利用周遭噪訊互相關函數探討日本九州地區淺部地殼之尾波衰減變化與潛在噪訊源

Abstract

噪訊地震學是利用連續周遭噪訊的互相關函數來重建類似震波於地球內部傳遞所產生的表面波體波訊號。其用於解析測站下方詳細的速度構造，已成為近20年來最熱門的研究領域之一。日本九州島位於菲律賓海板塊隱沒至歐亞板塊的火山前緣上，島內地震與火山活動頻繁。2016年規模6.5與7.3的熊本前震和主震於兩天內先後襲擊熊本市，並觸發鄰近震央位置的阿蘇火山於兩周內發生兩次小規模的噴發，同年十月再發生一次大規模的噴發。受益於噪訊的連續性，其互相關函數隨時間的演變能直接反映地下介質受到周遭環境因子或地體構造活動事件擾動的影響，故本研究將提取噪訊互相關的尾波能量來探討九州地區地殼衰減特徵以及地震前後衰減性質的變化，並同時分析周遭潛在噪訊源分佈和可能成因以及隨季節的演變。 首先利用日本防災科學技術研究所布放在九州地區內2014年至2017年的Hi-net與F-net地震觀測網垂直分量資料建立超過5000個測站對穩定的噪訊互相關函數，透過謹慎的測試選取表面波之後適當的尾波時窗，進行以2、4、8、15秒為中心週期的窄頻濾波後量測穩定可信的尾波衰減值，並將各週期量測到的衰減值均勻分佈於測站對之間的路徑以建立九州地區地殼衰減性質的側向變化，同時利用反投影和波束成形法對九州地區內部和外圍的潛在噪訊源進行定位與成因分析。衰減成像顯示九州上部地殼構造在側向與垂直向上都具備強烈的異質性(heterogeneity)，主要是受到區域內大量的活動斷層與火山裂隙存在液體或岩漿導致。另外我們也比較2016熊本地震前、後10個月噪訊互相關函數的平均疊加所建立的衰減構造特徵，發現在地震發生的斷層破裂帶與火山噴發位置附近在事件後的衰減有轉趨較強的傾向。而噪訊源定位結果指出九州地區全年次微震訊號主要來自北太平洋中心，但位置方向和強度會隨季節產生明顯變化，主微震訊號則來自九州西側的東海陸棚，除此之外，另有一獨立訊號源定位在阿蘇火山附近，其噪訊能量極有可能與當地發現長週期火山顫動活動有關。

Since ballistic waves (P, S, surface waves) extracted from cross correlation function of continuous ambient noise recorded at two stations have been proved equivalent to earthquake-generated signals, ambient noise seismology has become one of the most popular research disciplines in the last two decades. Kyushu island of Japan is located on the volcanic front where the Philippine Sea Plate subducts beneath the Eurasian Plate causing frequent earthquakes and volcanic activities on the island. In April of 2016, the Kumamoto foreshock and mainshock with magnitudes of 6.5 and 7.3 hit Kumamoto city successively within two days, triggering nearby Mount Aso Volcano erupted twice within two weeks, followed by a major eruption in October of the same year. Benefiting from the continuity of the noise, the evolution of its cross-correlation function over time can directly reflect the change of the subsurface structures being disturbed by environmental factors or tectonic events. In this study, we thus attempt to estimate the decay of coda energy from the interstation noise cross-correlation function (NCF) to investigate the frequency-dependent attenuation properties of the crust in the Kyushu area and their potential changes impacted by the quake damage and volcanic eruption. We also explore the distribution and possible physical causes of noise sources in and around Kyushu and their seasonal changes. Using the vertical-component records of the Hi-net and F-net seismic networks in Kyushu from 2014 to 2017 provided by the Japan Institute of Disaster Prevention Science and Technology, we construct daily NCFs of more than 5,000 station pairs. Selecting appropriate coda window following direct surface waves through thorough and careful evaluation, we measure the lapse-time decay of the selected coda energy after narrow-band filtering with the central periods of 2, 4, 8, and 15 s to obtain reliable and stable coda Q (Q_c) value for individual station pairs. The resulting Q_c values at each period are uniformly distributed on the 0.1ox0.1o cells intercepted by the great-circle ray paths between their respective paired stations. The lateral variations of attenuation properties in the crust are mapped by averaging all the Q_c values assigned within the cells. In addition, the back-projection and beamforming methods are employed to locate and analyze the potential noise sources inside and outside the Kyushu area. Results indicate that the attenuation property in upper crust of Kyushu exhibits strong lateral and vertical heterogeneity, which may attribute to the presence of fluids or magma in a large number of faulting cracks, fractures, and volcanic fissures in the region. The coda extracted from the stacks of the daily NCFs over 10-month period before and after Kumamoto mainshock are utilized to characterize the Q_c variations impacted by the earthquake, indicating that the attenuation tends to become stronger after the quake near the fault rupture zone and volcanic eruption site. The backprojection and beamforming results show that the dominant noise in the secondary microseism band (2-5 s) in the Kyushu area throughout the year originates from the center of the North Pacific Ocean, but the location, direction and intensity will change significantly with the seasons. In contrast, the relatively weaker noise in the primary microseism band comes from the East China Sea Shelf on the west side of Kyushu. Moreover, an additional noise source is located near the Aso Volcano, consistent with unusual large-amplitude signals with a negative time-lag moveout of about -3 km/s persistent over the period of the long-period volcanic tremor activities found there.