臺灣北部大屯火山群底下震波速度異常探討

Abstract

臺灣北部大屯火山群（Tatun Volcano Group, 簡稱TVG）地表地熱現象明顯，根據經驗定義很可能是活火山。大屯火山群臨近臺灣的首都臺北，如果大屯火山群有活動，對臺灣勢必有重大衝擊，因此學者們都希望能確認大屯火山群的活動性，以及底下有無岩漿庫存在。 地球物理學家研究火山區域常會利用區域地震層析法（local seismic tomography），得到地底下震波速度快慢的分佈，推測地下構造的物理特性。然而大屯火山群的地震主要都發生在淺部地殼，利用區域地震層析法無法得到清楚的深部資訊，因此本研究改利用遠距大型地震產生的地震波到時（the arrival time of tele-earthquakes）去觀察地下速度構造，如果震波穿過低速帶，就會有到時延遲的現象。 本研究使用大屯火山觀測站提供的2014年地震波形資料，利用交差相關函數（cross-correlation function）幫助判斷各個測站收到的地震波到時的相對時間，並以最小平方法得到理論到時，比較各個測站收到的地震波到時有無延遲現象以及延遲時間，去判斷該測站收到的震波在傳遞路程中是否受到其他物質影響造成震波到時延遲的現象，以尋找低速帶位置，並進一步利用有延遲現象的波傳路徑尋找波線交集，確認低速帶的位置與深度。 研究結果顯示：從地球對面來的地震（如；智利），以八煙測站為中心，八煙測站與附近測站（如：大油坑測站）收到的地震波到時皆有延遲的現象，顯示低速帶存在該區域下方。而透過其他方向來的地震，利用到時延遲的波線做交集，亦顯示了交集位置位在八煙測站偏西下方，在八煙與大油坑區域，深度約10 km。顯然在八煙、大油坑與附近區域下方存在一個低速帶，可能暗示著有部份融熔物質存在。

The Tatun Volcano Group (TVG) is situated in northern Taiwan, where hydrothermal activity is prevailing. According to the empirical definition of active volcano, TVG can be categorized as composed of active volcanoes. Therefore, scientists are eager to know whether there is a magma chamber beneath the TVG. Since earthquakes occurred in this area are too shallow to get a good local seismic tomography, it’s difficult to estimate possible velocity anomaly of magma chamber and/or hydrothermal system. Instead, we use natural seismic waves generated by large earthquakes worldwide to image the possible velocity anomaly beneath TVG. We systematically compare the differences of first arrival times from tele-seismic earthquakes in 2014 for finding any low-velocity zone within the crust. The waveform data was obtained from Taiwan Volcano Observatory – Tatun (TVO). If the seismic waves pass the low-velocity zone, the arrival-time will be delayed. We used cross-correlation function to get first-arrival times from those earthquake records, and calculated theoretical arrival times through least squares method. Then, we compared “observed” arrival times and “calculated” arrival times for examining possible delay as seismic waves passing through the low-velocity zones beneath TVG if there is any. The results show that arrival times of some of the tele-earthquakes occurred on the other side of the Earth appear to have significant delay, particularly for those recorded at Station Ba-Yan (BY) and near by. In addition, we drew the ray-paths of those earthquake waves with arrival-time delay, and tried to find the intersection of ray-paths. The obtained “crossing area” is under BY and Da-You-Keng (DYK) area, about 10 km deep. This may imply the existence of possible low-velocity zones, which is likely an indication of partial melting materials, existing beneath Stations BY and DYK.