Seismic reflection survey across the Kitakami lowland fault zone, northeast Japan: 2008 Hanaizumi and 2009 Nanshozan Profiles

The inner northeast Japan arc is characterized by arrays of north-trending thrust sheets, which accommodate the large east-west intraplate strain and deform Neogene basin deposits (Sato, 1994). The Kitakami lowland fault zone (KLFZ) is an active thrust zone that extends for about 80 km along the Quaternary volcanic front of the northeast Japan arc. The activity of the thrust zone after Quaternary is mostly recognized as a fault reactivation of Miocene normal faults in the area. We present seismic reflection data acquired along two profiles, the Hanaizumi08 and the Nanshozan09, to define the geological structure of the KLFZ. The Hanaizumi08 is 7.0 km-long seismic line across the Ishikoshi flexural line, which is located at the southern extension of the KLFZ. The Ishikoshi flexural line is the major geological structure of seismically active area where four earthquakes of magnitude over 6 occurred in past 150 years. However, the Quaternary deformation along the line is mostly invisible. The Nanshozan09 is 6.5 km-long seismic line across the Nanshozan fault and several west-dipping thrust faults parallel to it, which comprise the northern portion of the KLFZ. These faults deform late Quaternary fluvial terraces and alluvial fans in Kitakami lowland. In both seismic lines, the vibrator truck (IVI Minivib T-15000) is used as the seismic source. A 10-m source and geophone spacing yield a 5-m common midpoint (CMP) spacing on the final sections. Seismic reflection data was processed by using the standard CMP stacking method. The seismic profiles correlated with surface geologic mapping clearly show the along-strike variations of thrust geometries both at ground surface and at shallow depth, and document the detailed process of the active thrusting along the KLFZ.