Seismic Structure beneath Taal Volcano, Philippines
Date Issued
2014
Date
2014
Author(s)
You, Shuei-Huei
Abstract
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.
Subjects
塔阿火山
一維速度構造
三維速度層析成像
剪力波分離
Type
thesis
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