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 all earthquakes, have been a paradox since their discovery in the 1920s. The combined increase of confining pressure and temperature with depth inhibits frictional sliding, the principal earthquake mechanism for shallow earthquakes, along fault planes beyond depths of a few tens of kilometers, yet earthquakes occur to depths approaching 700 km. Brittle-like shear failure under high-confinement also occurs with impact cratering in both rock and ice and limits the steady state flow stress of highly confined rock+ice mixtures and thus the may provide important constraints on the thermomechanical evolution of certain moons and planets. This project includes new experiments and modeling aimed at elucidating the fundamental physical mechanism(s) that underlie localized brittle compressive failure under high confinement. Preliminary experiments demonstrate a transition in brittle-like failure mode with increasing confinement and that the high confinement faults appear not to be friction controlled. Nor do they appear to be related to other well established faulting mechanisms, including mode II cracking, dehydration embrittlement, or phase transformations. Their working hypothesis is that at play is a fundamentally different faulting mechanism called plastic faulting -- a non-frictional shear instability aided by adiabatic heating.
Spirited by past success in using ice as a model material for rock?ice was involved in the discovery of transformational faulting and our own discovery of what appears to be a universal mechanism of moderate-confinement brittle failure?and encouraged by its marked similarity to the behavior of rocks and minerals, this project consists of a three-year program of triaxial compression experiments to determine the effects, if any, of confinement, grain size and strain rate on both the terminal failure stress and the failure mode, with attention to microstructural detail, as observed using visual microscopy, cold stage SEM electron backscatter imaging, and cold room enabled micro-CT tomography. The researchers seek to reveal the physics of failure, and then to develop quantitative models of plastic fault initiation that only rely on independently measurable parameters. Such models are prerequisites for any conclusions concerning 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 theses and through the Dartmouth?s Women In Science (WISP) internship program. Students involved in this project will participate in professional development opportunities through Dartmouth's Center for the Advancement of Learning and in educational outreach programs, including Junior Science Cafés in regional high schools, Igloo Build programs sponsored by the Montshire Museum of Science, and those supported through Dartmouth's NSF-funded Polar Studies IGERT program, including courses on communicating polar science and the ethics and policy implications of polar research, and educational outreach activities with communities in Greenland.
