HR: 08:20h
AN: T11A-02    [PDF]
TI: A Holocene Seismic Event and Earthquake Potential of the Uemachi Fault in Metropolitan Osaka, Estimated From High-resolution Analyses of Continuously Cored Boreholes and Seismic Reflection Profiles
AU: * Sugiyama, Y
EM: sugiyama-y@aist.go.jp
AF: Active Fault Research Center, Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Nanayama, F
EM: nanayama-f@aist.go.jp
AF: Institute for Marine Resources and Environment, Geological Survey of Japan, AIST, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Miura, K
EM: miura.kenichiro@kiso.co.jp
AF: Kiso-jiban Consultants Co., Ltd., 3-22-6 Toyo, Koto-ku, Tokyo, 135-0016 Japan
AU: Yokota, H
EM: yokota@hanshin-consul.co.jp
AF: Hanshin Consultants Co., Ltd., 2-5-24 Nishihommachi, Nishi-ku, Osaka, 550-0005 Japan
AU: Kitada, N
EM: kitada@geor.or.jp
AF: Geo-Research Institute, 4-3-2 Itachibori, Nishi-ku, Osaka, 550-0012 Japan
AU: Cannon, E C
EM: eric.cannon@colorado.edu
AF: Department of Geological Sciences, University of Colorado, 2200 Colorado Avenue, Boulder, CO 80309-0399 United States
AB: We completed a high-resolution stratigraphic correlation of five continuously cored boreholes and identified a probable earthquake event dated as 9,200 to 9,500 calibrated 14C years BP on the 45-km-long Uemachi fault system traversing Metropolitan Osaka. The 50 to 75 m-spaced, 37 to 50 m-long five cores were extracted from a subsurface flexure zone of the northern part of the Uemachi fault, a main strand of the fault system. Correlation of the five cores were carefully undertaken based on densely measured vertical change of density, magnetic susceptibility, brightness, hue, pH, electric conductivity and pollen assemblage. Depth distribution of recognized correlation surfaces through the five cores indicates an about 2 m vertical uplift during the earthquake event in early Holocene time, and lack of postdating faulting events. High-resolution S-wave seismic reflection profiling across the 450-m-wide subsurface flexure has revealed that the flexure deformation associated with the last event is concentrated within the approximately 50-m-wide foremost part of the flexure zone. The seismic profile linked with the five borehole data also suggests the advancement of the front of flexure deformation at the time of the last faulting event. An average uplift rate of 0.3 m/ky in the past 3 My is estimated for the northern part of the Uemachi fault from 800 m vertical offset of the basal surface of the Plio-Pleistocene Osaka Group. The displaced basal surface is clearly imaged on the two existing deep seismic profiles (P-wave and S-wave profiles) across the entire flexure zone. An average uplift rate larger than 0.2 m/ky in the past 0.6 my is also estimated from the existing deep boreholes named OD-5 and OD-8, which are located on the downthrown side and in the midst of the flexure zone, respectively. If we assume average uplift rate to be 0.3 m/ky and coseismic uplift to be 2 m for the northern part of the Uemachi fault, the mean recurrence interval of the fault is estimated to be around 7,000 years. The elapsed time ratio of the Uemachi fault may have already exceeded 100 percent, and earthquake potential of the fault may be raised to nearly the same level as the Nojima fault immediately before the 1995 Kobe earthquake.
DE: 7221 Paleoseismology
DE: 7223 Seismic hazard assessment and prediction
DE: 8005 Folds and folding
DE: 8094 Instruments and techniques
SC: Tectonophysics [T]
MN: 2003 Fall Meeting