Xiangtang Downhole Array of Accelerometers-Instrumentation and Observation

Abstract

局所的なサイト特性が強震動に与える影響を検討するために, ボアホール地震観測システムが唐山市Xiangtang地域に構築された。地表, 地下-16m及び地下-32mに3成分加速度型強震計が設置された。観測可能最大地動加速度及び周波数範囲は, それぞれ, ±2g及び0-50Hzである。これまでに, 27個の局所地震が観測された。これらのマグニチュード(M)は1.7-5.9の範囲であった。また, これまでに観測された最大加速度は, M=5.9の地震時の60.1cm/s/sである。地表/地下の地動振幅スペクトル比の解析から, この地域の表層地盤の共振周波数は4.4及び8.8Hzあたりにあることがわかった。

The Xiangtang downhole array (XTDA) for the effects of local site conditions on strong ground motion is a three-element array of three-component, force balance accelerometers for measuring accelerations up to 2.0 g over a frequency range from 0 to 50 Hz. The accelerometers are placed at depths of 0 m, 16 m and 32 m respectively. The lithostratigraphy is about 12 m of soil overlying a 18 m layer of strong weathered granulite and granite below which is moderate-weathered granite. The array is located at the intersection of the Taoyuan fault and the Leting fault, it is only 18 km from the epicenter of the largest aftershock (Ms=7.1, July 28, 1976) of Tangshan earthquake (Ms=7.8, July 28, 1976). An ML =5-6 earthquake would be expected in the near future in this area. From its installation in September 1994 through December 1995, it recorded 27 local earthquakes with magnitudes ranging from 1.7 to 5.9. The maximum acceleration recorded at the soil surface is 60.1 cm/s/s from an ML =5.9 earthquake. The surface to downhole spectral ratios show that there are significant differences in the frequency contents between the soil surface and the underground rock measuring points.The surface deposits at XTDA site have two resonance frequencies, around 4.4 Hz and 8.8 Hz, where the spectral ratio amplitudes are more than 10, for horizontal ground motions. Amplifications of the surface soil layer exist up to 50 Hz. In contrast to the weathered granite layer (16-32 m), the surface soil deposits (0-16 m) play more important role in amplifications of ground motion.