菲律賓塔阿火山地區之地震構造

Abstract

菲律賓塔阿火山位於呂宋島上，馬尼拉市南方60公里處。塔阿火山是座非常活躍的活火山，基於它頻繁爆發的歷史，以及對於鄰近地區數百萬人口的高潛在危害度，塔阿火山被選為世界上15座最危險的火山之一。在2008年三月，我們在塔阿火山區域設置了一個由8個短周期地震觀測站所組成的臨時性地震觀測網，從2008年三月運行到2010年三月，記錄了超過2270個地震事件。在初期資料處理階段，我們利用周遭噪訊交互相關運算法，發現許多測站所記錄的資料具有線性偏移的誤差，並使用線性回歸方法修正所有地震事件的波相到時。地震的波相到時是經由仔細地人工挑選，並使用定位程式HYPO71，利用全球一維速度構造模型AK135測定初始的震源位置。之後我們挑選定位良好的地震，使用程式VELEST進行塔阿火山地區一維速度模型的反衍運算，並利用所得到的一維速度模型，改善地震事件的定位結果，並發現兩個地震群。一個地震群位於塔阿湖西岸呈現線性分佈的特性，另外一個地震群散佈於塔阿火山島東側，深度較淺的位置。之後我們利用改善之後地震定位結果，使用程式LOTOS進行三維速度模型的層析成像反衍計算，從反衍結果中我們發現幾個有趣的構造：在塔阿火山島的西北角，我們發現一個具有高速P波、高速S波，以及低P波-S波波速比值的構造，暗示了一個已經固化的舊有岩漿通道。而在塔阿火山島中央位置，我們發現一個低速S波以及高P波-S波波速比值的構造，可能代表一個大型的熱液儲存庫。另外我們在塔阿湖的西南角，發現一個具有低速P波、低速S波，以及高的P波-S波波值的構造，結合塔阿火山湖西側線性分佈的地震群，我們推測在塔阿火山湖西南角地底下可能存在著一個岩漿庫，並有一條岩漿通道從此處延伸到塔阿湖西北岸，如此的火山構造暗示著未來在遠離塔阿火山島上的歷史火山口的地區，可能會有新的火山口伴隨著岩漿噴出而形成。最後，我們量測地震剪力波分離的現象，藉以研究塔阿火山區域地殼中的方位非均向性，經過嚴格地篩選，我們得到40個有效的剪力波分離的量測結果，其結果顯示塔阿火山島底下的地殼方位非均向性相當複雜，可能反映著由島上眾多的歷史火山噴發口，以及反覆的岩漿入侵造成的地殼膨脹現象所形成的複雜應力非均向性，或是複雜的構造非均向性。

The very active Taal Volcano is situated 60 km south of Metro Manila in the southern part of Luzon Island. Based on its frequent explosive eruptions and high potential hazards to nearby population of several million, Taal Volcano is chosen as one of the 15 most dangerous “Decade Volcanoes” in the world. We deployed a temporary seismic network consisting of 8 stations since March 2008. The temporal network was operated from late March 2008 to mid March 2010 and recorded over 2270 local earthquakes. In the early data processing stages, unexpected linear drifting of clock time was clearly identified from ambient noise cross-correlation functions for a number of stations. The drifting rates of all problematic stations were determined as references to correct timing errors prior to further processing. Initial locations of earthquakes were determined from manually picking P- and S-phases arrivals with a general velocity model based on AK135. We used travel times of 305 well-located local events to derive a minimum 1-D model using VELEST. Two major earthquake groups were noticed from refined locations. One was underneath the western shore of Taal Lake with a linear feature, and the other spread at shallower depths showing a less compact feature around the eastern flank of Taal Volcano Island. We performed seismic tomography to image the 3D structure beneath Taal Volcano using a well-established algorithm, LOTOS. Some interesting features are noted in the tomographic results, such as a probable solidified past magma conduit below the northwestern corner of Taal Volcano Island, characterized by high Vp, Vs, and low Vp/Vs ratio, and a potential large hydrothermal reservoir beneath the central of Taal Volcano Island, characterized by low Vs and high Vp/Vs ratio. Combining the results of seismicity and tomographic images, we also suggest the potential existence of a magma chamber beneath the southwestern Taal Lake, and a magma conduit or fault extending from there to the northwestern shore of Taal Lake. Such magmatic signatures have never been reported in previous studies, suggesting that new eruption centers might be forming in places away from the historical craters on Taal Volcano Island. Finally, to investigate the anisotropy beneath Taal Volcano, we conducted shear wave splitting measurements using data from local earthquakes. With strict selection criteria, 40 valid measurements are remained. The resulting patterns of azimuthal anisotropy are difficult to interpret, and such complexity might be attributed to complex stress-induced anisotropy caused by numerous past eruptions and inflating behaviors induced by episodic intrusion of magma into a shallow reservoir, or complex structural anisotropy caused by crustal media with complicated patterns of prominent structures such as layers, parallel fractures, or lineated fabrics.