ABSTRACT

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.

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