Distributed Currents

A magnetic investigation has been carried out for the past five years at several CIEN stations, using induction coils of a diameter of about 80 - 100 cm, with 40 to 120 turns. The objective was to detect the magnetic component of the electric oscillations already revealed during intense seismic activity. The recording confirmed the absence of magnetic radiation correspondence for electric field oscillations. However, a large number of magnetic pulses were detected before the mainshocks of the seismic swarms occurred in 2016 and 2017 in Central Italy, see below.

Magnetic pulses had a time length of 0.1 sec and were concentrated

around the time of the Norcia, M = 6.5 on October 30, 2016, and the

Capitignano, M = 5.9 on January 17, 2017, seismic events.

To evaluate magnetic pulse intensities a test was used by adding a

further loop of the same diameter, a test loop. The two loops were

coupled side by side, see picture on the right, and a controlled electrical

current was injected in the test loop. The coupling magnetic field can be

calculated by (Landau and Lifsits 1986)

where R is the loop radius, J is the electric current density, K and E are

the elliptical integrals, r and x are the distances from the axis and from

the loop plain, respectively. This test confirmed that the measured pulse

intensities were in the range of 2 to 80 nT. To have an initial estimate of

the minimal electrical current flowing in the earth's crust, a simple model

was based upon the Biot-Savart law which considers an infinitely long line

conductor that is at some depth in the earth's crust.

Given that the magnetometer has the axis oriented approximately NNW-SSE, the idealized current flowing parallel to the ground plane (able to induce in the loop) will have approximately an ENE-WSW direction, which is perpendicular to the characteristic strike of Central Italy faults, see picture on the left. This configuration required currents of about 1 - 40 kA for the detected pulse from the Chieti Station, to produce the magnetic induction recorded from the loop at Chieti.

A more realistic model of distributed electric currents oriented ENE- WSW was hypothesized, where the concatenated flux to the loop was calculated with special functions, which gave a higher total electric current intensity of about 2 - 60 kA. The corresponding earth current densities at the Chieti Station were calculated to be of up to

40 micro A m^(-2). Electric field pulse intensities are therefore calculated to be well under the noise level of the electric field and should not be revealed by electric detectors used in this experiment in accordance with the results (see Fidani and Orsini, 2018).