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

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.