S-wave Velocity Structure Model of the Osaka Sedimentary Basin Derived from Microtremor Array Observations Takao KAGAWA Geo-Research Institute, Osaka Soil Test Laboratory, 4-3-2, Itachibori, Nishi-ku, Osaka 550-0012, Japan Sumio SAWADA Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji 611-0011, Japan Yoshinori IWASAKI Geo-Research Institute, Osaka Soil Test Laboratory, 4-3-2, Itachibori, Nishi-ku, Osaka 550-0012, Japan Atsushi NANJO Hanshin Expressway Public Corporation, 4-1-3, Kyutaro-machi, Chuo-ku, Osaka 541-0056, Japan (Received: November 18, 1997; Accepted March 20, 1998) Four Layered structure model with S-wave velocities of the Osaka Sedimentary Basin is presented in this paper to calculate long-period seismic response. We conducted microtremor array observation and obtained dispersion curves of Rayleigh waves at 16 sites in the basin. From the dispersion characteristics, we derived layered structure down to bedrock at each site. Compiling the results, we modeled a subsurface structure for the Osaka Sedimentary Basin. We found that the layered structure has common characteristics at all the sites in the Osaka Sedimentary Basin, and concluded that the basin structure can be modeled with four layers. Each layer has S-wave velocity of 0.35, 0.55, 1.0, and 3.2 km/s, respectively. Key words: The Osaka Sedimentary Basin, S-wave velocity structure, Microtremor array observation.
Fig. 1. Locations of the observation arrays used in this paper. Fig. 3. Equipment for microtremor array observation. We used vertical long-period seismograph, bridge circuit, amplifier, 14-bits digital data recorder and transceiver to receive clock code for each station. Fig. 2. Example of array formation conducted at Naruo-hama site. Basically, we used triangle shape formation for large arrays, and L shape arrangement along streets for small arrays.
Fig. 4. Example of a instrumental characteristics derived from a step response record. Step response is recorded before observation and natural period and damping factor are derived. They are used to correct instrument characteristics to insure common property. The right-top is a recorded step response. The left and the right-bottom are amplitude and phase spectrum of step response. In the spectrum, both of observed and fitted characteristic curves are indicated. Fig. 5. An example of vertical ground velocity from a large array at Mukonosoh site. 45 minutes high quality microtremor data were obtained at all stations.
Fig. 6. Phase velocities obtained from microtremor arrays at Amagasaki Port (rectangular symbol), Mukonosoh (circle), Itami (triangle). The symbols show mean of phase velocities and vertical bars indicate standard deviations. Fig. 7. Definition of sedimentary groups from seismic reflection section after YAMAMOTo et al. (1992). 8) b) d) Fig. 8. Results of inversion analysis assuming four-layered model for the sites along seismic reflection survey line. P-wave velocity, thickness, and density of each layer were fixed, S-wave velocity was inverted. Resulting S-wave velocities have common tendency at all the sites. The sites are a) Kansai International Airport, b) Amagasaki Port, c) Osaka North Port, and d) Konohana.
Fig. 9. Relationship between P-wave velocity and S-wave velocity in each layer. Empirical relation [OHTA et at. (1985)] is plotted on to compare paper. with the results in this Table 1. P-wave and S-wave velocities and density for each layer of proposed four-layered model of the Osaka Sedimentary Basin.
Table 2. Comparison of S-wave velocity structures between this study and NAKAGAWA et al. (1993), HORIKE et al. (1996). a) I)) (I) Fig. 10. Examples of inversion analysis for four-layered model at the sites a) Naruo-hama, b) Rokko Island, c) Sakai, and d) Kishiwada. P-wave, S-wave velocities, and density were fixed and the target was thickness. The symbols show mean of phase velocities and vertical bars indicate standard deviations.
Table 3. Thickness of each layer derived at the all observation sites. Table 4. Comparison of bedrock depths between this study and other previous studies.
Fig. 11. Relationship between basement depth and boundary depths. The upper figure a) is that of A/ B interface, and the lower is by B/C interface. Circles indicate the data derived from microtremor array observations and triangles are from interpretation of seismic reflection surveys. The ratios of A/B and B/C interface depths against basement depth are estimated as 0.193 and 0.473, respectively.
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