58 巻 (2005-2006) 3 号 p. 345-357
On October 23, 2004, a magnitude 6.8 earthquake occurred within the central part of Niigata prefecture; this seismic event is known as the Mid Niigata prefecture Earthquake in 2004. We investigate the subsurface structure in the area of the earthquake via receiver functions of teleseismic waveforms recorded at the Hi-net stations. The Hi-net stations located close to the aftershock region have been continuously recording ground motions since October 2000. For receiver functions estimated at stations located in areas of low relief, the arrival time of the initial pulse is clearly delayed, and its width greater than that estimated at stations in hilly and mountainous areas. These observations can be explained numerically using one- and two-dimensional model structure that contains near-surface sediments of very low velocity. To evaluate the detailed subsurface structure, we invert receiver functions for seismic velocity structure beneath each station. An extremely low velocity layer is detected beneath many stations. In the eastern part of the aftershock area, the thickness of this low velocity sediments is less than 2km, but in western areas the sediment thickness increases to more than 5km. Beneath KWNH station, on the southwestern edge of the aftershock area, the S wave velocity of the sediments is relatively low, and sediment thickness is estimated to be approximately 10km. The thickness and velocity of the sedimentary layer varies across the study area, consistent with variations in topography and surface geology. We evaluate the depth to the Moho from estimated velocity models. The depth contours of the Moho trend parallel to the coast beneath the central part of Niigata prefecture. A small ridge in the Moho is observed in the area of the boundary between the Nagano and Gunma prefectures. The gravity anomaly distribution in the study area can be explained by the variations in the Moho depths and the distributions of the low velocity sediments.