Earth Planets Space, Vol. 61 (No. 6), pp. 797-801, 2009LETTER
Toshiaki Mishima1, Tetsuro Hirono2, Norihiro Nakamura3, Wataru Tanikawa4, Wonn Soh4, and Sheng-Rong Song5
1Research Center for Inland Seas, Kobe University, Kobe, Japan
2Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, Japan
3Department of Geoenvironmental Science, Graduate School of Science, Tohoku University, Sendai, Japan
4Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology, Nankoku, Japan
5Department of Geosciences, National Taiwan University, Taipei, Taiwan
(Received September 5, 2008; Revised November 28, 2008; Accepted December 25, 2008; Online published July 27, 2009)
We carried out magnetic mineral analyses of samples from the shallowest major fault zone within the Chelungpu fault system, which is the zone that previous researchers believe slipped during the 1999 Taiwan Chi-Chi earthquake. Our aim was to gain an understanding of the changes to magnetic minerals during the earthquake. Magnetic hysteresis and low-temperature thermal demagnetization measurements showed that high magnetic susceptibilities in the black gouge zone within the major fault zone could be attributed not to fining of ferrimagnetic minerals but, rather, to their abundance. Thermomagnetic analyses indicated that the strata in and around the fault zone originally contained thermally unstable iron-bearing paramagnetic minerals, such as pyrite, siderite, and chlorite. We therefore concluded that frictional heating (>400°C) occurred in the black gouge zone in the major fault zone during the slip of the Chi-Chi earthquake and that the resultant high temperature induced thermal decomposition of paramagnetic minerals to form magnetite, resulting in the observed high magnetic susceptibilities.
Key words: Frictional heating, 1999 Taiwan Chi-Chi earthquake, Chelungpu fault, ferrimagnetic mineral.