以微地動分析宜蘭盆地S波速度構造;S-Wave Velocity Structure in the Ilan Basin from Microtremor Analysis

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Title:	以微地動分析宜蘭盆地S波速度構造;S-Wave Velocity Structure in the Ilan Basin from Microtremor Analysis
Authors:	方熙蒂;Fatimah,Siti
Contributors:	地球科學學系
Keywords:	microtremor;S-wave velocity;structure maps;H/V simulation
Date:	2016-07-22
Issue Date:	2016-10-13 13:22:36 (UTC+8)
Publisher:	國立中央大學
Abstract:	宜蘭盆地為一覆蓋著鬆散第四紀沖積層的沈積平原，其下是堅硬的中新世基盤，此高地層速度落差之盆地構造易放大地震動影響。由於此地區地震頻仍，是台灣高地震風險的區域之一，詳細的S波速度構造有助於地動場址效應之研究。宜蘭盆地已有密集的微地動量測分析資料，本研究利用微地動單站頻譜比擬合技術，以Haskell-Thomson法所計算出來的地層轉換函數與所觀察到的單站頻譜比作比對擬合，進而估算S波速度構造。研究中以氣象局強震站之接收函數分析結果和強震測站場址工程地質資料庫的鑽探岩心資料作為速度剖面之初始模型，利用基因演算法找到各測點最佳的S波速度剖面。宜蘭盆地的基盤和淺部三個地層之構造形貌皆透過本研究結果描繪出來，基盤等高線圖與過去研究結果相似，但透過密集且平均的測點分佈獲得更為平滑且詳細的形貌。最上層的鬆軟土層於宜蘭東部沿岸厚達250公尺，S波速度為150 至400m/s之間。其下的沖積層的速度範圍在300 至700 m/s。第三層為更新世地層，其厚度為10 至 800公尺間不等，越往東則越厚。盆地底部之基盤面深度在30 至 1300 公尺間，地層S波速度為900 至1500 m/s。根據歷史地震地表加速度峰值和速度峰值分布比對，顯示地表加速度峰值可能和盆地構造較無顯著關連，而是受震源效應或路徑效應影響，而地表速度峰值的分布則明顯由基盤構造所控制。 ;Microtremor measurements had been conducted in the Ilan basin, one of the most vulnerable area in Taiwan regarding earthquakes. The Ilan basin is a delta filled with unconsolidated Quaternary alluvium with underlying Miocene basement, which may give great effect in the ground shaking characteristic due to its strong impedance contrast. As the area is prone to earthquakes, detailed velocity structure is important for site effect studies. An approach is proposed in this research, to estimate S-wave velocity structure from microtremor data using horizontal-to-vertical (H/V) spectral ratio simulation. The theoretical transfer function of Haskell-Thomson matrix is calculated and compared to the observed H/V spectrum. The velocity profiles derived from Receiver Function (RF) analysis of the nearby Taiwan Strong Motion Instrumentation Program (TSMIP) stations and borehole data from Engineering Geological Database for TSMIP (EGDT) are used as the initial model. Genetic Algorithm (GA) searching method is applied to find the best fit solution of S-wave velocity profiles on each site. Structure maps are derived from the simulation results for top basement structure and the upper three layers. The basement contour derived in this study has similar characteristic with the basement contour presented by previous studies. Due to a dense and even measurement sites in this study, the structure obtained in this study is smoother and more detail. Based on the analysis, it is identified that the thickness of the uppermost soft soil layer reaches 250 m in the coastline in the east of Ilan with velocity between 150 – 400 m/s. This structure also represents the depth of top Alluvium layer whose velocity ranges from 300 – 700 m/s. The depth of Alluvium – Pleistocene layer boundary varies between 10 – 800 m, where the structure tends to gradually gets deeper to the east. The top basement has depth between 30 – 1300 m, with velocity of 900 – 1500 m/s. The PGA and PGV contour derived by Chang (2009) show that the PGA distribution may not related to the structure but rather affected by the source and/or path effect and that the PGV distribution value is controlled by the basement structure.
Appears in Collections:	[Graduate Institute of Geophysics] Electronic Thesis & Dissertation

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