隱沒帶地震震源參數分析及中美洲深部地幔D”不連續帶之構造探究

The source characteristics for subduction zone earthquakes and The nature of D” discontinuity beneath Central America

柯彥廷(Yen-Ting Ko)

指導教授 ： 郭本垣

共同指導教授 ： 洪淑蕙(Shu-Huei Hung)

國立臺灣大學/理學院/地質科學研究所/博士(2014年)

https://doi.org/10.6342/NTU.2014.01128

摘要

第一部分：
有效率的將波傳路徑之構造衰減效應與震源參數區分，有賴進一步的逆推技巧來實踐。在本研究中，我們發展一套全新的方法-群組地震法。主要概念是將周圍的地震視為群組，其群組地震至單一測站之路徑衰減效應Q視為相同，並且限定群組地震中每個單一地震僅能存在一個拐角頻率。這樣的假設，不僅符合震源基本假設，更大量降低逆推中需要推求的未知數。由於我們的逆推問題為高度非線性的關係，因此本研究採用模擬退火法來尋求最佳解。我們將群組地震法運用至日本隱沒帶70-150公里的地震，透過一系列的分析，我們的結果不論是在震源參數或是波傳構造的衰減參數都優於以往的方法。我們發現當地震波路徑大多穿過隱沒帶弧前地幔時，其Q值大約都在600以上，顯示震波受到構造之衰減效應較小。反之，當地震波穿越火山島弧或是弧後地幔時，其Q值大多落在300以下，顯示地震波受到波傳構造影響之衰減效應較強。另一方面，由我們的拐角頻率結果顯示，大部分的地震符合理論的地震自我相關性。然而不同震源深度，對於結果存在極大的差異。藉由Madariaga提供的模型參數計算出符合我們資料最佳的應力降為21.9±6.9 MPa，相較於前人研究中利用較淺源的地震所推算的最佳應力降為1.4±1.1 MPa來的大。
第二部分：
高頻率發生與廣泛分佈在日本隱沒帶的地震群提供我們珍貴的機會進一步探討震源參數隨深度變化的特徵。本研究運用群組地震法(CEM)，有效的減少逆推過程中的未知參數，進而確保逆推之結果的可性度。本研究中擷取813個發生在日本隱沒帶的地震，其深度範圍涵蓋0-150公里，地震規模包含3.5-6.5。根據震源深度，我們將地震區分為3個區塊(0-25公里、25-50公里、50公里以上)，分別對其計算靜態與動態之震源參數。由於在各個深度分區其周圍壓力與溫度、抑或應力場條件之差異，我們預期可能對於地震破裂機制產生不同程度的影響，進而影響地震破裂行為。從我們結果中顯示，拐角頻率，應力降，輻射能量等參數在每個深度分區皆遵照地震自我相關的理論公式分佈。但是，若是比較第二與第三分區的量測結果，各個參數間的變異性相當大。我們認為震源參數具有隨深度變化的趨勢，暗示地震破裂行為在各個深度分區的差異。
第三部分：
藉由剪力波三重波相的分析，發現在中美洲與其鄰近區域深部地幔存在一速度部連續面 (D”)，其深度僅在核慢邊界(CMB)上數百公里的位置。但是此不連續面是否廣泛存在於地球深部及其側向深度變化與不連續面的速度變化等皆為未解之迷。本研究最主要的目標即為討論D” 不連續面空間尺度的深度變化與其速度變化的關係。我們主要藉由USArray提供的豐富測站為依據，利用順推模擬的方式逐步尋找最佳的模型。我們將大範圍分佈的測站，利用方位角區分為數十條線性陣列，並且分析每條陣列測站從南美隱沒帶所接收到的地震波形資料。 藉由Scd-S走時殘差，我們可以有效的定義D”不連續帶深度位置，並且透過ScS-S走時殘差、ScS/S相對震幅、Scd/S相對震幅和波型擬合程度的分析，讓我們有機會對於D”不連續帶速度變化有更好的解析能力。經由我們的分析發現，D”不連續帶較深的位置都恰好對應剪力波波速較慢的區域，反之亦然。我們認為中美洲與鄰近區域之深部地幔之D”不連續帶存在明顯的側向變化，可能與古老隱沒至深部地幔的隱沒板塊有密切得關係。

關鍵字

震源參數 ； 隱沒帶地震 ； 震源自我相關性 ； 群組地震法 ； 剪力波三重波像 ； 波形模擬 ； D”不連續面 ； 隱沒板塊與熱柱交互關係

並列摘要

Part I.
An improved inversion technique is needed to effectively separate the frequency dependence of the source from the intrinsic attenuation of the medium. We developed a cluster-event method (CEM) in which clusters of nearby events, instead of individual events, pair with stations to form the basis for measurements of Q value and corner frequency (fc). We assume that the raypaths from one cluster to a station share an identical Q while each event in the same cluster is allowed for only one fc in the inversion process. This approach largely reduces the degrees of freedom to achieve a robust inversion. We use an optimization algorithm of simulated annealing to solve the non-linear inverse problem. The CEM was applied to events at 70 – 150 km depths in the Japan subduction zone recorded by F-net. We show that the method proposed here leads to better constraints on both source parameters and attenuation. The resultant Q’s in the mantle wedge increase from lower than 300 beneath the arc and back-arc to greater than 600 in the fore-arc region. The fc’s satisfy a self-similar scaling relationship with seismic moment of Mo ~ fc^-3 with a best-fit stress drop of 21.9±6.9 MPa in Madariaga’s form. This contrasts to the stress drop of 1.4±1.1 MPa for a global data set composed of prior measurements for crustal events. The results of this study agree with results from previous studies, except with an upward deviation due to higher corner frequencies and stress drops.
Part II.
Widespread and high rate of intermediate-large earthquakes in the Japan subduction zone provide an opportunity to systematically investigate depth-varying characteristics of earthquake sources. We utilize the cluster-event method to ensure a robust inversion for source and path parameters for 813 events spreading over a depth range of 0 – 150 km and seismic moments 4 orders of magnitude different. Static and dynamic source parameters are measured and compared for three depth regions: I, 0-25 km; II, 25-50 km; and III, > 50 km, where distinct rupture mechanisms may dominate. Our resulting corner frequency, stress drop, and radiated energy provide strong evidence of depth-dependent characterizations of earthquake sources. The estimations of higher corner frequency and stress drop at greater source depth with sufficient of short-period energy radiation may be attributed to earthquakes occurring on rougher and more immature faults.
Part III.
We constrain topography, impedance contrast, and shear velocity gradient of the D” layer beneath the Caribbean region using Scd-S and ScS-S times, Scd/S and ScS/S amplitude ratios, and the full waveforms between S and ScS recorded by the USArray. Our results reveal an east-west trending, V-shaped undulation of D” topography strongly correlated with shear velocity anomalies within the D” layer: The D” discontinuity elevates from the lowest point beneath the north South America by 80-180 km over a lateral distance of 600 km, while the shear velocity anomalies increase in the same trend. This can be explained by the fluctuation of temperature and post-perovskite phase boundary in the environment where a horizontally lying slab, probably from the ancient Farallon plate, is trapping heat and substantially warmed at its center.

並列關鍵字

source parameters ； subduction earthquakes ； earthquake self-similarity ； cluster event method ； shear wave triplication ； waveform modeling ； D” discontinuity ； slab-plume interaction

參考文獻

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