FRACTAL ANALYSIS OF SEISMOGENIC ULF EMISSIONS

Abstract

Fractal analysis has been performed on ultra-low-frequency (ULF) geomagnetic field data observed at the Izu peninsula. The first attempt of using fractal analysis for ULF geomagnetic field data during the large Guam and Biak earthquakes has been based on FFT. In order to quantitatively estimate the FFT-based fractal analysis, we first study a few fractal analyses, including the Burlaga–Klein method and the Higuchi method, as compared with the former FFT-based method. The accuracies of these three methods have been evaluated by applying them to the test data-sets of fractional Brownian motion with white noise. We conclude that the Higuchi method is superior to the FFT-based method and the Burlaga–Klein method. The S/N effect was also discussed. Then, we have applied the Higuchi method to the ULF geomagnetic field data during a big earthquake swarm (during June–August 2000) in the Izu peninsula. It is found that the fractal dimension exhibits a significant increase just before earthquakes with a magnitude Mj>6.0 associated with the Izu islands swarm with any of these three fractal analyses. This experimental finding will lend further convincing support to the presence of precursory ULF emissions.