我国东部地区非随机持续性微震背景噪声场源的特性及定位研究

我国东部地区陆海相连的独特区位是探究微震(2~20 s)背景噪声机理的天然场所.本文发展了一种基于噪声互相关非对称性能量走时差的场源定位技术, 并结合频域偏振等方法, 利用335个宽频带固定地震台站2015—2017年期间三分量的连续记录数据, 着重研究了我国东部陆地及近海地区微震背景噪声的非随机固有特性与场源位置, 并对及其激发响应机制进行了分析.获得的主要认识: (1)近海岸的涌浪作用是致使近岸台站DF(Double Frequency)微震谱峰出现双峰或多峰现象的主导因素, 其DF谱峰的频率和幅值明显强于陆地型台站.进一步获得的LPDFMs与SPDFMs偏振特性表明, LPDFMs主特征偏振功率的强弱与观测台站所处的地质构造环境密切相关, 而SPDFMs的强弱则取决于与海岸线的距离; 从极化程度、径向与垂向分量相位差结果看, 两者在0.25 Hz附近的分段特征非常清晰, 它们的产生机理并不完全一致, 可能来源于不同的场源; 在垂向和切向上的运动学特征差异不明显, 表明SF微震并不是纯态瑞利波; (2)研究区非随机持续性SF微震噪声源的方位主要指向印度洋方向, 很可能位于南半球印度洋南部至南极洲之间的深海区, 是由海浪驻波作用于远海海底所激发; 而DF微震噪声源的方位则主要指向东南向的太平洋方向, 其中LPDFMs很可能产生于开阔近海海域海浪与海底的相互作用, SPDFMs则可能源自于我国近海区域内稳定波-波相互作用形成的另一场源.(3)周期约10 s的主要持续性噪声源位置位于日本九州岛上的ASO火山附近区域; 另一个位于北太平洋的深海区, 但其所激发的信号强度相对较弱.

Characteristics and location of non-random persistent microseismic ambient noise sources in eastern China

The unique location where the land-sea are connected in the east of China is a natural place to explore the mechanism of microseismic (2~20 s) ambient noise. In this paper, a noise source location technique based on asymmetric energy travel-time difference of noise cross-correlation is developed, and combined with frequency domain polarization method, the continuous three-component recording data of 335 broadband fixed seismic stations from 2015 to 2017 are used to focus on the non-random inherent characteristics and field source location of microseismic ambient noise in the eastern continent and offshore areas of China. And the mechanism of excitation and response was analyzed as well. The main understandings obtained include: (1)The coastal swell action is the dominant factor causing the double or multiple peaks of DF microseismic spectrum at coastal stations, and the frequency and amplitude of DF spectrum at coastal stations are obviously stronger than those at continental stations. Further, the polarization characteristics show that the power of principal eigenvalue polarization of LPDFMs is closely related to the geological structure environment of the observation station, while the power of SPDFMs attributed to the distance from the coastline. According to the polarization degree β2 and the phase difference of radial and vertical components φVR, it can be seen that the segmented characteristics around 0.25 Hz are very clear, which indicates that their generation mechanisms are not completely consistent and come from different field sources.The difference of the kinematic characteristics between vertical and radial component is not obvious, which means that SF microseisms are not pure Rayleigh waves. (2) The direction of non-random persistent of SF microseismic noise source mainly points to the Indian Ocean, which probably comes from the deep sea area between the southern Indian Ocean and Antarctica in the southern hemisphere, and is excited by the standing gravity wave acting on the far seafloor. The direction of non-random persistent of DF microseismic noise sources mainly point to the Pacific Ocean. The LPDFMs are likely to be generated by the interaction between waves and the sea floor in the open coastal area, while the SPDFMs are likely to be generated by another source formed by the wave-wave interaction in the coastal area of China.(3)The main persistent localized noise source with a period of about 10 s is located in the vicinity of the ASO volcano on Kyushu Island, Japan; The other is located in the deep sea area of the North Pacific Ocean, but its excitation energy may be relatively weak.