Comment on “Modeling Low-frequency Magnetic-field Precursors to the Loma Prieta Earthquake with a Precursory Increase in Fault-zone Conductivity,” by M. Merzer and S. L. Klemperer

Abstract

MERZER and KLEMPERER (1997) present a model to explain ultra-low-frequency (ULF) magnetic noise anomalies associated with the Loma Prieta Earthquake (LPE). These authors state directly, ‘‘It is impossible to demonstrate that any claimed earthquake precursor was truly precursory, that is, forewarning of, rather than merely prior to, the earthquake, because the earthquakes and anomalies cannot be experimentally generated.’’ Nevertheless a sharp increase of the low-frequency contribution to the magnetic field at a distance of 7 km from the epicenter was observed three hours before the earthquake. This increase was superimposed on other changes in the low-frequency magnetic field in the previous weeks. The authors go on to suggest a plausible mechanism for the sharp increase in low-frequency magnetic field strength three hours before the earthquake as due to a roughly 15-fold increase in the electrical conductivity of the fault zone. The authors give three possible mechanisms for this anomaly: 1) An increase in the salinity of the fluids in the fault zone, 2) an increase in porosity, and 3) a change in the exponent of Archie’s law (ARCHIE, 1942) for the electrical conductivity as a power of the porosity from 2 to 1. The purpose of this comment is chiefly to provide support for the third scenario of the authors. Its purpose is not to defend the MERZER and KLEMPERER (1997) treatment against unrelated criticism (EGBERT, 2002) that the result of even a large increase in electrical conductivity may be mitigated through effects of the mutual inductance between the fault zone and the crust. In a recent contribution (HUNT, 2004a) I demonstrated that Archie’s law is a consequence of the relevance of continuum percolation theory to electrical conduction in porous media. The derivation was developed from the dependence of the electrical conductivity on volumetric water content. It was argued that the volumetric water content, h, was, in continuum percolation, the variable that could be tuned to sweep a system through a percolation transition and that, as a consequence, the electrical conductivity of a porous medium in which the solid