Analysis of coherence in global seismic noise for 1997–2012

The coherent behavior of four parameters characterizing the global field of low-frequency (periods from 2 to 500 min) seismic noise is studied. These parameters include logarithmic variance, kurtosis (coefficient of excess), width of support of multifractal singularity spectrum, and minimal normalized entropy of the distribution of the squared orthogonal wavelet coefficients. The analy)sis is based on the data from 229 broadband stations of GSN, GEOSCOPE, and GEOFON networks for a 16-year period from the beginning of 1997 to the end of 2012. The entire set of stations is subdivided into eight groups, which, taken together, provide full coverage of the Earth. The daily median values of the studied noise parameters are calculated in each group. This procedure yields four 8-dimensional time series with a time step of 1 day with a length of 5844 samples in each scalar component. For each of the four 8-dimensional time series, the frequency-time diagram of the evolution of the spectral measure of coherence (based on canonical coherences) is constructed in the moving time window with a length of 365 days. Besides, for each parameter, the maximum-frequency values of the coherence measure and their mean over the four analyzed noise parameters are calculated as a measure of synchronization that depends on time only. Based on the conducted analysis, it is concluded that the increase in the intensity of the strongest (M ≥ 8.5) earthquakes after the mega-earthquake on Sumatra on December 26, 2004 was preceded by the enhancement of synchronization between the parameters of global seismic noise over the entire time interval of observations since the beginning of 1997. This synchronization continues growing up to the end of the studied period (2012), which can be interpreted as a probable precursor of the further increase in the intensity of the strongest earthquakes all over the world.     

Variations in the Parameters of Background Seismic Noise during the Preparation Stages of Strong Earthquakes in the Kamchatka Region

The results of the long (2011–2016) investigation of background seismic noise (BSN) in Kamchatka by the method suggested by Doct. Sci. (Phys.-Math.) A.A. Lyubushin with the use of the data from the network of broadband seismic stations of the Geophysical Survey of the Russian Academy of Sciences are presented. For characterizing the BSN field and its variability, continuous time series of the statistical parameters of the multifractal singularity spectra and wavelet expansion calculated from the records at each station are used. These parameters include the generalized Hurst exponent α*, singularity spectrum support width Δα, wavelet spectral exponent β, minimal normalized entropy of wavelet coefficients En, and spectral measure of their coherent behavior. The peculiarities in the spatiotemporal distribution of the BSN parameters as a probable response to the earthquakes with М_w = 6.8–8.3 that occurred in Kamchatka in 2013 and 2016 are considered. It is established that these seismic events were preceded by regular variations in the BSN parameters, which lasted for a few months and consisted in the reduction of the median and mean α*, Δα, and β values estimated over all the stations and in the increase of the En values. Based on the increase in the spectral measure of the coherent behavior of the four-variate time series of the median and mean values of the considered statistics, the effect of the enhancement of the synchronism in the joint (collective) behavior of these parameters during a certain period prior to the mantle earthquake in the Sea of Okhotsk (May 24, 2013, M_w = 8.3) is diagnosed. The procedures for revealing the precursory effects in the variations of the BSN parameters are described and the examples of these effects are presented.     

The statistics of the time segments of low-frequency microseisms: Trends and synchronization

The problem of identifying the effects of synchronization in the parameters of low-frequency microseismic noise from the data of 77 stations belonging to the F-net broadband seismograph network in Japan for the period from the beginning of 1997 through August, 2009 is considered. The vertical components measured initially with a sampling rate of one second and subsequently converted into the signals sampled at 1 minute intervals by means of averaging and decimation are used in the analysis. Six statistics are taken as the parameters: the support width of the multifractal singularity spectrum; the generalized Hurst exponent; the asymmetry coefficient of the spectrum of singularity; the logarithmic variance; the spectral exponent; and the linear predictability index. These parameters are calculated from the realizations contained within consecutive daily time intervals. When using the moving time window with a width of one year for evaluating the multiple correlation, the daily variations in the median values of the statistics of the noise measured at five spatial clusters of stations exhibit a stable increase in the synchronization not long before the Hokkaido earthquake (September 25, 2003; M = 8.3), subsequently passing to the new level of high synchronization. Based on the analysis of the trends in the index of linear predictability it turned out possible to estimate the beginning of the enhancement in the synchronization with rather high accuracy as the middle of 2002. The effect revealed for the variations in the different parameters of microseisms is an independent argument for the earlier conclusion about the synchronization in the field of the microseismic noise on the Japan Islands.     

