Harry GreenDistinguished Professor of the Graduate DivisionEarth and Planetary Sciences hgreen@ucr.edu(951) 827-4508
Rheology of Nanocrystalline Materials: Clarifying the Sliding Mechanism of Earthquakes
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AWARD NUMBER
006941-003
FUND NUMBER
21286
STATUS
Closed
AWARD TYPE
3-Grant
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AWARD EXECUTION DATE
6/4/2015
BEGIN DATE
8/1/2014
END DATE
7/31/2016
AWARD AMOUNT
$136,859
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Sponsor Information
SPONSOR AWARD NUMBER
SPONSOR
SPONSOR TYPE
FUNCTION
Organized Research
PROGRAM NAME
Proposal Information
PROPOSAL NUMBER
13121178
PROPOSAL TYPE
New
ACTIVITY TYPE
Basic Research
PI Information
PI
Green, Harry W
PI TITLE
Other
PI DEPTARTMENT
Earth and Planetary Sciences
PI COLLEGE/SCHOOL
College of Nat & Agr Sciences
CO PIs
Garay, Javier;
Project Information
ABSTRACT
Earthquakes occur from just below the surface of Earth to almost 700km where they stop abruptly. Near the surface, earthquakes begin by overcoming friction on pre-existing faults but at depths greater than about 40km the pressure is so high that overcoming friction is impossible; the elevated temperatures at such depths allow rocks to flow at stresses lower than they could slip suddenly and cause an earthquake. Then why are there any deeper earthquakes? Twenty-five years ago the investigator discovered the physical mechanism by which faults (and by inference deep earthquakes) initiate and slide at high pressure. The key is formation of a "nanocrystalline" sliding zone that is weak because the crystals in the zone are so tiny-a few millionths of a millimeter in diameter. Recent high-speed friction experiments simulating shallow earthquakes have raised major questions concerning how shallow earthquakes actually slide. These experiments show that immediately after sliding begins, there is strong heating caused by the two sides of the fault rubbing against each other. The temperature rises rapidly until it initiates a mineral reaction that produces a very weak nanocrystalline sliding zone that looks just like the high-pressure fault zones. The PIs propose that this nanocrystalline zone slides by exactly the same physical process as the high-pressure faulting and that most earthquakes slide by this mechanism also. To test this hypothesis, they and a graduate student, with undergraduate help, will conduct experiments to determine the stresses needed to make nanocrystalline materials flow over a broad range of temperature, pressure, crystal size and sliding rate. If successful, they will have established for the first time a physical understanding of earthquake sliding. This physical understanding will enable more sophisticated modeling of predicted shaking damage caused by earthquakes and move us one step closer to the elusive goal of predicting earthquakes. Another result that should come from these studies is deeper understand-ding of flow of nanocrystalline ceramics that will provide valuable basic information for attempts to develop high-speed forming processes for a variety of ceramic products in industry. This project will provide a graduate student and undergraduate students with very valuable interdisciplinary experiences bridging between earthquake physics and materials science. UCR is the most ethnically diverse campus of the University of California and one of the most ethnically diverse universities in the USA. The institution supports the pursuit of state-of-the-art research including involvement of undergraduates making a vital contribution to their education. As a consequence, minority students at UCR graduate at the same rate as white students and at a much higher rate than that of "peer institutions".
Based upon high-pressure faulting and high-speed friction experiments, the PIs propose a universal sliding mechanism for most earthquakes. They hypothesize that in the first second or so of most earthquakes, a low-viscosity, nanocrystalline, "gouge" is generated, enabling sliding with a very low frictional resistance. They propose to test this hypothesis by synthesizing nanocrystalline materials and measuring their rheology as a function of grain size, strain rate, temperature and pressure. Deformed specimens will be analyzed by SEM and TEM accompanied by Electron Back-Scattered Diffraction (EBSD), selected-area electron diffraction, and energy-dispersive chemical analysis. The rheological experiments are straightforward and the PI and CoPI are experts in such procedures and have the necessary equipment in their laboratories. The critical step is in synthesizing fully-dense composites of controlled grain sizes. The CoPI is an expert in this step as well and has demonstrated success over the last 8 years. Therefore, the experimental program is essentially assured of success. There is already fragmentary existing knowledge suggesting that the materials chosen will flow by grain-boundary sliding at low viscosities under a subset of the conditions that will be examined. By creating an extensive data set, the PIs will be able to demonstrate whether or not high-speed friction experiments slide by this mechanism under some conditions, all conditions, or no conditions. Application to sliding of real earthquakes will be through comparison to the high-speed friction experiments and microstructures of deeply eroded faults analogous to the San Andreas Fault (SAF) of California. It is expected that these experiments will provide a quantitative mechanistic explanation of why friction falls precipitously very shortly after sliding initiates, and rises again as sliding slows. Additionally, the study will also demosntrate why the SAF shows no thermal anomaly (the so-called San Andreas heat flow paradox) and why pseudotachylytes are rare.(Abstract from NSF)
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