ELECTRIC EARTHQUAKE PRECURSORS: FROM LABORATORY RESULTS TO FIELD OBSERVATIONS

Abstract

The generation of transient electric potential prior to rupture has been demonstrated in a number of laboratory experiments involving both dry and wet rock specimens. Several different electrification effects are responsible for these observations, however, piezoelectricity cannot explain why quartz-free rocks can also generate precursory phenomena and electrokinetic phenomena are normally very weak to produce macro- and megascopic scale effects. Electrification is observed in dry, non-piezoelectric rocks meaning that additional, solid state mechanisms should be responsible. Herein we focus on a promising effect that is ubiquitous during brittle rock failure: the motion of charged edge dislocations (MCD) during crack opening and propagation (microfracturing). We report a series of laboratory experiments on dry marble samples and discuss their possible relationship to field observations of purported electric earthquake precursors (EEP). The experiments confirm the generation of pressure-stimulated currents (PSC) as expected by the MCD model. The PSC was linearly related to the stress rate, so long as the stressed material deformed elastically. Deviation from linearity arose when the applied stress drove the specimen into the plastic deformation range; this effect has been attributed to the dependence of the PSC on the stress rate and, ultimately, to the inverse of the changing (decreasing) Young’s modulus. The emitted current appears very intense and non-linear just prior to failure, where massive crack propagation implies massive MCD processes. Repeated cycles of deformation are associated with progressively weaker current emission, indicating the strong dependence of electrification on the residual damage. Overall, the results are consistent with, and render support to the concept of electrification by MCD/microfracturing. Other mechanisms cannot be excluded of course but are rather considered to accompany and supplement the drastic MCD process.