We deployed an extremely dense temporal seismic network in the source region of the 2004 mid-Niigata Prefecture Earthquake (thrust fault) on October 23, 2004, Japan. The seismic network consisted of 145 temporary seismic stations within a 30 km squared and had been kept within approximately a month after the mainshock. High accurate hypocenters of 708 events were determined by inverting the arrival times using double-difference earthquake location algorithm. The aftershocks along the mainshock (Mw = 6.6) and the largest aftershock (Mw = 6.3) rupture zones are distributed on two 60° westward-dipping planes, located approximately 5 km apart. Conversely, the Oct. 27 aftershock (Mw = 5.8) occurred on an eastward dipping plane with a dip angle of 25° that was conjugate to the mainshock fault plane. Most of aftershocks at both northeastern and southwestern edges occurred at shallow depths with eastward-dipping planes. Epicenters of aftershocks in the southwestern region are aligned along N15°E, and rotate approximately 20° counterclockwise from the strike of the mainshock fault. This rotation of the aftershock alignments coincides with the rotation of anticline axes in the southwestern area of the source region. Furthermore, distributions of station corrections for a one-dimensional velocity model suggest that the seismic velocity at the western side of the Muikamachi-fault is lower than that at the eastern side. It is also inferred that the velocity structures in the hangingwall vary along the fault strike. The average velocity in the mainshock rupture area is higher than the periphery in the hangingwall, especially compared with the southwestern side of the hypocenter.
- Dense seismic observations
- Mid-Niigata Prefecture Earthquake
- Station corrections