Articles | Volume 13, issue 4
Nonlin. Processes Geophys., 13, 409–412, 2006
https://doi.org/10.5194/npg-13-409-2006

Special issue: Nonlinear dynamics of Earth-Oceans-Space (EOS2005)

Nonlin. Processes Geophys., 13, 409–412, 2006
https://doi.org/10.5194/npg-13-409-2006

  11 Aug 2006

11 Aug 2006

Fractal analysis for the ULF data during the 1993 Guam earthquake to study prefracture criticality

Y. Ida and M. Hayakawa Y. Ida and M. Hayakawa
  • Department of Electronic Engineering, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo, 182-8585, Japan

Abstract. An extremely large earthquake (with magnitude of 8.2) happened on 8 August 1993 near the Guam island, and ultra-low-frequency (ULF) (frequency less than 1 Hz) electromagnetic fields were measured by 3-axis induction magnetometers at an observing station (with the epicentral distance of 65 km) with sampling frequency of 1 Hz. In order to study electromagnetic signature of prefracture criticality, we have undertaken the fractal (mono-fractal) analysis by means of the Higuchi's method for the ULF data during the 1993 Guam earthquake. Then, it is found that the fractal dimension exhibits five maxima 99, 75, 52, 21, and 9–4 days before the earthquake main shock, which suggests the ULF electromagnetic signature of nonlinear evolution (in the sense of self-organized criticality) taking place in the lithosphere just before the 1993 large Guam earthquake. That is, there take place step-like changes in the lithosphere during the long-term of the order of several months before the main shock.