以微地動量測方法研究順向坡之放大與極化效應;Detection of site amplification and directivity at dip-slope using microtremor measurements

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题名:	以微地動量測方法研究順向坡之放大與極化效應;Detection of site amplification and directivity at dip-slope using microtremor measurements
作者:	林子翔;Lin,Tsu-Hsiang
贡献者:	應用地質研究所
关键词:	微地動;方向性;場址放大;順向坡;單站頻譜比;Spectral ratio;Microtremor;Site Effect;Directivity;Dip-slope
日期:	2012-08-24
上传时间:	2012-09-11 18:26:46 (UTC+8)
出版者:	國立中央大學
摘要:	地振動在邊坡上的動態反應是地震誘發邊坡破壞的重要考量。過去因缺乏現地的地動觀測資料使此議題較少被了解，而多限於地形放大效應的少數觀測資料及相關之數值模擬。直到2007年，Del Gaudio及Wasoski才首次以實際觀測資料提出在邊坡上的震波極化現象。目前了解，在某些特定地形及地質構造之邊坡上可觀察到地振動放大與極化現象，而且常沿著可能滑移方向產生最大放大，這是以往未曾被瞭解而且被遺漏的重要地震誘發山崩議題。本研究在台灣碧山巖、草嶺、九份二山等順向坡進行寬頻地震儀佈設及微地動的量測，分析地動在順向坡之放大與極化情形。結果顯示，碧山巖在單一地質條件影響之下，放大效應主要受地形控制，坡頂處譜比值可達4至5，且低頻部分（1-3Hz）之方向性放大與地形相符合。綜合在順向坡坡面上之觀測結果，判定符合主頻與極化判定準則的測站，其極化方向平行於順向坡地層之傾向，譜比值可到3-5之間。然而部分同樣位於坡面上之測站受現地構造影響而呈現不同極化方向。這顯示微地動測量之結果易受局部地形地質特徵控制。另外在與CHY080強震站之強地動觀測結果比較中，可在同樣的頻率帶（1-3Hz）觀測到主頻放大，且從可判定極化方向之地震結果中發現，其極化方向與微地動之結果一致。但若單一比較集集地震與微地動資料，其極化方向差異約70度。主要因微地動反應現地局部地形與構造作用結果，但強震資料結果同時反應了震源特性與區域性構造，未來需要更多強地動資料進行歸納與比較分析場址極化效應。The influence of site effects can be an important factor in seismic triggering of landslides on unstable slopes. However, dynamic responses of unstable slopes to seismic shaking are poorly understood, because the scarcity of relevant seismometric data and its evaluation is made difficult by the complexity of the phenomena involved, link to surface geometry, local geology, amplified wave propagation, etc. The analysis presented by Del Gaudio and Wasowski (2007) show that slope response to seismic shaking can be characterized by directional variations with maxima amplification oriented along local topography features. The site amplification and directivity appears has occasionally been detected with maximum slope direction. This work attempt to explore the phenomenon based on microtremor measurement, and the test were performed at Bishan, Jiufengershan and Tasoling in Taiwan. At Bishan, all measurements are located on the same stratum, it points out that the amplification of these sites are influenced by topography and the value of HVNR of these sites near ridge can be 4-5. Further, we observed the response directivity at 1-3Hz are coincided with local topography. Summarize all directivity measured at dip-slope faces, most results are with dip direction and some exceptions are linked to the local site features. It shows that microtremor is easily affected by local geology and topography conditions. A comparison of microtremor results with records of an accelerometer CHY080 station at Tasoling shows that the microtremor and the strong motions have the same dominant frequency (1-3Hz). The directivity of the earthquakes which are met the criteria for site response directivity identification presence the same amplification oriented with microtremor. However, the maximum amplification direction of Chi-Chi earthquake event provides a different of 70 degrees from microtremor results. It might due to the nature different between microtremor and seismic, the former is more related to local structures and the latter combines source properties and site effect. Considering the non-linearity of site response, it would need more investigation with moderate-strong earthquake data recorded at those unstable slopes to understand dynamic responses.
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