Chairs
Dr Alexandre Schubnel, Ecole Normale Supérieure, France
Dr Alexandre Schubnel, Ecole Normale Supérieure, France
Alexandre Schubnel has a double appointment as an associate researcher at the Centre National de la Recherche Scientifique, and as an associate professor at the laboratoire de Géologie of Ecole Normale Supérieure in Paris, France. His research interests lie in Rock Physics and Rock Mechanics. His major contribution is the development of laboratory seismology. Indeed, fracture experiments performed on rocks at in-situ conditions demonstrated that accelerations recorded in the kilohertz range on centimeter-sized samples were self-similar to the ones one can expect at the kilometric scale during a large earthquake, and as such, were successful at reproducing supershear ruptures on laboratory rock samples for the first time. Contemporaneously, AS and colleagues performed experiments at Earth’s mantle conditions, which successfully reproduced deep-focus earthquakes in the laboratory. Put together, these two studies demonstrate that dynamic rupture propagation is self-similar, and thus, laboratory earthquakes are not simple earthquake analogs, but real -yet tiny earthquakes. As such, his work has opened the door to “laboratory seismology”, i.e. the possibility of reproducing earthquakes and studying their source complexity, in the laboratory.
AS was the recipient in 2014 of both the bronze medal of CNRS and the Gouilloud-Schlumberger (young researcher prize in Earth Sciences) of the French National Academy of Sciences.
09:00-09:45
Subduction zone earthquakes: A case study on the application of lab data to understand earthquake processes
Professor Greg Hirth, Brown University, USA
Abstract
Earthquakes within the oceanic lithosphere, and the slab-wedge interface, occur over an exceptionally wide range of conditions. Seismicity is observed at normal stresses ranging from ~200 to ~8000 MPa (even greater for deep focus events), temperatures from less than 100oC up to at least 600oC and under both dry and water saturated conditions. The range of conditions where these events occur provides the opportunity to explore the efficacy of using lab-derived mechanical properties to investigate processes responsible for seismicity. Furthermore, we know more about the rheological properties of olivine - the most abundant mineral in the oceanic lithosphere - than perhaps any other mineral that is stable where earthquakes occur. Thus, we can directly test models for how asperity-scale deformation processes are manifest in frictional behavior, at both laboratory and geologic conditions. Finally, the occurrence of earthquakes at conditions where well-calibrated phase transitions occur provides the opportunity to evaluate the role of reactions for promoting a wide range of slip behaviors observed in subduction zone settings. In this presentation, I will first outline the conditions where earthquakes occur within the outer rise regions of subduction zones and compare these data with predictions based on extrapolation of lab data on the high-temperature frictional behavior of olivine. Using these observations as a constraint, I will then discuss mechanisms responsible for intermediate depth earthquakes, including the role of dehydration reactions in both peridotite and gabbroic lithologies.
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Professor Greg Hirth, Brown University, USA
Professor Greg Hirth, Brown University, USA
Greg Hirth studies experimental rock mechanics, deformation mechanisms and frictional properties in both crustal and mantle lithologies, structural geology, and the application of lab data to geophysical and geological problems. He focuses on the processes that control the mechanical behavior of rocks using optical and electron microscopy in conjunction with theoretical considerations. He received his BS from Indiana University, and PhD from Brown University working on the experimental deformation of quartz aggregates. After two years as a Postdoctoral Researcher at the University of Minnesota, he spent 14 years as a Research Scientist at the Woods Hole Oceanographic Institution. He returned to Brown as a Professor in 2007. He is a Fellow of the Mineralogical Society of America and the American Geophysical Union
09:45-10:30
Superplasticity in rocks
Professor Takehiro Hiraga, University of Tokyo, Japan
Abstract
Uniaxial constant-displacement-rate tensile tests were conducted on samples of polycrystalline forsterite + periclase, forsterite + diopside and forsterite + enstatite + diopside. At subsolidus temperatures of 1250-450 degree, a pressure of one atmosphere and a strain rate of 10-5~10-4 /sec, elongation up to 500% was achieved, exhibiting the superplastic behavior (Hiraga et al. 2010 Nature). Grains of the same phases aggregated perpendicular to the tensile direction which is identified in natural mylonite composed of fine-grained minerals (Hiraga et al. 2013 Geology). During superplastic flow, dynamic grain growth and constant relative grain size among the phases which is determined by Zener relationship are observed, the latter of which is also identified in nature (Tasaka et al. JGR 2014). Crystallographic preferred orientation develops during superplastic flow at a certain condition (Miyazaki et al. 2013 Nature) which we consider the result of significant grain rotation during superplasticity. To confirm this prediction, we conducted experiments to observe such grain rotation directly. The result indicates that grain-boundary-sliding is a primary source for grain rotation. We will also discuss some issues on extrapolating flow laws constructed at experimental conditions to nature that can vary a wide range from 10-15 (mantle flow) to 102 /sec (seismic friction) in strain rate (!), for example.
