VLF signals emitted from rocks under different experimental conditions with particular reference to seismic precursors.

Abstract

The monitoring of ELF and VLF radio spectrums (between 0.8 and 12 kHz) during the process of the breaking of rock for uniaxial compression has drawn attention to the presence of electromagnetic impulses with two characteristic modalities in a dynamic, which develops before, during and immediately after the breaking, culminating in a paroxysmal, concomitant episode at the point of breaking. The occurrence of this phenomenon appears to be independent from the lithology, which nevertheless conditions it, the evidence in function of the mechanical and structural characteristics of the material. The presence of saturation liquids has significantly influenced this phenomenon. The emission appears to be directly tied to the process of micro and macro fracturing that precedes and determines the breaking of the sample. An analogue emission process has been verified also in conditions of relaxed stress in the settlement process of the lithoid sides after the knocking down of open pits. Finally, a monitoring system of the natural radiation in the atmosphere, followed by a few sensors over the course of two years, found only three signals compatible with the models observed in the laboratory. However for each of these a quake of greater or equal 4.5 magnitude was identified, manifesting itself on national territory with an average delay of 3.6 days.