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

Abstract

Shear tests on quartz rocks and single quartz crystals have been conducted to understand the possible relationship between the intensity of detectable stress in fault areas and the energy released in the form of electromagnetic waves in the range 30 KHz-1 MHz (LF–MF). For these tests, a new type of piston-cylinder has been developed, instrumented to collect the electromagnetic signals generated by the quartz during shear stress tests and that allows energy measurements on electromagnetic emissions (EMR) to be performed. The data obtained indicate that shear-stressed quartz crystals can generate electromagnetic emissions in the LF–MF range. These emissions represent a tiny fraction of the total energy dissipated in the fracturing process. The spectrum of radio emissions consists of continuous radiation and overlapping peaks. For the first time, a characteristic migration of peak frequencies was observed, proportional to the evolution of the fracturing process. In particular, the continuous recording of the radio emission spectra shows a migration of the peaks toward higher frequencies, as stress continues over time and smaller and larger fractures form. This migration could be used to distinguish possible natural signals emitted by quartz in tectonically active environments from possible signals of other geophysical and possibly anthropogenic origin.

Fracture Analysis of α-Quartz Crystals Subjected to Shear Stress

“…The data obtained in this study should be seen in the light of the most recent literature on the effects of shear stress on alpha quartz, literature to which the authors have also contributed [5,6]. The fractures we have observed show an evolution which, we believe, depends substantially on the stress rate rather than on the pressure itself.…”

“…The task of this experimental work is to understand the mechanisms that link the progressive demolition of the crystal structure of quartz and the appearance of polymorphic phases during the application of sliding stresses. In two previous works [5,6], the authors of this work reported details on the polymorphic phases of silica that are formed during fracturing and on the correlations between fractures and the amount of very low frequency electromagnetic energy emitted by the crystals during the fracturing.…”

“…The automated scanning system allows one to measure and analyze about 1 million particles per hour and to know the values of the perimeter, area and volume of each particle, from 300 µm to 1 µm. This scanning microscope has already been used previously by the authors for investigations on asbestos fibers in soils and recently on quartz crystals [5,6]. To determine the morphologies, the particles are dispersed on the surface of the optical plate.…”

“…In a recent publication, we formulated another hypothesis to explain the lowering of the coefficient of friction in experiments on quartz-containing rocks. This hypothesis is based on the progressive formation of nanocrystalline cristobalite, which works as a solid lubricant [5,6]. The silica polymorph called cristobalite has the particularity of being an auxetic material, i.e., with a negative Poisson ratio, both in the form of low temperature (alpha) and in that of high temperature (beta).…”

“…The accumulation of this phase in the interface between the sliding surfaces would cause the friction coefficient to decay more or less rapidly, depending on the intensity of the dynamometamorphic action [6]. Furthermore, quartz's structural decay is marked by the decay of the electromagnetic signals generated by its fracturing, which diminishes over time as fracturing and damage of the original crystalline structure proceed [5,6]. A key element is missing in this theory: how and when the patina of cristobalite is generated.…”