基于中国内陆大孔径地震台阵的Rayleigh面波噪声源分布特征研究

Distribution characteristics of Rayleigh wave noise sources derived from records of a large-aperture seismic array in Northwest China

Rayleigh面波地震背景噪声成像技术已被成功运用到全球范围不同尺度的地球内部结构的研究中,并以背景噪声场是时空均匀分布为前提假设.然而真实的噪声源分布的时空非均匀性将导致经验格林函数提取存在偏差,最终影响噪声成像结果的精准性.近年来,噪声源分布特征研究逐步成为提高噪声成像精准度、深化地震背景噪声成像的关键问题.本文利用频率-波束域分析法对中国西北地区的一个大孔径台阵(WuTan Array,简称WTA)在2014全年的垂直分量连续记录做了聚束分析,研究了Rayleigh波噪声源分布特征.结果显示:WTA台阵成功探测到了10~20s周期范围的来自于全球不同方位的Rayleigh波噪声信号,其源区分布具有明显的季节变化特征:冬季集中分布在北大西洋方位,而夏季则转为印度洋方位噪声信号最强.此外,Rayleigh波噪声源区空间分布还表现出一定的频率依赖性,即在较低频段(0.0488~0.0635Hz)在北大西洋、北太平洋、印度洋及西太平洋四个方位均有分布;而在频率较高频段(0.0928~0.1025Hz)则集中分布于西太平洋方位.Rayleigh波噪声源时空分布特征和频率依赖性与海洋活动本身的季节性变化和频谱特征有关.并初步推测本文所观测到的Rayleigh波是由加剧的海浪运动直接作用于海岸、大陆架或海底而激发产生的第一类地脉动噪声信号.

Ambient Noise Tomography(ANT)has been successfully applied in studying the structure of the earth interior.The theory behind this technique presupposes a diffuse symmetric noise field.However,the actual anisotropy usually exists in the noise which causes the bias of Green′s functions and accordingly leads to errors of corresponding ANT results.Recently,the characteristic of microseisms sources distribution has become a critical issue to improve accuracy of the ANT.This paper presents a study on Rayleigh wave microseisms distribution characteristic based on the vertical component continuous seismogram recorded by a large-aperture array(WuTan Array,WTA)deployed in northwest China during 2014 using the conventional frequency-wavenumber(f-k)method.It is noted that Rayleigh wave microseisms at periods of 10~20 s are globally observed in various directions and have distinct seasonal variations.In winter,the North Atlantic is the strongest microseism source while in summer,the intense wave activity across the Indian Ocean dominates the Rayleigh microseism generation.Additionally,the azimuth distribution of the Rayleigh microseisms displays obvious frequency dependence.That is,Rayleigh microseisms are scattered in the orientations of the North Atlantic,the North Pacific,the West Pacific and the Indian Ocean at the lower frequency band while concentrating only in the direction of the West Pacific region at the relatively higher frequency band.The seasonality and frequency dependence of Rayleigh microseisms sources distribution is due to seasonal variation and the spectrum characteristic of the ocean activity itself.It is inferred that Rayleigh waves are the primary microseisms which are created by the direct action of ocean waves on the coast,continental shelf or sloping seafloor.