Nanoscale evidence for transient rheology during an earthquake

Alexis K. Ault, Jordan L. Jensen, & Robert G. McDermott

Submitted August 13, 2018, SCEC Contribution #8429, 2018 SCEC Annual Meeting Poster #173

Nanoscale textures record the evolution of fault rock rheology and strength during the earthquake cycle. Earthquakes occur on thin fault surfaces and require a dramatic reduction in fault friction, potentially aided by a concomitant spike in fault temperature. Here, we apply novel nano-imaging and nano-chemistry tools to reveal evidence for transient rheological changes on a hematite fault mirror or high gloss, light reflective fault surface. We target a fault that crosscuts megacrystic specularite with minor quartz in the Pleistocene El Laco Fe-ore deposit, northern Chile. Scanning and transmission electron microscopy methods, including electron backscattered diffraction (EBSD) of the natural fault surface, indicate the mirror comprises a 50 micron zone of nanoscale, sintered polygonal hematite grains (<2 micron in diameter) with no shape or crystallographic preferred orientation, rhombohedral twins and misoriented magnetite nanoparticles in some hematite crystals intersecting the slip surface, and decreasing grain size away from the fault mirror. Locally, amorphous interstitial silica connects sub-5 nm silica films detected by electron energy loss spectroscopy that encase hematite grains. A network of topographic ridges defining a polygonal pattern transects the polygonal hematite crystals on the mirror. These ridges are hematite that impinges or protrudes into the polygonal grain boundaries of material on opposing side of the fault that is no-longer preserved. We suggest friction-generated heat from a dense network of geometric asperities resulted in amorphization of hematite and quartz and observed high-temperature reduction of Fe at the fault surface, a process that is aided by crystal comminution and the increased surface area of the nanoparticles created during incipient earthquake propagation. Following dynamic weakening, hematite crystals regrow and interlock across the slip surface, aiding in strength recovery post-earthquake. These results imply the interplay between transient heat and evolving fault rock rheology – even at the nano-scale – controls fault strength and may promote stick-slip behavior.

Citation
Ault, A. K., Jensen, J. L., & McDermott, R. G. (2018, 08). Nanoscale evidence for transient rheology during an earthquake. Poster Presentation at 2018 SCEC Annual Meeting.


Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)