利用近、遠震P波與S波資料聯合反演探查北台灣三維速度構造

Abstract

台灣北部正經歷板塊碰撞的轉變時期，從前期的斜向碰撞到後期的弧後張裂，形成複雜且不同階段的岩漿作用與火山構造。其中，大屯火山群緊鄰大台北都會區，是否會造成火山災害長期備受關注。大屯火山群在過去被認為是一座死火山，但在近期的研究證據指出，最近一次的噴發時間為6000年前左右，並且在火山群下方可能有岩漿庫的存在。為了探究大屯火山群下的岩漿系統，在本研究中結合原有觀測網與新布設的北台灣密集寬頻地震陣列－福爾摩沙陣列－的資料來進行三維速度構造的聯合反演。 在資料處理上，本研究將近震及遠震分開進行前處理。近震的部分，使用了集成的地震觀測網，共計671個測站在研究區域內。地震資料經篩選自1991年1月到2017年12月共計60081筆近震事件。為了避免事件分布不均造成反演結果不穩定，本研究進一步將地震事件以半徑兩公里為標準，在三維空間中進行分布均勻化的處理 (Liang et al, 2004; Huang et al., 2014a)，最後事件數量為3587筆。遠震的部分，使用福爾摩沙陣列 (Formosa Array) 的寬頻地震站 (Academia Sinica, Institute of Earth Sciences, 2017)。本研究共蒐集了2018/4-2019/10震央距在30-90度之間、規模大於5.0共371筆遠震事件。在波形的前處理上，P波及S波均經過帶通濾波0.08-0.15 Hz 並利用adaptive stacking method (Rawlinson and Kennett, 2004) 來進行相對走時的計算，在經過多次迭代後留下訊噪比大於5且互相關值大於0.9的事件，P波留下148筆S波留下66筆的事件數量。 本研究使用三維速度模型近、遠震聯合反演法 (Huang et al., 2014a,b; Huang et al., 2015) ，除了利用近震約束淺層速度構造外，並加入遠震資料來增加在深部構造的解析能力，使得速度構造整體可解析的範圍更廣更深。從反演後的結果顯示，相較於前人在過去只使用近震的研究結果 (Huang et al., 2014)，在加入了遠震後明顯地增強了對於北部區域的解析能力。此一新的速度模型中顯示出了大屯火山群下的岩漿庫幾何形狀，為一半徑約10公里的橢球狀，在垂直方向上為狹長狀落於5-25公里深，比前人研究所提出的30公里 (Lin, 2016) 來得淺。 此密集且均勻分布的福爾摩沙陣列提供了一獨特的機會去獲得較高解析的大屯火山群火山系統及北台灣的深部速度構造。根據本研究P波與S波速度構造的結果顯示，此岩漿庫為低P波與S波波速比值，並非如常見因部分融熔岩漿產生的高P波與S波波速比值的構造，暗示可能是由氣體為主的組成。岩漿庫的成像表示大屯火山應是一座活火山。因此，持續的監控大屯火山群各類火山活動與徵候 (例如: 火山微震、地熱活動、氣/液體排放、CO2/SiO2通量、地表變形等。)，對其火山災害的防制及減輕至關重要。

Northern Taiwan has experienced a tectonic transition from oblique collision to post-collisional extension with multiple-stage and complex volcanisms. Among which, the Tatun volcano group (TVG) proximate to the 7-million-population metropolis Taipei has long been a central concern in volcanic hazard. While the TVG has been previously considered an extinct volcano, recent evidences indicated a much younger age of the last eruption event (~6000 years) and the possible existence of magma reservoir beneath the TVG. To examine and unveil the TVG magmatic plumbing system in detail, the local P-wave travel time data and the teleseismic waveform data from a new island-wide Formosa Array Project are combined for a 3D joint inversion. We do the data preprocessing of local events and teleseismic events separately. We use both local earthquake and teleseismic data for a joint inversion in this study. The local earthquake data are compiled from an integrated regional seismic network. A total of 671 stations and 60,081 events are sorted out from January 1991 to December 2017 in the study area. For better conditioning the inversion, a 3-D event grouping method (Liang et al, 2004; Huang et al., 2014a) with a 2 km radius is applied to homogenize the event distribution and reduce the event number to 3,587 in the end. For the teleseismic data, we collect the waveform data of 371 teleseismic events (ML≥5.0) in a distance range of 30°-90° recorded by the newly-deployed Formosa Array (Academia Sinica, Institute of Earth Sciences, 2017) from April 2018 to October 2019. The vertical component of waveform data is bandpass-filtered in 0.08-0.15 Hz to measure the P- and S-wave relative travel times between stations using the adaptive stacking method (Rawlinson and Kennett, 2004). Through the iterative measuring process, only those signal-to-noise ratio (S/N) larger than 5 and the cross-correlation coefficient (CC) greater than 0.9 are retained. After the final visual screening, we obtain a total of 148 events and 66 events for P- and S-wave respectively in the end. The new model derived from the joint inversion with both local earthquake and teleseismic data confirms the existence of the TVG magma reservoir and further unveils its geometry and extent. The magma reservoir is ~10 km in diameter and a vertically elongated shape at the depth range of 5-25 km, shallower than the previously proposed depth of ~30 km (Lin, 2016). The 120 uniformly-distributed broadband stations of Formosa Array provide a unique opportunity to illuminate the Tatun volcanic plumbing system and deep subsurface velocity structure beneath northern Taiwan in greater detail. The unveiled Tatun magma reservoir characterized by low Vp/Vs ratios contains likely volcanic gases rather than partial melts that would result in high Vp/Vs ratios. Though Monitoring all kinds of activity in the TVG (e.g. hydrothermal, microseismicity, gas/fluid chemistry, CO2/SiO2 flux, ground deformation, etc.) as now the Tatun volcano observatory operates is therefore of great importance for volcanic hazard forecasting and mitigation.