Multifractal Analysis for the ULF Geomagnetic Data during the Guam Earthquake

Abstract

In our previous papers we have shown that the fractal (monofractal) dimension (D_o) showed a significant increase before the Guam earthquake occurred on August 8, 1993. In order to have a further support to this precursory effect, we have carried out the corresponding multifractal analysis (by means of detrended fluctuation analysis) for the same data to study the statistical self-similar properties in a wide range of scales. We have analyzed the ULF geomagnetic data (the most intense H component) observed at Guam observatory. As the result, we have found that we could observe significant changes in the multifractal parameters at Guam. α_min showed a meaningful decrease about 25 days before the earthquake, but α_max exhibited no significant change at all, so that Δα showed a significant increase before the earthquake. Correspondingly, the generalized multifractal dimension Dq (q>1) showed a significant decrease (whereas Dq (q<0) showed no change) and D₀ (= Dq (q=0) (as already found in our previous papers) is reconfirmed to increase before the earthquake. These multifractal characteristics seem to be a further support that these changes are closely associated with the earthquake as a precursor to the Guam earthquake.