Use of wavelet analysis for detection of seismogenic ULF emissions

Abstract

Wavelet analysis is applied to high‐resolution magnetic ULF data in a seismoactive region to determine whether there is evidence of ULF electromagnetic emissions that precede or accompany earthquakes. We have developed an algorithm that is specially adapted to the single‐station wavelet detection of geomagnetic events. For this purpose we have constructed wavelet‐based magnetic signatures of certain earthquakes. Namely, we have used the distribution of energies among blocks consisting of coefficients of wavelet packet transforms. Our computer experiments have shown that common features preceding two strong earthquakes appear in geomagnetic fields recorded close (around 20 km) to the epicenter of the earthquakes. The anomalies occupy a wide range of periods (from 10 s to 250 s). Evidence was also found on the presence of short period seismogenic pulses associated with a strong earthquake (M = 5.6) on March 26, 1997, in Kyushu, Japan. A comparison of extracted geomagnetic variations at two observatories located in its epicentral zone has indicated that seismogenic geomagnetic disturbances occurred 6 to 7 hours prior to the earthquake.