西准噶尔地区地震背景噪声源分析

西准噶尔是我国大陆远离海岸带最远的地区.利用频率域聚束(或称f-κ分析)方法对布设在西准噶尔地区的两个不同尺度的三分量宽频地震台阵61天和31天的连续记录分别进行了低、高频背景噪声源分析.通过台阵响应函数的计算,确定了分析背景噪声源的最佳频带范围分别为0.04~0.1 Hz和0.5~3 Hz.对于低频背景噪声,分析了初次地脉动的震源,结果说明西准噶尔大尺度台阵(WJLA)在观测时间范围内可以接收到来自亚欧大陆周边几个海洋活动强烈的海岸带的背景噪声,尤其以来自北太平洋西海岸带和北大西洋东海岸带的信号最强.通过分析由两个强温带气旋引起的北大西洋的海浪剧烈运动产生的地脉动信号,证明了这两个强温带气旋与北大西洋东海岸带相互作用的区域有所不同,并发现了当这两个强温带气旋结束后,该海域依然会在较长的一段时间内保持活跃状态.对于高频背景噪声,在1~2.5 Hz频带范围内有一个持续而稳定的噪声源,来自于西准噶尔小尺度台阵(WJSA)中心北偏东60°方向,主要由克拉玛依市区及附近的人类活动产生;除此之外,在较低频段有时还会在270°~300°方位产生一个能量相对更强的噪声源,其信号传播速度更快,分析认为该类震源为测区西北部的多个矿山的采矿活动.本项实验研究证明:即使在远离海岸带的我国西北部地区,背景噪声仍具有足够强的信号,但噪声来源存在强烈的方向性,因此在该地区利用背景噪声对地球内部进行成像时,需要考虑噪声源方位特性对成像的影响.

Source analysis of seismic ambient noise in the western Junggar area

Western Junggar is the most remote area from coastal zones in the mainland China. This article analyzed the source of ambient noise in the western Junggar area by using frequency-domain beamforming (f-k analysis). We deployed two three-component arrays of different sizes there and acquired two groups of records (one is 61-day-long, and the other is 31-day-long). By calculating their array response functions, we identified the optimal frequency ranges for this analysis, which are 0.04~0.1 Hz and 0.5~3 Hz. For the lower-frequency ambient noise (recorded by Western Junggar Large-scale Array, or WJLA), it mainly refers to the primary microseism. WJLA is able to receive ambient noise that is generated from the surrounding coastal zones of Eurasia. Especially, signals from the west coastal zone of the North Pacific and east coastal zone of the North Atlantic are strongest. By analyzing ambient noises from two extra-tropical cyclones, we proved that their influence districts are different, and found that the ocean will stay active for a long period (about 20 days) after the cyclones died down. For higher-frequency data, the result indicates that there is a persistent and stable noise source within 1~2.5 Hz in the western Junggar area. It comes from 60 degrees east of north (NEE), which is produced by various kinds of human activities in and near Karamay city. Besides, a source with relatively stronger energy and faster propagation speed occurs in the orientation of 80 degrees west of north (NWW) sometimes, possibly being the result of mining activities. This research manifested that the ambient noise is strong enough even in Northwest China. As there exists strong azimuthal anisotropy of noise sources, the orientation feature should be taken into consideration when applying ambient noise methods to image the Earth's interior in this region.