Earth Planets Space, Vol. 57 (No. 5), pp. 441-446, 2005LETTER
Makoto Uyeshima1, Yasuo Ogawa2, Yoshimori Honkura3, Shigeru Koyama1, Naoto Ujihara3, Toru Mogi4, Yusuke Yamaya4, Makoto Harada5, Satoru Yamaguchi6, Ichiro Shiozaki7, Tatsuya Noguchi7, Yoshihiro Kuwaba7, Yoshikazu Tanaka7, Yuji Mochido7, Noriko Manabe7, Masanori Nishihara7, Mamoru Saka1, and Masato Serizawa1
1Earthquake Research Institute, University of Tokyo, Japan
2Volcanic Fluid Research Center, Tokyo Institute of Technology, Japan
3Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Japan
4Graduate School of Science, Hokkaido University, Japan
5Earthquake Research Center, Tokai University, Japan
6Department of Earth and Planetary Sciences, Kobe University, Japan
7Department of Civil Engineering, Tottori University, Japan
(Received February 25, 2005; Revised April 20, 2005; Accepted April 21, 2005)
Across the source region of the 2004 Mid-Niigata Prefecture earthquake, wideband magnetotelluric (MT) survey was performed just after the onset of the mainshock. Owing to the temporal stop of the DC powered railways around the area together with intense geomagnetic activity, we obtain MT records with excellent quality for both short and long period data, as long as 10,000 s. Two dimensional regional strike is evaluated with the aid of the Groom-Bailey tensor decomposition together with induction vector analysis. As a result, N15°W is determined for the strike. This strike is oblique to the local geological trend and also to the strike of the main shock source fault together with aftershock distribution of N35°E. Two dimensional resistivity structure is determined with the aid of an ABIC inversion code, where static shift is considered and estimated. Characteristics of the structure are as follows. (1) About 10 km thick sedimentary layer exists on the top. (2) A conductive body exists in the lower crust beneath the source region. The mainshock occurred at the boundary of the conductive sedimentary layer and a resistive basement beneath it and aftershocks occurred in the sedimentary layer. From geological studies, it is reported that the sedimentary layer was formed in the extensional rift-structure from Miocene to Pleistocene and has been thickened by compressional tectonic regime in the late Quaternary. Interstitial fluids or clay minerals, which reduce the sedimentary layer resistivity, control the reactivation of the normal fault as the mainshock thrust fault and aftershock activity. The second conductive body probably indicates existence of fluids in the depths as well. Such a conductive layer in the lower crust was also revealed by previous MT experiments along the Niigata-Kobe Tectonic Zone and probably plays a main role in concentration of strain rate along the zone.
Key words: Resistivity structure, magnetotelluric sounding, 2004 Mid-Niigata Prefecture earthquake, water, clay minerals, Niigata-Kobe Tectonic Zone.