Deep Earthquakes : from the Laboratory to the Field – DELF
Deep Earthquake: from the Laboratory to the Field (DELF)
Deep earthquakes cannot be mechanically explained like shallow earthquakes and it has been proposed that at least part of the subducting slab seismic activity could be triggered by phase transformations and mineral reactions. However, the way mineral reactions can modify the deformation regime of deep rocks, from ductile to brittle (embrittlement) is poorly understood and remains one of the outstanding unsolved problems of geophysics and rock mechanics
Origin of deep earth seismicity: an experirmental frontier
Seismological, geophysical, field and experimental observations all seem to point towards the same direction: a high pressure (>GPa) high temperature (>500°C) domain exists where although deformation should be ductile, the arousal of phase transitions triggers mechanical instabilities which favor dynamic fracture propagation and earthquake generation. It seems thus necessary for experimentalists to build the right experimental tools in order to attain the s -P-T (deviatoric stress - pressure – temperature) conditions to characterize the mechanics of a certain number of phase transitions. This should be performed in a systematic manner, by investigating the full s-P-T field. This research project proposes to tackle this issue by studying experimentally, under non-hydrostatic stress conditions, the following reactions:<br />1. Serpentine dehydration<br />2. Eclogitization <br />3. Olivine – Spinel <br /> Our goal will be to experimentally describe the exact s-P-T envelopes of the phase transitions by combining the rock mechanics approach (plastic yield envelopes in the s-P domain) with the thermodynamics and petrology approach (PT plots of mineral stability field). We will also try to quantify the respective role of strain rate and water activity on the transformation kinetics. Within the s-P-T field prone to instabilities, we will address some additional general questions in order to understand the nature of the mechanical instability:<br />- What are the respective roles of the reaction enthalpy ?H and the volume variation ?V?<br />- What is the grain size of reaction products? Do they behave superplastically?<br />- What are the respective roles played by the deviatoric stress and the strain rate on the phase transition kinetics?<br />- What is the role of water, both in terms of mechanics and reaction kinetics?<br />- Are the AEs observed true indicators of shear fracture? What are their statistics, their focal mechanisms?<br />
The project is subdivided into one technical task (Task 2) consisting in building a new type of rock deformation apparatus, three thematic tasks (Tasks 3, 4 and 5) each tightly focused on the three phase transformations mentioned above and a mechanical modeling and seismological data integration task 6. Beside experiments, Task 3 and 4 will involve field works subtasks, as we will need to perform sampling and also believe it is crucial to compare our experimental results to natural microstructures and textures. Obviously, task 5 cannot involve any fieldwork, but it will include mounting an acoustic set-up on the DDIA 30 in Chicago in order to reach pressures of the order of 15 GPa on natural olivine samples large enough to perform acoustic monitoring. The preliminary phases of task 5 will consist in performing experiments at lower pressure on the germanate olivine. Task 6 is instrumental in order to integrate our data and upscale it, by performing mechanical modelling and looking at real earthquake catalogs and tomographic maps.
Two Ph-D students, following a Master 2 training period, will be hired over the course of the project. Ph-D and Master students will be involved in Tasks 3-6. The first PhD. project will be focussed on experimental work on natural samples (Serpentine and Eclogite) and field work, while the second one will be focussed on studying single phase transformations (Qz-Coes, Ol-Sp) and modelling.
Four field trips will be organized during these 4 years. We will begin the project by a joint fieldtrip in the Voltri area. All group members will meet together once a year for internal progress workshop in Paris. A financial update will be performed annually before this meeting.
As a deliverable, we will build up a new generation solid pressure medium apparatus equipped with acoustics (GRAAL). We will patent this apparatus, in collaboration with the manufacturing company to which we will provide the initial drawings. This will be a valuable research resource for the team, as for the community, well beyond this research project. It will open the door to a better understanding of most processes happening under stress within the first hundred kilometres of the Earth.
Internal workshop will be organised once a year. Findings will be reported in presentations at national and international conferences and in peer-reviewed journal articles. Intellectual property of each participant will mainly be expressed as publications. Expected publications list - see deliverable table (5.3) below.
Presentation of results: at AGU 2013, 2014, 2015 and 2016, EGU 2014, 2015, 2016 and the Gordon Conference on Rock deformation in 2014 and 2016. In addition, PhD students will also attend and present their results at the RST 2014 and 2016. Finally, several seminars will be given in France and abroad. A website will be set up to make available new results, summary figures, and data set: www.geologie.ens.fr/~delf. There is no doubt that this project will increase the international visibility of the French expertise within the Rock deformation community, as well as within the earthquake mechanics and high-pressure communities.