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Professor Takehiro Hiraga, University of Tokyo, Japan
Professor Takehiro Hiraga, University of Tokyo, Japan
Takehiko Hiraga is an Associate Professor at the Earthquake Research Institute, University of Tokyo since 2006. He has established a unique laboratory, focused on crystal defects and in particular, on the structure, composition, and mechanical properties of grain boundaries in earth materials. Through several years of research and development, his lab has refined techniques that allow them to produce nano-materials which are ideal for investigating the properties of grain boundaries in rocks. Accomplishments with this synthetic geomaterial include developing a grain growth law for a polyminerallic system, superplastic deformation, and the development of a crystallographic preferred orientation during superplasticity. All these processes are controlled by grain boundary phenomena such as grain boundary diffusion, grain boundary sliding, and grain boundary migration. The goal of his research is to more fully understand grain boundary processes based on nano-scale structure and chemistry. This understanding can be used to explain a number of dynamic processes in the earth’s interior including flow and brittle failure of the crust and mantle.
11:00-11:45
Can grain size sensitive flow lubricate faults during the initial stages of earthquake propagation?
Dr Nicola de Paola, Durham University, UK
Abstract
Recent friction experiments carried out under upper crustal P-T conditions have shown that microstructures typical of high temperature creep develop in the slip zone of experimental faults. These mechanisms are more commonly thought to control aseismic viscous flow and shear zone strength in the lower crust/upper mantle. In this study, displacement-controlled experiments have been performed on carbonate gouges at seismic slip rates (1 ms-1), to investigate whether they may also control the frictional strength of seismic faults at the higher strain rates attained in the brittle crust. At relatively low displacements (< 1cm) and temperatures (≤ 100 °C), brittle fracturing and cataclasis produce shear localisation and grain size reduction in a thin slip zone (150 μm). With increasing displacement (up to 15 cm) and temperatures (T up to 600 °C), due to frictional heating, intracrystalline plasticity mechanisms start to accommodate intragranular strain in the slip zone, and play a key role in producing nanoscale subgrains (≤ 100 nm). With further displacement and temperature rise, the onset of weakening coincides with the formation in the slip zone of equiaxial, nanograin aggregates exhibiting polygonal grain boundaries, no shape or crystal preferred orientation and low dislocation densities, possibly due to high temperature (> 900 °C) grain boundary sliding (GBS) deformation mechanisms. The observed micro-textures are strikingly similar to those predicted by theoretical studies, and those observed during experiments on metals and fine-grained carbonates, where superplastic behaviour has been inferred. To a first approximation, the measured drop in strength is in agreement with our flow stress calculations, suggesting that strain could be accommodated more efficiently by these mechanisms within the weaker bulk slip zone, rather than by frictional sliding along the main slip surfaces in the slip zone. Frictionally induced, grain size-sensitive GBS deformation mechanisms can thus account for the self-lubrication and dynamic weakening of carbonate faults during earthquake propagation in nature.
