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標題: | 南加州地區地震b值與Vp/Vs關係之研究 A study of the relationship between earthquake b value and Vp/Vs in Southern California |
作者: | Ya-Chi Liu 劉雅琪 |
指導教授: | 吳逸民(Yih-Min Wu) |
關鍵字: | 地震活動不同規模大小之頻率,地震b值,Vp/Vs值,孔隙壓力,南加州, earthquake-size distribution,earthquake b value,Vp/Vs ratio,pore pressure,Southern California, |
出版年 : | 2020 |
學位: | 碩士 |
摘要: | 地震活動不同規模大小的頻率 (即地震b值) 最主要受到地殼不同應力區的應力變化而改變;高地殼應力能導致較高的大地震發生頻率 (即b值較低),反之亦然。許多研究指出仍有其他次要物理因素能影響地震b值,例如孔隙壓力。根據地殼淺部注水實驗結果已知於地殼小於5公里處,地震b值與孔隙水壓兩者之間可同時呈現正比或反比關係,然而在較深部地殼,孔隙水壓能如何影響地震b值目前尚不明瞭。近期研究發現台灣地區地殼內地震b值與Vp/Vs呈現反比相關,經由實驗室岩石三軸應力試驗結果得知Vp/Vs可以反映地殼孔隙壓飽和度變化,這項研究提供一個於整個地殼尺度觀察地震規模大小分布與孔隙壓力變化的先例。基於南加州地區擁有高品質、高密度的地震觀測資料以及較完整的地震目錄,本研究使用上述研究相同的方法檢驗南加州地區是否同樣觀察得到地震b值與Vp/Vs之間的反比關係。本研究使用南加州地震觀測網 (SCSN) 1981年4月至2017年12月的地震資料,將地震資料去除地震群集以回復到背景地震值,地震b值的計算使用最大似然法以0.1° × 0.1° × 10公里格點中心的半徑20公里地震事件計算。Vp/Vs計算使用SCSN以及Harvard的速度模型資料,重新空間取樣至與地震b值相同大小的格點,並將這兩者觀測資料空間上重疊者進行比較。研究結果顯示南加州地區地震b值與Vp/Vs兩者反比關係沒有台灣地區的觀察明顯,南加州地區Vp/Vs分布範圍比起台灣地區集中緊密,b值相對應分布變化也因此較小。本研究發現南加州地區地殼0-5公里範圍,b值與Vp/Vs有負線性相關,地殼5-10公里範圍則為b值與Vp/Vs線性關係轉換過渡帶,地殼10-20公里範圍b值與Vp/Vs呈正線性相關。本研究推論南加州地區地殼0-5公里範圍內b值與Vp/Vs負相關較弱與線性斜率較緩的原因,可能與其地殼含水量較低、軸差應力較低、孔隙水壓變化較小有關。此結果有助於了解不同規模天然地震發生頻率的變化與物理機制。 Earthquake-size distribution (b value) is reported to be governed by crustal stress which depends on different stress regimes (faulting types). Several studies suggest that b value can be influenced by secondary factors locally, e.g., pore fluid pressure. Injection experiments have shown that b value can both be directly and inversely proportional to pore pressure in shallow crust (< 5 km). However, how pore pressure changes affect b value in general and at deeper depths is still unclear. Recently, b values in the whole curst of Taiwan orogen are founded to be inversely proportional to pore pressure, which is estimated from seismic Vp/Vs ratios. Also, degree of crustal pore fluid saturation can be evaluated from Vp/Vs ratio that is supported by laboratory rock experiments. In this study, we apply the same approach in Southern California examining whether we can see the same relationship in the crust, where high-quality seismic observations exist. We declustered the seismicity during Apr. 1981 to Dec. 2017 from the Southern California Seismic Network (SCSN) for calculating crustal b values under the state of background seismicity. We estimated b values in a radius of 20 km from the center of 0.1° × 0.1° × 10 km grids using the Maximum likelihood approach. Then we estimated seismic Vp/Vs ratios in the same grids as b values for comparison which are derived from SCSN and Harvard. We noticed that both of the b value and Vp/Vs ratio distributions in general are denser than the case of Taiwan. In Southern California, the b values decrease with increasing Vp/Vs ratios in the crustal range of 0-5 km, conversely, the b values increase with increasing Vp/Vs ratios at the 10-20 km depths. There is a transition of the linear relationship between b values and Vp/Vs ratios in the 5-10 km depths. Overall, the negative correlation between b value and Vp/Vs ratios founded in the 0-5 km depths is weaker and much shallower than that has been observed in Taiwan. We inferred that the Southern California crust is mainly composed of igneous and metamorphic rocks with low fluid content, and this condition cause less pore pressure effect while the crust is under low differential stress. Our results contribute the understanding of variation in natural earthquake-size distribution and the physical mechanisms. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/65671 |
DOI: | 10.6342/NTU202000401 |
全文授權: | 有償授權 |
顯示於系所單位: | 地質科學系 |
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