HR: 1340h
AN: T43A-1365    [Abstracts]
TI: Characteristics of Inhomogeneous Structure Around the Mid Niigata Prefecture Region Inferred From Receiver Functions
AU: * SHIOMI, K
EM: shiomi@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba-Shi, Ibaraki-Ken, 305-0006 Japan
AU: OBARA, K
EM: obara@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba-Shi, Ibaraki-Ken, 305-0006 Japan
AB: On October 23, 2004, an M6.8 earthquake (known as the Mid Niigata Prefecture earthquake in 2004) occurred in the central part of Niigata prefecture, central part of Japan. We investigate receiver functions of teleseismic waveforms recorded at NIED Hi-net, High Sensitivity Seismograph Network, and F-net broadband seismograph stations to construct the subsurface structure around the epicentral region. These stations have been continuously recording ground motions since October 2000, at latest, and their spacing is about 20 km. We observed that the arrival time of the initial pulses of the receiver functions estimated at stations located in low relief areas are clearly delayed, and their width are greater than that estimated at stations in mountain areas. These observations may 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 commonly detected beneath many stations. In the eastern part of the epicentral region, the thickness of this low velocity sediments is less than 2 km, but in the western area, the sediment thickness increases to more than 5 km. Beneath the southwestern edge of the epicentral region, the S wave velocity of the sediment is relatively low, and sediment thickness is estimated to be approximately 10 km. The thickness and velocity of the sedimentary layer varies across the study region, coincident with variations in topography and surface geology. We evaluate the depth of the Moho from estimated velocity models. Depth contours of the Moho trend parallel to the coast beneath the central part of Niigata Prefecture. Almost all large earthquakes with M5.5 or greater are located at the northern part of the aftershock region, where a small ridge in the Moho is observed. In the area of the boundary between the Nagano and Gunma Prefecture, 40 km south of the aftershock region, the Moho also has a small ridge. The gravity anomaly distribution in the study region can be explained by variations in the Moho depths and the distributions of the low velocity sediments.
DE: 7218 Lithosphere (1236)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8015 Local crustal structure
DE: 9320 Asia
SC: Tectonophysics [T]
MN: Fall Meeting 2005