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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 1/2
Nat. Hazards Earth Syst. Sci., 1, 33–36, 2001
https://doi.org/10.5194/nhess-1-33-2001
© Author(s) 2001. This work is licensed under
the Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License.

Special issue: Double Issue: Seismic hazard evaluation - Part I

Nat. Hazards Earth Syst. Sci., 1, 33–36, 2001
https://doi.org/10.5194/nhess-1-33-2001
© Author(s) 2001. This work is licensed under
the Creative Commons Attribution-NonCommercial-ShareAlike 2.5 License.

  30 Jun 2001

30 Jun 2001

A precursory ULF signature for the Chi-Chi earthquake in Taiwan

Y. Akinaga1, M. Hayakawa1, J. Y. Liu2, K. Yumoto3, and K. Hattori4 Y. Akinaga et al.
  • 1Dept. of Electronic Engineering, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu Tokyo 182-8585, Japan
  • 2Institute of Space Science, National Central University, Chung-Li 320, Taiwan
  • 3Dept. of Earth and Planetary Sciences, Kyushu University, 33 Hakozaki Fukuoka 812-8581, Japan
  • 4Marine Biosystems Research Center, Chiba University, 1-33 Yayoi, Inage Chiba 263-8522, Japan

Abstract. ULF emission data at Lunping (epicentral distance, 120 km) have been analysed for the Chi-Chi earthquake (with magnitude 7.6 and depth of 11 km) in Taiwan which occurred on 21 September 1999. Simple intensity analyses have not yielded any significant results but we have found, based on the analysis of polarization (the ratio of vertical magnetic field component Z to the horizontal component G), that the polarization (Z/G) showed a significant enhancement for two months before the earthquake. This kind of temporal evolution of polarization seems to be very similar to previous results, so that it is highly likely that this phenomenon may be associated with the Chi-Chi earthquake. Also, the comparison of the results of polarization analyses, by changing the signal threshold, has given us an approximate intensity of the seismogenic emission of the order of the monthly mean value.

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