DataSheet1_Characterization and spatiotemporal variations of ambient seismic noise in eastern Bangladesh.docx

This study analyses the ambient noise field recorded by the seismic network, TREMBLE, in Bangladesh, operational since late 2016. Horizontal-vertical spectral ratios confirm the placement of stations on sediment, many situated on thick sedimentary columns, consistent with local geology. Noise across the broadband spectrum is systematically examined. A high amplitude local microseism (0.4–0.8 Hz) is recorded, originating near the coast and modulated by local tides. The secondary microseism (0.15–0.35 Hz) correlates strongly with wave height in the Bay of Bengal and varies with seasons, with greater power and higher horizontal amplitude in the monsoon season when the wave height is highest. The microseism increases in amplitude and decreases in frequency as a tropical depression moves inland. The primary microseism (∼0.07–0.08 Hz) exhibits no seasonal changes in power but display strong horizontal energy which changes with seasons. Low frequency (0.02–0.04 Hz) noise on the horizontal components has a 24-h periodicity, due to instrument tilt caused by atmospheric pressure changes. A station located next to the major Karnaphuli River shows elevated energy at ∼5 Hz correlated to periods of high rainfall. Anthropogenic noise (∼4–14 Hz) is station-dependent, demonstrating changing patterns in human activity, such as during Ramadan, national holidays and the COVID pandemic. Our work holds implications for seismic deployments, earthquake, and imaging studies, while providing insights into the interaction between the atmosphere, ocean, and solid Earth.

本研究分析了自2016年末起投入运行的孟加拉国境内TREMBLE地震台网所记录的环境噪声场（ambient noise field）。水平-垂直谱比（Horizontal-vertical spectral ratios）证实，所有台站均布设于沉积层之上，其中多数位于厚沉积柱之上，这与当地地质特征相符。研究人员对宽频带频谱范围内的噪声开展了系统性分析。研究记录到一处振幅较高的局地微震（microseism，0.4–0.8 Hz），其起源于近海区域，且受当地潮汐调制。次生微震（secondary microseism，0.15–0.35 Hz）与孟加拉湾（Bay of Bengal）的浪高呈显著正相关，且随季节动态变化：在浪高达到峰值的季风季节（monsoon season），其能量更强、水平振幅更高。随着热带低压（tropical depression）向内陆移动，该类微震的振幅逐渐增大，频率逐渐降低。原生微震（primary microseism，约0.07–0.08 Hz）的能量无明显季节变化，但其水平方向的强能量会随季节发生改变。水平分量（horizontal components）上的低频噪声（0.02–0.04 Hz）呈现24小时周期性，这一现象由大气压力变化（atmospheric pressure changes）引发的仪器倾斜所导致。紧邻卡尔纳普里河（Karnaphuli River）的一处台站，其在约5 Hz频段处的能量显著升高，且与强降雨时段高度相关。人为噪声（anthropogenic noise，约4–14 Hz）具有台站特异性，其变化模式反映了人类活动的节律特征，例如斋月（Ramadan）、国家法定假日以及新冠疫情（COVID pandemic）期间的活动变迁。本研究成果不仅可为地震台网布设、地震及成像研究提供参考，同时也为大气、海洋与固体地球（solid Earth）之间的相互作用机制提供了新的认知视角。