Magnetic field variations associated with the SES before the 6.6 Grevena-Kozani earthquake

Abstract

The 6.6 Grevena-Kozani EQ that occurred on May 13, 1995 was preceded by two strong SES activities recorded (with a sampling rate 1 sample/sec) at Ioannina station on April 18 and April 19, 1995. These activities were accompanied by observable variations of the horizontal magnetic field (unfortunately, the vertical magnetometer was out of order). The magnetic recordings were found to“arrive”1-2 [s] after the electric field variations. The same value of the time-lag is found for both the whole SES activities and portions of them. A careful inspection shows that this time-lag cannot be attributed to the instrumentation; for example, MT disturbances recorded just before (or after) the SES activities exhibit an opposite behavior, i.e., the magnetic recordings“precede”the electric ones as naturally expected.1) The fact that SES electric field variations“precede”the accompanied magnetic ones by a time-lag of the order of one second excludes any possibility of attributing them to a nearby artificial source. This time-lag can be understood only if we consider that the trasmission of the fields obey diffusion type equations and the epicentral distance was about 80 [km]. The measurable time-lag between these variations may provide a tool for (1) improving the prediction of the epicenter and (2) distinguishing true SES from artificial noise (the latter distinction becoming possible if the epicentral distance is significant, e.g., several tens of km).