Matej Pec, Massachusetts Institute of Technology

Deformation of all materials necessitates the collective propagation of various microscopic defects. On Earth, fracturing gives way to crystal-plastic deformation with increasing depth resulting in a “brittle-to-ductile” transition region that is key for estimating the integrated strength of tectonic plates, constraining the earthquake cycle, and utilizing deep geothermal resources. Here, we show that the crossing of a brittle–to–ductile transition in marble is accompanied by profound changes in the frequency of acoustic emissions suggesting changes to the size and propagation velocity of the active defects. We further identify dominant classes of emitted waveforms using unsupervised learning methods and show that their relative activity systematically changes as the rocks cross the brittle-ductile transition. As pressure increases, long-period signals are suppressed and short-period signals become dominant. At highest explored pressures signals frequently come in avalanche-like patterns. We propose that these classes of waveforms correlate with individual dominant defect types. Complex mixed-mode events indicate that interactions between the defects are common over the whole studied pressure range in agreement with post-mortem microstructural observations. Our measurements provide first experimental data on in-situ microscale dynamics of a brittle-to-ductile transition that can inform micromechanical models for semi-brittle deformation. 

Short Biography
Matěj Peč studies the mechanical and microstructural response of rocks to various driving forces associated with plate tectonic movements and human activity. To this end, he performs experiments in the laboratory and compares the results to natural observations and theoretical models. His lab specializes on developing ultrasound probes for in-situ monitoring of deformation at high pressure and high temperature conditions. His work so far has focused on elucidating the processes occurring at the base of the seismogenic layer, the coupling between flow and reaction in magmatic systems, and the influence of carbon mineralization on physical and mechanical properties of rocks. He is particularly interested in the coupling between deformation and chemical reactions.

Matěj has a B.Sc. in Geology and a M.Sc. in Structural Geology from the Charles University in Prague, Czech Republic. During his master studies, he spent a year at the Université Montpellier II in France as an ERASMUS scholar. He then pursued a Ph.D. in rock mechanics under the supervision of Prof. Renée Heilbronner and Prof. Holger Stünitz at the Universität Basel in Switzerland, where he graduated “summa cum laude” in 2012. After his Ph.D., he worked in the laboratory of Prof. David L. Kohlstedt at the University of Minnesota as a postdoctoral researcher. Matej joined the Department of Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology in January 2017 as an Assistant Professor.