Seismic Surveys of the Surface Faults in Fukushima Iwaki area, Eastern Japan, Appeared at the Induced Earthquake of the M9.0 2011 Great Tohoku Earthquake

Abstract:

Ten and odd kilometers long earthquake surface faults appeared along two known active faults in Fukushima Iwaki area, eastern Japan, at the Fukushima Hamadori earthquake (M7.0) on April 11, 2011. This earthquake is considered as an induced earthquake of the 2011 March great Tohoku earthquake (M9.0). To reveal subsurface structure around the faults from several ten meters to a few kilometers in depth, we conducted seismic surveys along four lines which cross the Itozawa fault (line1), Yunodake fault (line2) and inferred extension area of the Yunodake fault (line3). In line1, the CMP stacked time section is relatively more reflective in the east side of the surface fault and lacks continuous reflectors in the west side of it between 0.3s and 1.5s in two way time. In deeper part, amplitude of reflectors decreases below 7km in depth. In line2, a sedimentary basin is well imaged in the area of Tertiary and Quaternary sediments. The top of the basement shows complicated shape and reaches 700m in depth. The velocity structure by a ray tomography technique corresponds very well to the sedimentary structure by the reflection survey. In line3, the basement is 500m deep at the southern edge, almost flat and gently dips at two parts and is 800m deep at the northern edge. The sediments gently dip northward. Large stratigraphic throws are not perceived on the line3 seismic section. Line 4 crosses the Itozawa fault and the target depth is from near surface to about 30m. In line4, very strong and continuous reflector is imaged down to 20m in depth and it is interpreted as the top of the metamorphic rock. Several faults are interpreted from the step-like structure of the reflectors near the surface fault.