-axial deformation apparatus named after his inventor Mervin Paterson, was installed at the ENS in November of 2018. This new state-of-the-art experimental device weighs several tons and took a week of effort to assemble. The first successful deformation test under high pressure and temperature conditions was performed early 2019. The original Paterson press is capable of generating confining pressures of up to 400 MPa and axial loads up to 100 kN on cylindrical samples with typical diameters of ~10 mm and lengths of ~20 mm. The new-generation used here was designed to host much larger samples (50 mm length and 25 mm diameter) but reach similar stresses (up to several GPa’s), implying much greater axial loads. Unlike the original Paterson, the vessel of the one installed at ENS is therefore framed in a structure rated for 1000 kN. The vessel was tested at confining pressures up to 700 MPa and certified for routine experiments at pressures (using Ar gas) of 400 MPa. The current temperature capability is 650 °C.
The first tests revealed very fine control upon displacement and strain rate during the experiment (monitored and recorded at 1 Hz). A first deformation experiment was carried out on Carrara marble at 400 °C, 100 MPa and strain rates ranging from ~3 × 10–5-2 × 10–4 s–1. The sample was shortened by ~15 %, until failure was observed.
The seismic efficiency of an earthquake is a measure of the fraction of the energy that is radiated away into the host medium. We estimated the first complete energy budget of an earthquake and show that increasing heat dissipation on the fault increases the radiation efficiency. We develop a novel method to illuminate areas of the fault that get excessively heated up. We finally introduced the concept of spontaneously developing heat asperities, playing a major role in the radiation of seismic waves during an earthquake.
Using acoustic recordings high-pass filtered, we applied at the laboratory scale an observation technique in seismology called “back-projection” to image, on the fault, zones of high-frequency energy release during rupture propagation. Our results showed that the high-frequency radiation originates behind the rupture front during propagation and propagates at a speed close to that obtained by our rupture velocity inversion. From scaling arguments, we suggested that the origin of high-frequency radiation lies in the fast dynamic stress-drop in the breakdown zone together with off-fault co-seismic damage propagating behind the rupture tip. The systematic application of the back-projection method at the laboratory scale provides new ways to locally investigate physical mechanisms that control high-frequency radiation.
A major part of the seismicity striking the Mediterranean area and other regions worldwide is hosted in carbonate rocks. Recent examples are the destructive earthquakes of L’Aquila 6.5 2016 in Central Italy. Surprisingly, within this region, fast (≈3km/s) and destructive seismic ruptures coexist with slow (≤10 m/s) and non- destructive rupture phenomena. We reproduced in the laboratory the complete spectrum of natural faulting on samples of dolostones representative of the seismogenic layer in the region. The transitions from fault creep to slow ruptures and from slow to fast ruptures, occurred at conditions encountered at 3-5 km depth (i.e., P = 100 MPa and T = 100°C) and is explained by the activation of flash weakening at asperity contacts which induce the propagation of fast ruptures radiating intense high frequency seismic waves.
Dry faults weaken due to degradation of fault asperities by frictional heating (e.g. flash heating). In the presence of fluids, theoretical models predict faults to weaken by thermal pressurization of fault fluid. However, experimental evidence of rock/fluid interactions during dynamic rupture under realistic stress conditions remained poorly documented. We demonstrated that the relative contribution of thermal pressurization and flash heating to fault weakening depends on fluid thermodynamic properties because water’s liquid–supercritical phase transition buffers frictional heat. The heat buffer effect has maximum efficiency at mid-crustal depths (~2–5 km), where many anthropogenic earthquakes nucleate.
We deciphered the mechanism of intermediate-depth earthquakes (30–300 km) earthquakes by performing deformation experiments on dehydrating serpentinized peridotites (synthetic antigorite-olivine aggregates, minerals representative of subduction zones lithologies) at upper mantle conditions. Experimentally produced faults, observed post-mortem, were sealed by fluid-bearing micro-pseudotachylytes. Microstructural observations demonstrated that antigorite dehydration triggered dynamic shear failure of the olivine load-bearing network. These laboratory analogues of intermediate depth earthquakes demonstrated that little dehydration is required to trigger embrittlement. We proposed an alternative model to dehydration-embrittlement in which dehydration-driven stress transfer, rather than fluid overpressure, causes embrittlement.
Southern Tibet is the most active orogenic region on Earth where the Indian Plate thrusts under Eurasia, pushing the seismic discontinuity between the crust and the Earth’s mantle to an unusual depth of ~80 km. Numerous earthquakes occur in the lower portion of this thickened continental crust, but their triggering mechanisms remain enigmatic. Both field observations and geophysical data reveal a causal link between brittle seismic failure and eclogitization, a set of mineral phase transformations, of the lower continental crust. We demonstrate that mineral reactions lead to brittle deformation in situations where reaction rates are slow compared to the deformation rate. This work has been published in Geology in 2019.