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Dr Nicola de Paola, Durham University, UK
Dr Nicola de Paola, Durham University, UK
Nicola De Paola graduated in 2005 with a PhD in structural Geology at Durham University (UK), where he is currently employed as a Senior Lecturer in Structural Geology and Rock Mechanics. He has recently established and is currently managing the new Rock Mechanics Laboratory, in the Earth Sciences Department at Durham University (UK). The low to high velocity rotary shear apparatus (LHVRS) installed in the Rock Mechanics Laboratory, is one of the most up-to-date and efficient high velocity shear apparatus anywhere in the world. His research activity is related to the following topics:
A) Mechanics of exhumed shear zones: Fault zone structure and architecture; Deformation processes and inferred slip behaviour; Fluids involvement in seismic faulting.
B) Rock mechanics and laboratory experiments: Porosity and permeability evolution; Rheology of intact and fractured rocks; Fluids involvement in earthquake mechanics; Investigation of seismic faulting by high-velocity friction experiments.
11:45-12:30
Phase-Transformation-Induced Nanometric Lubrication of Earthquake Sliding
Professor Harry W. Green, University of California, Riverside, USA
Abstract
Frictional sliding is strongly dependent on normal stress, hence the increase of pressure with depth in Earth prohibits slip initiation deeper than ~50km and requires that deeper earthquakes initiate by a mechanism other than overcoming of static friction. However, there is no similar constraint on the sliding mechanisms. High-pressure faulting is initiated by phase transformation and yields a thin nanocrystalline oxide “gouge” exhibiting extraordinarily low friction, even in the absence of a fluid. The only known physical mechanism that can explain this lubrication is flow of the “gouge” by grain-boundary sliding (gbs). High-speed friction experiments on a wide variety of rock types show similar very low frictional resistance of topologically indistinguishable, fully-dense, randomly-oriented nanocrystalline gouge consisting of oxides not present initially, suggesting that lubrication occurs by the same mechanism as in high-pressure faulting – flow by gbs of a nanocrystalline “gouge” formed by phase transformation in the first second(s) of sliding. Microstructures within the Punchbowl Fault, a deeply-eroded ancestral branch of the San Andreas Fault, are consistent with this argument. This mechanism intrinsically resolves two major conflicts between earlier laboratory results and natural faulting -- lack of a thermal aureole around major faults (San Andreas Fault heat-flow paradox) and the rarity of pseudotachylytes (shear-heating-induced endothermic reactions prevent temperature rise to melting). Despite this strong indirect evidence of extremely weak rheology of nanocrystalline oxides, there is no direct experimental confirmation. Accordingly, we are investigating the rheology of fully-dense nanocrystalline MgO. Preliminary results show that at moderate temperatures viscosity drops profoundly.
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Professor Harry W. Green, University of California, Riverside, USA
Professor Harry W. Green, University of California, Riverside, USA
Harry Green is Distinguished Professor of Geology and Geophysics at University of California, Riverside. He received all academic degrees at UCLA and postdoc in Materials Science at Case Western Reserve University. Taught 22 years UC Davis and last 21 years UC Riverside. Elected Fellow of AGU, MSA and AAAS. Recipient Bowen Award, AGU; Roebling Medal, MSA; numerous named lectureships. Professor Green investigates high-temperature, high-pressure rheology and petrology, principally of mantle rocks, and how stress and deformation interact with metamorphism and phase transformations. Concerning earthquakes, he and students/postdocs discovered faulting during the olivine®spinel phase transformation and its relevance to deep-focus earthquakes; determined that dehydration embrittlement is a viable earthquake mechanism, even if ∆V of reaction negative; and most recently, has demonstrated that the profound weakening observed in modern earthquake experiments is a result of phase transformation and consequent generation of a nanocrystalline gouge. He currently investigates the physics underlying this phenomenon.