Strain constraint on the source of the alleged Varotsos‐Alexopoulos‐Nomicos (VAN) precursor of the 1995 Aigion earthquake (Greece)

Abstract

We present the data continuously monitored at the Institut de Physique du Globe de Paris/National Technical University of Athens Galaxidi geophysical observatory before the 1995 Aigion earthquake (Greece) and analyze their implications for the source of the alleged Varotsos‐Alexopoulos‐Nomicos (VAN) precursor (seismic electric signals (SES)) of this earthquake. On April 30, 1995, an electrotelluric anomaly of 10 mV km−1 was observed at the VAN station of Volos, located 150 km NE of Aigion. The VAN group attributed it to the preparation for the 1995 Aigion earthquake. The same day, the N‐S tiltmeter of the Galaxidi observatory, located 30 km NE of Aigion, did not detect any tilt above its noise level, 7.5 × 10−8 rad. Assuming all conditions which provide the greatest electric signal at Volos but the smallest local deformation, we model the characteristics of the source that is required to generate the SES observed at Volos with a N‐S tilt at Galaxidi compatible with our observations. We suppose, as is usually done for possible earthquake precursors, that the electrotelluric signal results from streaming potentials induced in response to the pore fluid flow accompanying an aseismic creep event near the hypocenter of the upcoming earthquake. We also suppose that the electrical structure between Aigion and Volos is composed of a favorable two‐dimensional (2‐D) horizontal conductive layer at midcrustal depth, excluding a local amplification at Volos (the SES appeared with comparable ΔV/L values on both the short and long measurement lines), as well as a highly speculative ad hoc 1‐D channel between Aigion and Volos (as proposed by VAN). With these assumptions, we find that a coupling coefficient greater than 500 V MPa−1 would explain the reported anomaly. Typical values of this coefficient measured in the laboratory are of the order of 100 mV MPa−1 The source of the SES of April 30, 1995, is hence very unlikely to have been located in the hypocentral zone of the 1995 Aigion earthquake, whatever its actual link with the earthquake.