Identification of Sources of Geomagnetic Variations Using Detrended Fluctuation Analysis (DFA)

This report is an attempt to use Detrended Fluctuation Analysis (DFA) to relate scaling properties of ground-based ULF magnetic field data to occurrence of earthquakes. The magnetic field data are from the South European Geo-magnetic Array (SEGMA) (Vellante et al., 2002). Several moderate earthquakes (4.3 < M < 5.5) occurred in the Adriatic region during November-December 2004. Thus a seven month period of magnetic field measurements: 01 August 2004 – February 2005 is analyzed. The DFA approach is applied to ULF range (10-450 sec) of data and their scaling properties are studied depending on geomagnetic and local conditions. We investigate scaling properties of the vertical and horizontal magnetic field components (Z and HOR) both in day and night conditions. Both regional (at all stations) and local effects (at one observatory only) in the DFA scaling index dynamics are found: i) regional effects correlate with the geomagnetic activity controlled by Kp scaling index, and ii) local ones are mainly characterized by DFA scaling index decrease in the Z component observed only in the shorter time window 10-90 sec. The latter persists days over diurnal trends. Introduction In this study our goal is a detection of long-range correlations in ground-based magnetic field data. An approach of Detrended Fluctuation Analysis (DFA) is applied to data from magnetometric array SEGMA. The SEGMA array includes magnetic field measurements from Central and SouthEastern Europe: three observatories in Italy (L’Aquila, Ranchio, Castello Tesino, one in Hungary (Nagycenk) and one in Bulgaria (Panagyurishte). All three components of the Earth’s magnetic field are recorded with sampling rate 1 second. It is well-known that the Earth’s magnetic field variations represented by several planetary geomagnetic indices (Dst, Kp, Ap, etc.) are indicative for the geomagnetic activity (magnetospheric and/or ionospheric processes) that occur in the near-Earth environment. Recently, strong earthquakes are observed that have been preceded by intense magnetic noises recorded in the ULF range (FraserSmith, 1990; Molchanov et al., 1992; Kopytenko et al, 1993,1994; Hayakawa et al, 1996; Hayakawa, 2005). The ‘seismogenic’ ULF signals prove to emerge hours, days or several weeks (to month) prior to earthquakes. These signals have local character and have been registered at distances less than 100 km from earthquake epicenter. A problem to be solved is: how the ‘seismogenic’ ULF noises can be detected and discriminated from the magnetospheric/ionospheric ones and used as a precursor signature of impending earthquake? The ratio between the vertical (Z) to horizontal (HOR) components of the magnetic field variations, i.e. Z/HOR, has been used for such purposes. The Z/HOR ratio and its increase however are influenced by geomagnetic activity. Figure 1 depicts a strong correlation between the Kp and the Z/HOR variations (at frequency 0.5 Hz). The arrows indicate earthquake moments occurred on 01-25 WDS'07 Proceedings of Contributed Papers, Part II, 93–100, 2007. ISBN 978-80-7378-024-1 © MATFYZPRESS