A critical review of Electric Earthquake Precursors

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, but how these may scale up co-operatively in large heterogeneous rock volumes, to produce observable macroscopic signals, is still incompletely understood. Accordingly, the nature and properties of possible Electric Earthquake Precursors (EEP) are still inadequately understood. For a long time observations have been fragmentary, narrow band and oligo-parametric (for instance, the magnetic field was not routinely measured). In general, the discrimination of purported EEP signals relied on experience and ad hoc empirical rules that could be shown unable to guarantee the validity of the data. In consequence, experimental studies have produced a prolific variety of signal shape, complexity and duration but no explanation for the apparently indefinite diversity. A set of inconsistent or conflicting ideas attempted to explain such observations, including different concepts about the EEP source region (near the observer or at the earthquake focus) and propagation (frequently assumed to be guided by peculiar geoelectric structure). Statistics was also applied to establish the beyond chance association between presumed EEP signals and earthquakes. In the absence of well constrained data, this approach ended up with intense debate and controversy but no useful results. The response of the geophysical community was scepticism and by the mid-90's, the very existence of EEP was debated. At that time, a major re-thinking of EEP research began to take place, with reformulation of its queries and objectives and refocusing on the exploration of fundamental concepts, less on field experiments. The first encouraging results began to appear in the last two years of the 20th century. Observation technologies are mature and can guarantee reliable electric field measurements, although improvements are still possible with new generation electrodes and smart measurement schemes facilitating noise suppression. It is increasingly apparent that simultaneous electric and magnetic measurements are indispensable and conducted in most new experiments. There is also an emerging trend towards multi-parametric, broadband observations that should provide far better data and constraints on the source processes. The physics of electrification mechanisms are beginning to clarify, as also is the potential of solid state effects: charge and current densities under controlled conditions are such, that if scaled up to the size of seismogenic zones, they would yield observable EEP. However, there are still many unknowns, requiring careful experimentation and theoretical development. Research is also directed towards decoding the physics of stress/strain changes that cause electrification, exploiting properties such as are the fractal nature of faulting and Self-Organised Criticality (SOC). The first evidence of possible electromagnetic precursors due to a SOC system has been published recently. Modelling of the source processes from first principles is stepping up and certain classes of observed signals can now be predicted by theory, providing new and more rigorous means of data authentication; such models have also established the feasibility of long range EEP signals. Although progress is apparent, the knowledge is still grossly incomplete and EEP data are not indisputable, if tested with the full rigour of scientific verification methods. The new research philosophy requires time and vigilance before it begins to pay off, but it appears to have taken a more promising course.