Report
Teaser, summary, work performed and final results
Periodic Reporting for period 2 - REALISM (Reproducing EArthquakes in the Laboratory: Imaging, Speed and Mineralogy)
Teaser
The project REALISM (Reproducing Earthquakes in the Laboratory: Imaging, Speed and Mineralogy) proposed a simple idea: to reproduce earthquakes in the laboratory. Indeed, earthquakes being spectacular examples of uncontrollable catastrophes, the opportunity to study them under...
Summary
The project REALISM (Reproducing Earthquakes in the Laboratory: Imaging, Speed and Mineralogy) proposed 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 unique and is, in fact, the only way to understand the details of the 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. We have already shown that both deep and shallow laboratory earthquakes are not mere `analogs’ of earthquakes, but are real – though very small - seismic events obeying the same 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 constrains ubiquitously observed seismological statistical laws (Omori law for foreshocks and aftershocks, Gutenberg-Richter of earthquake magnitude statistics) and produced 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 has already a valuable research asset for the European community, as it 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.
Detailed scientific context:
Meteorologists have both a deep understanding of atmospheric physics and high-quality observations so that they can, nowadays, predict the weather reliably on short time scales, as well as try to predict its long-time trends. In seismology however, although observations have much improved in recent years, the situation is very different. Not knowing exactly when and where an earthquake will happen restricts the possibility of installing instruments in the proper places. Moreover, except near the surface, it is not even possible to make the necessary observations. Nevertheless, the improvement on observations was such in recent years that it revealed how much our understanding of the earthquake source physics was limited. And this took place with the three following, distinct, major, observational revolutions:
• The disastrous Tohoku-oki earthquake, which occurred in Japan on March 11th 2011, was the best recorded truly great earthquake of modern times. This unprecedented record of a Mw 9.0 earthquake and its complexity, which propagated for approximately 200 seconds along the subduction interface over a 400 km long and 70 km wide area, left the community baffled at first, and revealed that our understanding of earthquake mechanics is still rather poor. Indeed, the earthquake propagated partly in a region – seismologists refer to asperities - previously believed to be aseismic. In addition, the coherent short-period seismic wave radiation preferentially emanated from the deeper portion of the fault, whereas the largest fault displacements - up to 50 meters - occurred at shallower depths and generated the tsunami. Thus, what controls the distribution of seismic and aseismic asperities at depth, their relative size and their seismic wave radiation during
Work performed
I. New rock deformation apparatus:
A 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 took a week of effort to assemble. The first successful deformation test under high pressure and temperature conditions was performed early 2019. This new Paterson press is capable of generating confining pressures of up to 400 MPa on large samples (50 mm length and 25 mm diameter). 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 has already a valuable research asset for the European community, as it 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.
II. Performing the full energy budget of laboratory earthquakes:
a. 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.
b. It is possible to observe in the field the fossilized traces of earthquakes that have occurred several kilometers deep, tens of millions of years ago. Micrometric crystals, which formed from the magma upon the rupture, record crucial information unveiling its focal mechanism and its local energy budget.
c. 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.
d. 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.
III. Reproducing deep seismicity in the laboratory:
a. 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. These laboratory analogues of intermediate depth earthquakes demonstrated that little dehydration is required to trigger embrittlement.
b. We also successfully reproduced the deep seismicity under Tibet. Indeed, 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. We demonstrated that mineral reactions lead to brittle deformation in situations where reaction rates are slow compared to the deformation rate.
c. The deepest earthquakes recor
Final results
Progress:
I. A new generation rock deformation apparatus:
A new-generation Paterson press, a gas-medium tri-axial deformation apparatus named after his inventor Mervin Paterson, was installed at ENS in November of 2018. This new state-of-the-art experimental device took a week of effort to assemble. The first successful deformation test under high pressure and temperature conditions was performed early 2019. This new Paterson press is capable of generating confining pressures of up to 400 MPa on large samples (50 mm length and 25 mm diameter). 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 has already a valuable research asset for the European community, as it 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.
II. Performing the full energy budget of laboratory earthquakes:
a. 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.
b. It is possible to observe in the field the fossilized traces of earthquakes that have occurred several kilometers deep, tens of millions of years ago. Micrometric crystals, which formed from the magma upon the rupture, record crucial information unveiling its focal mechanism and its local energy budget.
c. 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.
d. 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.
III. Reproducing deep seismicity in the laboratory:
a. 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. These laboratory analogues of intermediate depth earthquakes demonstrated that little dehydration is required to trigger embrittlement.
b. We also successfully reproduced the deep seismicity under Tibet. Indeed, 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. We demonstrated that mineral reactions lead to brittle deformation in situations where reaction rates are slow compared to the deformation rate.
c. The deepe
Website & more info
More info: http://www.geologie.ens.fr/.