Faulting under high confinement: Experimental investigation and model development

点击翻译按钮获取中文摘要

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Deep earthquakes, some of which are among the largest and most damaging of allearthquakes, have been a paradox since their discovery in the 1920s. The combined increase ofconfining pressure and temperature with depth inhibits frictional sliding, the principal earthquakemechanism for shallow earthquakes, along fault planes beyond depths of a few tens ofkilometers, yet earthquakes occur to depths approaching 700 km. Brittle-like shear failure underhigh-confinement also occurs with impact cratering in both rock and ice and limits the steadystate flow stress of highly confined rock+ice mixtures and thus the may provide importantconstraints on the thermomechanical evolution of certain moons and planets. This projectincludes new experiments and modeling aimed at elucidating the fundamental physicalmechanism(s) that underlie localized brittle compressive failure under high confinement.Preliminary experiments demonstrate a transition in brittle-like failure mode with increasingconfinement and that the high confinement faults appear not to be friction controlled. Nor dothey appear to be related to other well established faulting mechanisms, including mode IIcracking, dehydration embrittlement, or phase transformations. Their working hypothesis is thatat play is a fundamentally different faulting mechanism called plastic faulting -- a non-frictionalshear instability aided by adiabatic heating.Spirited by past success in using ice as a model material for rock?ice was involved in thediscovery of transformational faulting and our own discovery of what appears to be a universalmechanism of moderate-confinement brittle failure?and encouraged by its marked similarity tothe behavior of rocks and minerals, this project consists of a three-year program of triaxialcompression experiments to determine the effects, if any, of confinement, grain size and strainrate on both the terminal failure stress and the failure mode, with attention to microstructuraldetail, as observed using visual microscopy, cold stage SEM electron backscatter imaging, andcold room enabled micro-CT tomography. The researchers seek to reveal thephysics of failure, and then to develop quantitative models of plastic fault initiation that only relyon independently measurable parameters. Such models are prerequisites for any conclusionsconcerning the possible role of a particular failure process in geophysical processes.The proposed work also will contribute to Dartmouth's long tradition of involving undergraduates in research, through senior honors thesesand through the Dartmouth?s Women In Science (WISP) internship program. Students involvedin this project will participate in professional development opportunities through Dartmouth'sCenter for the Advancement of Learning and in educational outreach programs, including JuniorScience Cafés in regional high schools, Igloo Build programs sponsored by the MontshireMuseum of Science, and those supported through Dartmouth's NSF-funded Polar StudiesIGERT program, including courses on communicating polar science and the ethics and policyimplications of polar research, and educational outreach activities with communities inGreenland.