2001 Volume 77 Issue 6 Pages 87-92
Small amplitude Seismic Electric Signals (SES), are not accompanied by readily detectable variations of the horizontal components of the magnetic field. This does not hold for strong SES activities, i.e., before earthquakes with M-6.5 at epicentral distances of -100 [km]. The instrumentation used to obtain such conclusions is described. Attention is focused on a careful calibration of the coil magnetometers that were in operation at loannina station when the strong SES activities associated with the 6.6 earthquake at Grevena-Kozani were recorded. These magnetometers act as dB/dt detectors for periods longer than around half a second; furthermore their output is“neutralized”after 200 [ms] from the“arrival”of a fast step magnetic variation. In other words, a signal recorded by these magnetometers should correspond to a magnetic variation that has“arrived”at the sensor less than 200 [ms] before the recording. This property is critical for the study of the time difference between the variations in the electric and the magnetic fields. The variations in these fields“arrive”simultaneously (within experimental accuracy) from the usual nearby artificial sources, while for magnetotellurics the magnetic recordings“precede”those of the electric field. Both behaviors differ from that observed in the case of strong SES activities, a matter described in the accompanying paper.