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Electromagnetic Emissions from Quartz Subjected to Shear Stress: Spectral Signatures and Geophysical Implications
Open AccessArticle

“Pre-Earthquake” Micro-Structural Effects Induced by Shear Stress on α-Quartz in Laboratory Experiments

INGV National Institute of Geophysics and Volcanology, Via Ugo La Malfa 153, 90146 Palermo, Italy
Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730000, China
Key Laboratory of Petroleum Resources, Lanzhou 730000, China
CNR-IGAG, Institute of Environmental Geology and Geoengineering, Research area of Rome, 00015 Monterotondo, Italy
Author to whom correspondence should be addressed.
Geosciences 2020, 10(5), 155;
Received: 5 March 2020 / Revised: 20 April 2020 / Accepted: 22 April 2020 / Published: 27 April 2020
(This article belongs to the Special Issue Electromagnetic and Radon Pre-earthquake Precursors)
This paper presents the results of measurements performed on α-quartz subjected to shear stress in dry conditions, to understand the relationship between the shear intensity and the resulting physical and chemical effects. If a shear stress of intensity higher than 100 MPa is applied continuously to alpha quartz crystals, they will tend to lose their crystallinity, progressively reduce their friction coefficient (Cof) and change into a low-order material, apparently amorphous under X-ray diffraction, but with a structure different from silica glass. Raman and Pair Distribution Function analyses suggested a structure like cristobalite, a silica polymorph well-known for its auxetic behavior, i.e., having a negative Poisson ratio. This elastic parameter pre-eminently controls the friction coefficient of the material and, if it is negative, the Cof lowering. As a result, the increase in low crystallinity cristobalite is sufficientto explain the lowering of the quartz friction coefficient up to values able to contribute, in principle, to the triggering processes of active faults. This allows hypothesizing a slip induction mechanism that does not include the need to have the interposition of layers of hydrated silica, as invoked by many authors, to justify the low friction coefficients that are achieved in shear stress tests on rocks abundant in quartz. View Full-Text
Keywords: phase transitions; defects; fracture and flow; rheology of faults; quartz; friction phase transitions; defects; fracture and flow; rheology of faults; quartz; friction
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Martinelli, G.; Plescia, P.; Tempesta, E. “Pre-Earthquake” Micro-Structural Effects Induced by Shear Stress on α-Quartz in Laboratory Experiments. Geosciences 2020, 10, 155.

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