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Natural Hazards and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Volume 1, issue 3 | Copyright

Special issue: Seismic hazard evaluation - Part II

Nat. Hazards Earth Syst. Sci., 1, 119-126, 2001
https://doi.org/10.5194/nhess-1-119-2001
© Author(s) 2001. This work is licensed under
the Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License.

  30 Sep 2001

30 Sep 2001

Scaling characteristics of ULF geomagnetic fields at the Guam seismoactive area and their dynamics in relation to the earthquake

N. Smirnova1, M. Hayakawa2, K. Gotoh2,1, and D. Volobuev1 N. Smirnova et al.
  • 1St. Petersburg University, St. Petersburg 198504, Russia
  • 2The University of Electro-Communications, Chofu, Tokyo 182, Japan

Abstract. The long-term evolution of scaling (fractal) characteristics of the ULF geomagnetic fields in the seismoactive region of the Guam Island is studied in relation to the strong (Ms = 8.0) nearby earthquake of 8 August 1993. The selected period covers 10 months before and 10 months after the earthquake. The FFT procedure, Burlaga-Klein approach and Higuchi method, have been applied to calculate the scaling exponents and fractal dimensions of the ULF time series. It is found that the spectrum of ULF emissions exhibits, on average, a power law behaviour S(f ) α f -b , which is a fingerprint of the typical fractal (self-affine) time series. The spectrum slope b fluctuates quasi-periodically during the course of time in a range of b = 2.5–0.7, which corresponds to the fractional Brownian motion with both persistent and antipersistent behaviour. An tendency is also found for the spectrum slope to decrease gradually when approaching the earthquake date. Such a tendency manifests itself at all local times, showing a gradual evolution of the structure of the ULF noise to a typical flicker noise structure in proximity to the large earthquake event. We suggest considering such a peculiarity as an earthquake precursory signature. One more effect related to the earthquake is revealed: the longest quasi-period, which is 27 days, disappeared from the variations of the ULF emission spectrum slope during the earthquake, and it reappeared three months after the event. Physical interpretation of the peculiarities revealed has been done on the basis of the SOC (self-organized criticality) concept.

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