As most of the participants of this proposal are also involved in teaching graduate courses, the transfer of scientific knowledge to graduate students will be done during lectures, at the Master level. The obtained data will be used to support various laboratory activities related to rock deformation processes and earthquake mechanics. During these labs and lectures, students will learn to think critically about the role of experimental rock mechanics in Earth Science.
D’un point de vue purement technologique, notre projet permettra de monter un appareil de Griggs de nouvelle génération en France, et le seul au monde équipé en acoustique. Cet appareil ouvrirait un certain nombre de perspectives expérimentales qui ne se limitent en aucun cas à ce projet. L’étude de la fusion partielle à haute pression in situ, aujourd’hui impossible, est déjà envisagée. Sa mise en place au Laboratoire de Géologie de l’ENS permettrait en outre de conforter la place occupée par celui-ci sur ces thématiques, à la fois sur le plan national et international. Le financement de ce projet consisterait donc en un soutien fort et une incitation pour le laboratoire à continuer dans cette direction.
Deep focus earthquake analogs recorded at high pressure and temperature in the laboratory, A. Schubnel, F. Brunet, N. Hilairet, J. Gasc, Y. Wang and Harry W. Green II, Science, 341, 1377-1380, 2013.
Deep earthquakes cannot be mechanically explained like shallow earthquakes and it has been proposed that at least part of the subducting slab seismic activity could be triggered by phase transformations and mineral reactions. Indeed, the generation of intermediate depth earthquakes occurs within the pressure and temperature (PT) where serpentine and other hydrous minerals break down. A significant seismic activity is also evidenced between 400 and 700 km (i.e., in the transition zone), which correlate with the PT range of olivine transitions towards its high-pressure polymorphs. However, the way mineral reactions can modify the deformation regime of deep rocks, from ductile to brittle (embrittlement) is poorly understood and remains one of the outstanding unsolved problems of geophysics and rock mechanics. Nevertheless, seismological, geophysical, field and experimental observations all seem to point towards the same direction: a high pressure (>GPa) high temperature (>500°C) domain exists where although deformation should be ductile, the initiation of phase transformations triggers mechanical instabilities, thus favoring dynamic fracture propagation and earthquake generation.
From the experimental point of view, being able to deform rocks in a controlled manner at HP-HT conditions, while contemporaneously monitoring the reaction progress, proves to be a challenging interdisciplinary task that lies at the frontier between the High Pressure and the Rock Mechanics communities. This is precisely the aim of this project: we propose to develop a new generation Griggs type apparatus equipped with continuous acoustic monitoring, which will enable us to attain the right s -P-T (deviatoric stress - pressure – temperature) conditions to characterize the mechanics of a number of phase transitions (Serpentine dehydration, Eclogitization reactions, Olivine to Spinel transformation) typical of subduction zones.
This new setup will be instrumental to achieve the main goal of this study, which is to understand the coupling between earthquake source mechanics and mineral physics, i.e. how does shear stress affect mineral reaction equilibrium and kinetics, and under which conditions can it trigger a dynamic mechanical instability. Our objectives are to experimentally describe the exact s-P-T envelopes of the phase transitions by combining the rock mechanics approach (plastic yield envelopes in the s-P domain) with the thermodynamics and petrology approach (PT plots of mineral stability field). We will also try to quantify the respective role of strain rate and water activity on the transformation kinetics. Complementary experiments will be performed on a solid pressure medium multi-anvil deformation apparatus (D-DIA) with a continuous acoustic monitoring system under synchrotron Xray light source. Indeed, scientists involved in this project have been working on this set-up in the last years and this experimental system is now fully operational at the GSECARS beam line (Advanced Photon Source). Preliminary results, on both serpentine dehydration and olivine – spinel transformation, are more than promising, which proves not only the feasibility of the proposal, but also its pertinence.
It is crucial for experimentalists to be forced into thinking at a larger scale. In such a way, this project also involves two field studies in order to remember what kind of microstructures we are to reproduce in the laboratory. In the same spirit, we aim at comparing nano-earthquakes (or so called Acoustic Emissions) source signals, catalogs and P and S elastic wave velocity maps obtained in the laboratory to that recorded by Seismology. Finally, upscaling experimental results needs modeling, and thermo-chemo-mechanical models will be developed in order to potentially bridge the gap between experimental data, field observations and large-scale subduction zone processes.
Project coordination
Alexandre SCHUBNEL (Laboratoire de Géologie - Ecole Normale Supérieure)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
Partnership
LG ENS Laboratoire de Géologie - Ecole Normale Supérieure
Help of the ANR 361,300 euros
Beginning and duration of the scientific project:
December 2012
- 48 Months
