Zagreb Earthquakes on 22nd March 2020 and Geomagnetic Information

Earthquakes in the Zagreb area on 22nd March 2020 were an occasion to estimate earthquake-related noise in minute magnetograms and its significance to the geomagnetic information renewal i.e.,survey of geomagnetic networks, reduction, and modelling of the geomagnetic information (that is, declination and its annual variation). Firstly, the distinction of geomagneticaly quiet and disturbed days was vividly portrayed by comparison of Q1 (quiet) and D1* (disturbed) days of March 2020 in the geomagnetic observatory Lonjsko polje magnetograms. The day of 22nd March was rather quiet geomagnetic day, therefore the comparison of available magnetograms of the observatories Lonjsko polje and Nagycenk was performed. Identification of earthquakes on magnetograms was limited by the uncertainties of observatory and epicentre coordinates, magnitudes of the earthquake waves\u27 velocities, times of their occurrence at observatories, rounding of earthquakes times to whole minutes, and filtering observatories\u27 records into minute (and preliminary) magnetograms. Comparisons of observatories minute magnetograms during the observed day did not unequivocally indicate earthquakes; seemingly the earthquakes noise was less than the effects of the quiet days\u27 geomagnetic disturbances. The differences of variations, obtained by removing the diurnal variation from centred magnetograms, in Lonjsko polje and Nagycenk, at the earthquakes\u27 times were found negligible, less than 10" for the declination, 5" for the inclination, and 0.6 nT for the total intensity. Thus, the contribution of the noise of earthquakes of comparable features was estimated negligible in the context of the geomagnetic information renewal