| 摘要: | GPS的連續位移記錄可以提供許多地表運動資訊，如平均速度、季節性週期振盪、同震位移等，其中，暫態訊號（transient signal）能夠反映地殼在受到短期應力作用時所產生的變形行為，例如震後變形、火山活動或地下水位變化等。因此，將這些變形訊號從觀測數據中分離出來，以提供後續的模式分析，是測地學中重要的課題。在這項研究中，我們挑選了2013至2021年間位於大屯火山區的6個GPS測站記錄，旨在檢測可能存在的暫態變形訊號。首先，我們使用最小二乘法移除時間序列中的長期運動趨勢和已知的錯移，再使用Kondrashov and Ghil (2006)提出的奇異譜分析（singular spectrum analysis, SSA）插值方法來處理缺失的數據。隨後，我們採用了多通道奇異譜分析方法（Multichannel SSA）來提取這些測站在時間和空間上相關的暫態位移事件。研究顯示，在2015年6月至2020年4月期間存在兩個明顯的暫態運動時期，分別是2015年6月至2016年10月期間的垂直下沉和水平收縮以及2015年6月至2016年10月期間的垂直抬升和水平擴張，可能與當地的火山活動相關。因此，我們將同時段的暫態位移結果與Pu et al. (2021)根據大油坑地區的微震破裂特性，以及噴氣中的總硫含量與二氧化碳比值隨時間變化所提出的模型進行了比較。研究發現，在2015年10月至2016年7月期間淺部正斷層地震和總硫含量與二氧化碳比值的顯著增加，可能表示熱液和氣體向上運動。因此，我們推測第一個暫態時期可能是由於大量的流體和氣體流出所導致的地殼解壓效應。此外，對於三個長期運行的GPS測站（自2006年以來），MSSA分析顯示，大屯火山區在2008年至2015年之間亦呈現出地表抬升和下沉的類似模式，但幅度較小。因此，利用MSSA可以有效分析該區域的GPS時間序列，作為未來監測大屯火山區地表活動特性的方法。然而，大部分的GPS測站集中在火山活動較為活躍的西半部區域，但東半部區域的YM03測站亦檢測到顯著的位移量，或許也暗示著東半部區域可能也會有火山活動所造成的地表變形。;GPS position time series can be utilized as an efficient method for monitoring surface displacement changes over time. In general, these time series can capture various ground motions such as long-term variations, short-term transient displacement, offsets, and seasonal signals. Differentiating between these different signals within GPS time series is crucial for comprehending the underlying geophysical or environmental processes. In this study, we selected six GPS stations located in the Tatun volcanic group (TVG) in northern Taiwan from 2013 to 2021 to explore potential spatially and temporally correlated transient deformations. Initially, we removed the secular trend and offsets using the least square method and addressed missing data using the singular spectrum analysis imputation method proposed by Kondrashov and Ghil (2006). Subsequently, the multichannel singular spectrum analysis (MSSA) was applied to extract the common transient displacements across these stations. Our analysis revealed two distinct periods of transient motion between June 2015 and April 2020: vertical subsidence and horizontal contraction from June 2015 to October 2016, and vertical uplift and horizontal expansion from June 2015 to October 2016, likely associated with local volcanic activity. To validate our findings, we compared our results with a hydrothermal fluid activity model proposed by Pu et al. (2021), which relied on seismic data and the ratio of surface sulfur emission (S_t) to {\rm CO}_2 at Dayoukeng fumarole from 2014 to 2017. We suggested that the notable increase in shallow normal faulting earthquakes and the S_t/{\rm CO}_2 ratio between October 2015 and July 2016 might indicate the upward movement of volcanic fluids and gases. Therefore, we infer that our first transient period, characterized by vertical subsidence and horizontal contraction from June 2015 to October 2016, may correspond to the regional crustal decompression effect resulting from the substantial outflow of hydrothermal fluids and gases. Additionally, MSSA analysis of three long-operated GPS stations (back to ~2006) revealed similar patterns of ground uplift and subsidence in the TVG from 2008 to 2015, albeit with smaller amplitudes. Utilizing MSSA effectively analyzes the GPS time series in this region as a method for monitoring future volcanic surface activity characteristics. However, most GPS stations are concentrated in the western region, and the significant displacement at station YM03 in the eastern region may suggest potential surface deformation due to volcanic activity in the eastern region. |
