大屯火山群單頻波訊號之搜尋與分析

Abstract

大屯火山群位於臺北盆地北方，是世界各國距離首都最近的火山之一。林等人 (Lin et al. 2016) 利用S波陰影和P波延遲兩個現象，推估大屯火山群下方可能依然存在著活躍的岩漿庫，為此，進一步探究該地區的地殼構造細節及監測大屯火山群的活動等相關研究一時成了臺灣地球科學界的顯學。 除了火山構造地震事件，火山區域的熱液或岩漿活動所伴隨的應力變化可在岩隙間引發特有的單頻訊號，為了確認這些外形如螺絲釘或水滴狀的訊號，其強度與出現頻率是否可作為火山活躍度的重要指標之一。本研究利用單頻訊號在頻率域的特性設計一套自動搜尋系統，對新近佈設的福爾摩沙陣列 (Formosa Array) 內14個地震觀測站約九個月 (自2018年4月至2019年3月) 的垂直向短周期連續紀錄，在1.2~9 Hz區間進行完整搜尋。 以60秒作為單次的搜尋時間窗長度，本研究共偵測到超過五十萬筆的單頻訊號事件。除了螺絲釘或水滴狀等單獨事件，絕大多數 (~99%) 為強度微弱的連續訊號 (tremors)。初步的分析結果顯示：一、各測站均可測得重覆出現的單頻訊號，且其特徵頻率在不同測站有明顯差異，顯示各測站所測得的訊號均來自近距離震源，且強度微弱，無法被其他站所偵測到，故推測本區域此類訊號的來源皆是靠近淺部地層，並沒有辦法與岩漿庫之活動產生足夠的相關性，較無法作為火山活躍度的重要指標。二、統計各個測站之搜尋結果及時頻圖做堆疊分析，單頻訊號事件在時間上的出現頻率具有明顯的日週期，這顯示偵測此類訊號的門檻或許會受到人為活動影響。

Recent studies have suggested that there is potential active magma chamber underneath the Tatun Volcano Group (TVG), which is located at the northern tip of Taiwan, and is only about 15 km away from the capital city Taipei. To better access the proposed possibility, we conduct a full search for the single frequency seismic signals, which are closely related the activities of the plumbing system in the volcanic area, using the vertical continuous data (04/2018-03/2019) recorded by 14 representative stations of the recent deployed Formosa Array. Using 60 second length as the single searching time window, more than 500,000 signals meet our selection criteria in the frequency band from 1.2 to 9 Hz. Besides the characteristic single events, tornillos (Spanish for “screw”) and monochromatic signals, the vast majority of the detected events are weak continuous signals, i.e., single-frequency tremors. Our preliminary analysis of the auto-picking results demonstrates that: (1) Repeated single-frequency waves are observed at each station, and the characteristic frequency is obviously different from stations to stations, suggesting these events are excited by very weak near-by sources. (2) Stacking spectrogram analysis shows that there is strong daily cycle of the excitation rate and intensity of the single-frequency events, implying that the detection of these events might be infected by human activities.