Periodic Reporting for period 4 - REALISM (Reproducing EArthquakes in the Laboratory: Imaging, Speed and Mineralogy) Reporting period: 2021-04-01 to 2022-09-30 Summary of the context and overall objectives of the project 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. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far 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). 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. 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 recorded occur in subducting slabs at depths corresponding to the base of the upper mantle (i.e. 400-660 km), in the so-called transition zone. Their origin has long been debated, mainly because the high PT conditions in which they occur do not allow standard fracture mechanics to explain their nucleation. To provide insights into the physics of DFEs and better constrain the conditions needed to obtain transformational faulting, a series of deformation experiments was performed with the GRAAL apparatus, a new-generation Griggs-type device equipped with Acoustic Emissions (AEs) recording capability. IV. A research group involving three PhD students, three post docs, a research engineer, three permanent researchers as well as associated researchers and visiting scientists from collaborating groups has been established.V. Outreach: The PI, some post-docs and PhDs were involved in outreach towards the greater public.a. A book of fine art photography (The Fault, by Grégoire Eloy, RVB Books, 2017) was produced after the field study performed by Dr. Ferrand in Northern Italy. b. A second book of fine art photography, possibly produced by Xavier Barral editions, the leading fine art photography editor in France (Editor, in collaboration with ESA and NASA of a the book on Mars landscapes) is currently in production to document electronic microscope photography of laboratory earthquakes microstructures and landscapes. This work is performed using the scientific images produced by M. Jérôme Aubry during his PhD. in collaboration with a renowned photographer, M. Thomas Gizolme.c. Numerous sound artists have contacted the PI to provide original recordings of laboratory earthquakes. A digital sound library is currently being produced. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) Detailed and numbered expected scientific results:1- A better understanding of rupture nucleation and acceleration, providing insights for earthquake hazard mitigation; 2- Constrain the relationships between in-situ rupture processes, rupture and sliding velocities on one hand, and frictional weakening, stress drop and roughness, on the other hand. 3- Constrain statistical laws (Omori, Gutenberg-Richter) power-law exponents to various loading scenarios. 4- Obtain unprecedented data sets on rock fracture dynamics at in-situ conditions to test seismic slip inversion and dynamic rupture modeling techniques.5- A better understanding of the mechanical conditions and processes that drive the rupture speed, and the transitions from slow, to sub-Rayleigh, to supershear; Figures showing the New Generation Rock Deformation apparatus and the first successful experiment
