SCEC Award Number 14054 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title A New Creep Instability at Intermediate Homologous Temperatures with Application to Slow Earthquakes and Non-Volcanic Tremor
Investigator(s)
Name Organization
Charles Sammis University of Southern California
Other Participants Terence Langdon, Marshall Rogers-Martinez
SCEC Priorities 5c, 5d, 3c SCEC Groups FARM, Seismology, Geodesy
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
 A ductile instability that has the potential to produce slow earthquakes and associated tremor has recently been observed during the High-Pressure Torsion (HPT) processing of metals. To date, this instability has only been observed in metals having high stacking fault energies and only over a narrow range of homologous temperatures. We began a series of HPT tests on metals and alkali halides designed to test our hypothesis that this mechanism is controlled by homologous temperature and stacking fault energy, thus providing a physical basis for the extrapolation from metals and alkali halides to silicates. Our most significant result to date is the observation of strain weakening in the aluminum alloy (Al 6061) that we were using to test the apparatus (Fig 3a). Previous observations of strain weakening in HPT testing have been limited to pure metals and hydrated NaCl. Our observation in an alloy with a significant (>5%) fraction of alloying elements, shows that the weakening is not limited to pure materials, and thus may also occur in rock.
Intellectual Merit Slow earthquakes and non-volcanic tremor are poorly understood yet potentially important deformation mechanisms that occur at the base of seismogenic zones at both transform and subduction plate boundaries and may significantly influence the timing of large earthquakes in the overlying brittle crust. We are pursuing the hypothesis that these newly discovered phenomena are due to creep instabilities at the brittle-ductile transition.
Broader Impacts Understanding the mechanics of slow earthquakes and non-volcanic tremor will advance our understanding of the seismic cycle and associated timing of large earthquakes. This grant supported the work of a graduate student at USC (Marshall Rogers-Martinez) and has facilitated cross disciplinary research between the Earth Sciences and Material Sciences departments at USC.
Exemplary Figure Figure 3. Strain weakening in Al 6061. Panel (a) on the left shows shear stress. Panel (b) on the right shows microhardness. Note that a peak occurs in both at about the same strain.
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