以東沙潟湖海底地震儀研究地殼速度不連續面與周遭噪訊源

Abstract

東沙環礁座落於南海北緣陸棚上，為一直徑約25 公里的巨大圓形礁體。由於環礁內水深較淺(~22 公尺)且孕育多樣珍稀生物，故不利進行主動源海上震測，使東沙環礁的地體構造始終是個未解之謎。本研究為解析其地體構造，自2017年3月至9月建立一直徑約15 公里的地震儀陣列，包括埋於潟湖內海床的四顆海底地震儀及一座中研院架設的島上寬頻地震永久站，為因應測站少、陣列直徑小的限制，我們使用遠震P波接收函數法解析環礁下地殼速度不連續面深度及速度。模型正推結果顯示下部地殼的P波速度為每秒6.9 ± 0.1 公里、厚度16 ± 1 公里，綜觀前人研究，在南海北緣的下部地殼普遍存在高速層且厚度大於10 公里，分佈多鄰近東沙環礁，證實埋於潟湖內海底地震儀資料所提供資訊可用以解析環礁下的一維地殼構造。且推測此高速下部地殼可能為岩石圈拉張導致荷重減輕誘發岩漿厚底作用的結果，如北大西洋鄰近Hatton Bank的被動大陸邊緣。然而雖有岩漿侵入證據卻未發現較厚的海洋地殼(> 15 公里)及向海傾斜反射層等被動大陸邊緣富含岩漿的指標形貌，符合IODP團隊2018年的探查結果，指出南海被動大陸邊緣不同於典型富含岩漿或岩漿匱乏的型態。此外，本文從兩兩測站的連續資料(共101天)計算交互相關函數，利用震幅在正負時窗的不對稱性，解析其噪訊源。結果顯示主要來源為環礁西側的兩個主要交換水道，次要噪源在春季呈東北向，但夏季轉西南向，可能與季風、內波折射或與更加複雜的交互作用相關連。

Dong-Sha Atoll is situated at the continental shelf of the northern margin of the South China Sea (SCS), with about 25 km in diameter. The lagoon's deepest water depth is ~22 m; therefore, it is too shallow to do active seismic imaging. Due to the limited observations for the crustal structure below, the tectonics of Dong-Sha Atoll is still poorly understood. This study utilized the seismic data recorded in 4 ocean-bottom seismographs (OBSs) buried beneath the Dong-Sha lagoon and one BATS- station on the Dong-Sha Island to image the crustal structure. With this new seismic array ~15 km in aperture, for the first time, we can provide local constraints on the velocity model beneath Dong-Sha Atoll and estimate the directionality of the ambient seismic sources in the study region. Previous studies have shown the existence of a high-velocity layer in the lower crust beneath the continental shelf-slop of the northern SCS margin, interpreted as the igneous intrusion. Here, we mapped the crustal seismic discontinuity and estimated its depth beneath the Dong-Sha region by retrieving teleseismic receiver functions (RFs) from the seismic array. By comparing observed and synthetic RFs, the forward modeling results revealed a thick and high-velocity layer from the depth of 8 km to Moho (26±2 km) with a P-wave velocity of 6.9±0.1 km/s beneath Dong-Sha Atoll, which may be related to the magmatic underplating. In addition, we analyzed 101 days-long continuous data to investigate ambient seismic sources. The spatiotemporal variations mapped from the amplitude asymmetry of CCFs indicate the noise excitation mostly from the west and some from the north-east (NE). The main water exchanging channels between the lagoon and ocean water is located at the west, which excites microseism signals. Noise excitations from NE might be due to the northeast monsoon or internal wave refraction. This data set shows convincingly that the burial of seismic sensors in soft sediment can increase data fidelity to resolve the crustal structure beneath the shallow lagoon.