利用P波接收函數法探討台灣北部大屯火山群地殼速度不連續面

Crustal Seismic Discontinuities Beneath The Tatun Volcano Group in Northern Taiwan from P Receiver Functions

鄭至為(Chih-Wei Cheng)

指導教授 ： 陳勁吾

國立臺灣大學/理學院/海洋研究所/碩士(2017年)

https://doi.org/10.6342/NTU201703185

摘要

大屯火山群 (Tatun Volcano Group) 位於臺灣最北端區域，因沖繩海槽之前緣向西延伸擴張造成北台灣由過去擠壓應力環境轉變成拉張環境，形成造山崩解而噴發熔岩流之複式火山，其主要噴發年代為80至20萬年前，約至10萬年前噴發活動漸趨停止。根據鑽井資料分析，雖有多處火山噴發，但為同一岩漿來源。此區域之微震集中於深度小於5 公里的淺部，一般認為與熱水或氣體通過重力斷層、裂隙的造成壓力突變有關。近期研究發現大屯火山群東北方底下 ~20公里有岩漿庫。我們分析大屯火山觀測站 (Taiwan Volcano Observatory at Tatun) 寬頻地震站陣列的遠震波形資料，進一步探討大屯火山及鄰近區域底下之地殼構造。我們利用遠震P波接收函數法 (receiver functions) 約制地殼中的速度不連續面及莫荷面。我們分析2012至2013年間，震央距30至90度，規模大於6.0之遠震事件，由時間域迭代法 (iterative deconvolution) 計算接收函數。觀察Ps轉型波相及複反射訊號的到時與波形特徵。結果發現大屯火山區域莫荷面深度約為30公里，由西向東有漸深的趨勢，至東南方五分山測站一帶又變淺。Vp/Vs值於七星山及大屯火山群東北方約1.76至2，推測地殼溫度偏高，在大屯火山群東北方區域約20至25公里有一明顯低速層，推測是火成岩脈之部分熔融體。此外，大屯火山群東北方與東南方地殼可能存在非均向性，暗示地殼仍受到熱的作用，而在東南方地殼中有傾斜不連續面的訊號，應與火山活動有密切關聯。

關鍵字

大屯火山群 ； 接收函數 ； 速度不連續面 ； 低速層

並列摘要

The Tatun volcano group (TVG) in northern Taiwan was formed during the last 0.8 to 0.2 Ma thought to be associated with the turning of the regional stress from compression to tension between the arc-continent collision and subduction. The cause of eruption has been thought to mainly due to extensional collapse. The last is eruption estimated to occur ~6000 years ago. Post volcanic activities such as micro-earthquakes and gas emissions are still abundant and active in this region, especially near Tayoikeng areas. It has been long debated whether a magma chamber still exists beneath TVG. A recently study, proposed that a low-velocity zone, likely a magma reservoir, may reside in the lower crust beneath northern coast of Taiwan, to the northeast of TVG. To better image the crustal structure beneath TVG and its adjacent regions, we analyze waveform data recorded at 16 TVO (Taiwan Volcano Observatory at Tatun) dense broadband stations from 140 magnitude >6.0 teleseismic events during 2012-2013, generating receiver functions (RF) by time domain iterative deconvolution to detect seismic discontinuities. Our results show that the crustal structure in this region is heterogeneous and anisotropic likely due to the volcanic activities in the crust and cracks in the subsurface. The Ps converted phases are manifest at 3-4 s from most back-azimuth, indicating the presence of Moho discontinuity. The H-κ results confirm that the Moho is at 25-30 km across the region with increasing depth toward northeast, and the Vp/Vs ratio is 1.76-2 around ChiShin volcano. We further detect a low velocity zone layer to the northeast of TVG at ~22 km depth with thickness of 5 km, likely associated with volcanic activities.

並列關鍵字

Tatun Volcano Group ； receiver functions ； crustal seismic discontinuities ； low velocity zone

參考文獻

廖文豪 (2007) 大屯火山群長週期單頻訊號完整搜尋之結果與隱示。國立台灣大學地質科學研究所碩士論文，共65頁。

Ammon, C. J., G. E. Randall, and G. Zandt (1990), On the nonuniqueness of receiver function inversions, Journal of Geophysical Research, 95(B10), 15303, doi:10.1029/JB095iB10p15303.

Bannister, S., C. J. Bryan, and H. M. Bibby (2004), Shear wave velocity variation across the Taupo Volcanic Zone, New Zealand, from receiver function inversion, Geophysical Journal International, 159(1), 291-310, doi:10.1111/j.1365-246X.2004.02384.x.

Belousov, A., M. Belousova, C. H. Chen, and G. F. Zellmer (2010), Deposits, character and timing of recent eruptions and gravitational collapses in Tatun Volcanic Group, Northern Taiwan: Hazard-related issues, Journal of Volcanology and Geothermal Research, 191(3-4), 205-221, doi:10.1016/j.jvolgeores.2010.02.001.

Bianchi, I., N. Piana Agostinetti, P. De Gori and C. Chiarabba (2008), Deep structure of the Colli Albani volcanic district (central Italy) from receiver functions analysis, Journal of Geophysical Research, 113(B9), doi:10.1029/2007jb005548.