Fractal analysis of ULF geomagnetic data associated with the Guam earthquake on August 8, 1993

This paper presents a fractal analysis of the ULF electromagnetic data obta ined at Guam observatory during a strong (Ms=8.0) earthquake occurred on Au gust 8, 1993 near the Guam island. We have already reported the presence of seismogenic ULF emissions for this quake (Hayakawa et al., GRL,1996). The chosen period covered four months before and two months after the earthquak e. It was found that the spectrum of emissions at noon exhibited a power la w behavior f(-beta) which is typical for self-organized critical dynamics. The slope of the spectrum in the noon sector fluctuates in a range from 2.5 to 0.7 with a tendency to gradually decrease during the process of the ear thquake preparation. We consider this to be a candidate for a precursory si gnature. The similar effect was found for the night sector. We suggest that the focal zone was at the critical stage of self-organized criticality pro cess before the quake, related to active microfracturing processes followed by the generation of ULF emissions as we detected.