CSEDI Collab. Research: A joint mineral physics and nano-seismological study on high-pressure faulting in metastable olivine and harzburgite with implications to deep earthquakes

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Worldwide, the number of earthquakes per year decreases rapidly with depth down to ~300 km, then peaks around 550 - 600 km, before terminating abruptly near 700 km. Deep-focus earthquakes (DFEQs), i.e., those occurring at depths below 300 km, are particularly mysterious, as we know that rocks generally deform by creep and flow, rather than by brittle fracture, at these depths, where pressures and temperatures are both very high. Understanding the mechanisms of DFEQs is important because these quakes occur in subduction zones and pose significant seismic hazards in many regions around the globe. It also helps understand properties and behaviors of rocks and how plate tectonics works in the Earth's interior. The experimental capabilities developed in the project will find broad applications in disciplines far beyond earth science, including materials science, physics, and engineering.In this project, the investigators will combine advanced experimental techniques and state-of-the-art seismological analytical tools to obtain information on the physical mechanisms of fracturing under high pressure and high temperature. The materials to be studied are (Mg,Fe)2SiO4 olivine (the dominant mineral in the oceanic lithosphere and the upper mantle) and harzburgite (the dominant rock assemblage of the oceanic lithosphere). Samples will be deformed in a new class of deformation apparatus equipped with in-situ acoustic emission (AE) monitoring as well as x-ray diffraction and imaging, under a wide range of conditions of pressure, temperature, differential stress, strain, and strain rate. Controlled deformation will be conducted on these materials at pressures up to 14 GPa. A suite of state-of-the-art seismological methods of event detection, location, and source characterization will be applied to the nanoseismograms of AE events to determine rupture mechanisms. Our goal is to understand the physics that connects earthquake mechanics and minerals/rocks at laboratory scales, to provide fundamental insight as to how and under what conditions shear localization occurs, affecting, and affected by, mineral reaction equilibrium and kinetics, and triggers dynamic mechanical instability. Attention will be paid to controlling oxygen fugacity and minimizing water content during the experiments. It must be kept in mind the vast difference in scales between laboratory and subduction zone processes. The team will conduct comparison studies to examine AE source characteristics against those of DFEQs. Thermo-chemo-mechanical models will then be developed and evaluated based on experimental data and seismic observations, and large-scale subduction zone processes. Combining these approaches, the investigators anticipate a significant enhancement of our understanding of the mechanisms for DFEQs by establishing physical models for DFEQs whose testability and scalability can be further examined by computational simulations.