Field experimentation on the detectability of co-seismic electric signals

Abstract

Simultaneous electric, magnetic and seismic measurements in Greece show that, for epicentral distances of the order of 100km and for M6 class earthquakes, no electric disturbances in the frequency range DC to 50Hz are recorded at earthquake origin time. Electric disturbances are recorded only during the arrival of seismic waves and have the following properties: (a) The amplitude does not scale with the dipole length and may significantly differ for parallel horizontal short dipoles located at neighboring sites. (b) The amplitude of the vertical component does not exceed the strongest horizontal one. (c) The data collected with a sampling rate of 50Hz, show that strong frequency content lies in the range up to around 2.5 Hz and the disturbances vanish at frequencies less than 0.1Hz and more than 5Hz, providing an explanation on the absence of these disturbances in the usual VAN real time records. These disturbances are accompanied with magnetic variations recorded either by coil and/or by torsion photoelectric magnetometers. It is not yet clear whether the magnetic variations are solely due, or not, to“vibrations”of the magnetometers upon the arrival of seismic waves. Electric disturbances were also recorded upon the arrival of seismic waves of the Izmit, Turkey, earthquake of August 17, 1999 in most of our stations. However, no pronounced electric disturbances were recorded during the Athens earthquake of September 7, 1999, neither for the Turkish event of November 12, 1999. The basic results of this paper are compatible with those reported by Uyeda et al.1)