Coherence of the ULF fields in the seismoactive zone of Japan

The work is devoted to a numerical analysis specifically adapted for multi-point detection of geomagnetic events, which precede or accompany earthquakes. Both numerically simulated data and geomagnetic perturbations during real earthquakes were investigated. The objective of this paper is to study the correlation properties of the ULF fields on small distances in seismoactive zone of Japan. By applying a combination of the Fourier, Hilbert and wavelet transforms to a single component and to their combinations we developed five correlation techniques and analyzed the geomagnetic variations at five observation points for two-year permanent observations. Our study was in two main directions: correlation properties of the field before an earthquake, and a coherency of geomagnetic pulsations launched by the earthquake already taken place. Five different coherency criteria have been examined: (1) the cross correlation function; (2) semblance; (3) in-phase; (4) σ; and (5) frequency stability criterion. We found that the signal detection times determined by the cross correlation, semblance and in-phase criteria are much more stable compared to that found with the help of the sigma correlation technique which allows detecting when the signal to noise ratio is about ∼3. The study revealed anomaly signals in the period range from 15 min to 1.5 h independently at all observatories. The signature of the anomaly signal is a wave train usually consisting of 2–3 of 0.5–1 h pulses which appears 3–4 h and 8 h prior to the earthquake time if we study correlations within 90 km radius. Intensity of the signals is around 0.5–1 nT.