Abstract
AbstractAcross 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.
Highlights
The 2004 Mid-Niigata Prefecture Earthquake of M6.8 occurred at 17:56 on 23, Oct., 2004 in the backarc area of the central main Japan island
(1) About 10 km thick sedimentary layer exists on the top
The mainshock occurred at the boundary of the conductive sedimentary layer and a resistive basement beneath it and aftershocks occurred in the sedimentary layer
Summary
The 2004 Mid-Niigata Prefecture Earthquake of M6.8 occurred at 17:56 on 23, Oct., 2004 in the backarc area of the central main Japan island. One reason to conduct such a survey urgently was that DC-powered railways surrounding the area were not in operation (black broken lines in Fig. 1) due to the problems of landslides and aftershock ground motions. We set azimuth of the survey line as N55◦W–S55◦E by considering trend of surface geology, mechanism of the mainshock and aftershock activities (Sakai et al, 2005; Hirata et al, 2005b). A moderately conductive layer of several tens of m is detected in the lower crust from the center of the profile to the east
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