Field experimentation on the detectability of co-seismic electric signals

Simultaneous electric, magnetic and seismic measurements in Greece show tha t, for epicentral distances of the order of 100 km and for MF class earthqu akes, no electric disturbances in the frequency range DC tu 50 Hz are recor ded at earthquake origin time. Electric disturbances are recorded only duri ng the arrival of seismic waves and have Me following properties: (a) The a mplitude does not scale with the dipole length and may significantly differ for parallel horizontal short dipoles located at, neighboring sites. (c) T he amplitude Of the vortical component does not exceed the strongest horizo ntal one. (c) The data collected will a sampling rate of 50 Hz, show that s trong frequency content lies in the range up to around 2.5 Hz and the distu rbances vanish at frequencies less than 0.1 Hz and more than 5 Hz, providin g 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 i s not yet clear whether the magnetic variations are solely due, or not, to "vibrations" of the magnetometers upon the arrival of seismic waves. Electr ic disturbances were also recorded upon the arrival of seismic waves of the Izmit, Turkey, earthquake of August 17,1999 in most of our stations. Howev er, no pronounced electric disturbances were recorded during the Athens ear thquake of September 7, 1999, neither fur tile Turkish event of November 12 , 1999. The basic results of this paper are compatible with those reported by Uyeda et al.