以噪訊干涉技術監測地殼地震波速度變化

Noise-based monitoring on crustal seismic velocity variations

馮冠芙(Kuan-Fu Feng)

指導教授 ： 黃信樺

共同指導教授 ： 吳逸民(Yih-Min Wu)

國立臺灣大學/理學院/地質科學研究所/博士(2022年)

https://doi.org/10.6342/NTU202200093

摘要

地震噪訊干涉法是很有潛力的研究技術，透過分析地殼地震波速度的時序變化，可以進一步探究地殼隨時間的行為表現。然而，地殼地震波速度的時序變化會受到地殼內部（構造活動）與外部（環境變動）作用，反映複雜的地殼行為，而這樣的行為反應更是因地而異。本研究分別分析了在活躍火山地區（夏威夷基拉韋厄火山）與活躍造山帶地區（臺灣）的地殼地震波速度隨時間變動的行為表現，分別探討2018年基拉韋厄火山噴發前岩漿侵入地殼的過程，以及臺灣地殼對於環境因子影響的季節性反應。
在火山的案例中，為了瞭解基拉韋厄火山在2018年噴發事件前岩漿侵入地殼的過程，使用噪訊干涉技術搭配雙站交相關函數的時序觀測，分析12個寬頻地震站近一年半的垂直向連續地震紀錄，並透過頻率相依的特性，以三個頻段（0.3-0.6、0.6-0.9、0.9-2.0赫茲）來分析該區地震波速度的時序變化。藉由地震波速度變化頻率相依的特性，發現地震活動相關的速度變化和與岩漿相關的速度變化發生在不同的深度。透過速度變化在時空分佈上的分析，結果揭示了三個時期的岩漿侵入過程，最初期的岩漿侵入活動可能於火山噴發至地表前六個月就已經開始。而針對臺灣的案例，同樣使用地震噪訊干涉技術，分析臺灣寬頻地震網自1998年至2019年的長期連續紀錄。為了直接與鄰近地震測站的氣象測站資料進行比對，選用單站異軸向的交相關函數法，分析頻段0.1-0.9赫茲的訊號。地殼地震波速度變化結果顯示了數起同震震波速度下降以及明顯的年週期訊號。年週期的地震波速度變化有明顯的地域特性，經由與各項環境因子（雨量、氣溫、氣壓、風速）的比對，發現降雨所造成的地殼內液壓變化是主要影響地殼地震波速度變化的原因，其他氣象因子的影響則相對次要。了解與修正降雨對於地殼地震波速度變化影響的同時，能讓地震相關的速度變化訊號變得更加清楚容易辨識，有助於進一步更精確的分析。
以上述兩個研究案例為基礎，本研究計畫建立臺灣地殼地震波速度變化監測系統。為了即時處理與分析資料，提出移動參考法，測試與展示運用此方法進行近即時監測的可能性。在透過一系列的時間與空間解析力的測試與考量後，現階段的監測系統預期將有49個地震站，其中25個來自臺灣寬頻地震網、6個來自中正大學西南部觀測站、以及8個來自氣象局。以每小時為計算單位運行。期望未來可以透過多計算核心進行循序分散式運算的方式，朝向近即時速度變化監測的目標邁進。

關鍵字

地震噪訊干涉 ； 地殼地震波速度變化 ； 岩漿侵入過程 ； 基拉韋厄火山 ； 臺灣 ； 監測系統

並列摘要

Seismic noise interferometry is a powerful approach to studying temporal crustal behaviors through continuously measuring crustal seismic velocity changes (dv/v). However, the interpretation of such dv/v variations is not straightforward because both internal (tectonic/magmatic) processes of the crust and external (environmental) factors could affect dv/v simultaneously. The relationship between these potential factors and crustal dv/v is complicated and varies from place to place globally. In this dissertation, we investigate crustal dv/v variation in an active volcano, Kilauea volcano, and an active orogenic belt, Taiwan. In these two case studies, the pre-eruptive intrusion processes prior to the 2018 Kilauea eruption and the seasonal dv/v responses to the environmental effects in Taiwan are investigated, respectively.
In the case study of the Kilauea volcano, seismic noise interferometry accompanied with frequency-dependent analysis is used to investigate precursory magmatic intrusion process before the May 2018 eruption. The 1.5-year vertical-component seismic data from 12 broadband seismometers distributed around the summit and along the East Rift zone are analyzed. Daily correlation functions have been constructed to compute dv/v variations in three frequency bands (0.3-0.6, 0.6-0.9, and 0.9-2 Hz). The frequency-dependent dv/v responses clearly show different origins related to local earthquakes and magma activity at different depths. Temporal and spatial distribution of dv/v shows three periods of pre-eruptive magmatic processes, which began almost a half year before the eruption. In the case study of Taiwan, to investigate the long-term crustal dv/v behaviors, the seismic noise single-station cross-component (SC) method is applied to continuous seismic data of 15 broadband stations from 1998 to 2019. The daily SC functions are constructed to compute dv/v variations in a frequency band of 0.1 to 0.9 Hz. The dv/v results reveal not only co-seismic velocity drops associated with regional moderate-to-large earthquakes but also strong seasonal variations. Through a series of spectral and time-series analyses with the nearby weather data, this study suggests that the rainfall-induced pore-pressure change plays a predominant role in driving the dv/v seasonality. The effect of other factors is relatively local or secondary. This study also further demonstrates how correcting such rainfall effects can improve the detection accuracy of internal processes related to crustal damage caused by earthquakes.
Based on the success and experience from the two case studies, this study is now establishing a noise-based crustal dv/v monitoring system in Taiwan. To handle the real-time data transmission and processing, we introduce a moving reference method and demonstrate its viability and performance compared to traditional fixed reference method. Throughout a series of spatiotemporal resolution tests, the monitoring system will consist of 49 stations, including 25 from the BATS, 6 from the CCUSW, and 8 from the CWB at this stage and run at hourly basis for detecting crustal dv/v. The system will be utilized for in-sequence distributed computing with multiple cores to further increase the temporal resolution toward a near real-time monitoring system in the future.

並列關鍵字

seismic noise interferometry ； crustal seismic velocity changes ； magmatic intrusion ； Kilauea volcano ； Taiwan ； monitoring system

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