新疆数字地震台站观测动态范围和台基噪声的分析

介绍了新疆数字地震台仪器的配置,对新疆23个数字地震台站台基噪声进行分析计算。台站的观测动态范围,反映了观测仪器本身的性能和台基环境干扰背景的水平,有效动态范围大小反映记录地震信号的最大能力。台站(台网)的监测能力不仅取决于仪器的性能,而且还与台基的噪声背景有关。得出了各地震台台基的脉动噪声的均方根值、观测动态范围、地噪声功率谱。有效观测动态范围的大小与数字地震仪的种类和配置有关。新疆23个数字地震台中的21个台的台基地动噪声在1~20 Hz内符合中国数字测震台网技术规程的要求,而另外2个台,即二宫和石河子数字地震台没有达到规定指标。     

不同深度布设台站的噪声及信号幅值特征分析

为探究地震观测中地震计检测到的噪声和信号强度均受其布设深度的影响,本文首先对CPUP和LPAZ两台站布设的不同深度地震计所得到的数据进行噪声水平和地震信号对比;其次采用对比功率谱密度的方法对两台站不同通道采集到的不同时段的噪声数据进行分析;最后比较两台站不同通道采集到的整月数据的噪声幅值、信号幅值、信噪比特征。结果显示:深度较大的通道,其噪声功率均值较小;当事件信号到来时,较深通道的地震计检测到的信号和噪声幅值比较浅通道均有所减小,在信号和噪声幅值均减小的共同影响下,信噪比有一定程度的变化,其中LPAZ台站的信噪比提高较为明显。      

顾及共性误差的南极半岛地区连续GPS站坐标时间序列分析

共性误差是区域连续GPS网络中存在的一种与时空相关的主要误差源.对区域网络使用空间滤波能够有效减小这种误差并改善坐标时间序列的精度.本文使用GAMIT/GLOBK 10.5软件解算南极半岛区域11个GPS测站2010—2014的数据,各测站独立估计的单天解组成坐标时间序列.利用堆栈法(stacking)、主分量分析法(principal component analysis,简称PCA)和Karhunen-Loeve展开法(KLE)对其进行空间滤波,在利用PCA和KLE方法进行空间滤波时,使用前两个主分量作为共有模式分量.结果表明:三种方法均能有效提取南极半岛区域共性误差,PCA方法的滤波效果明显优于stacking方法,略微优于KLE方法;空间滤波能够有效减小残差时间序列的振幅、功率和RMS;对坐标时间序列进行空间滤波能够有效降低其线性项和周期项误差,从而提高线性项和周期项估计的精度.共性误差的谱分析结果显示在U方向上存在9.4天、13.7天等短周期的信号,推测共性误差组成成分中可能含有与海潮相关的误差源.     

华北地区地震环境噪声特征研究

利用华北流动地震台阵观测的垂直分向连续波形数据, 通过计算功率谱密度和相应的概率密度函数, 对华北地区地震环境噪声特征进行了分析研究. 结果表明, 东部平原和沉积盆地2 Hz以上的高频环境噪声水平与全球新高噪声模型（NHNM）相近, 周期3——18 s的平均噪声水平低于NHNM和新低噪声模型（NLNM）的平均值; 山区及西部高原的高频噪声水平明显低于NHNM, 周期1——18 s的噪声水平大多明显低于NHNM和NLNM的平均值; 不同区域18 s以上周期的噪声水平差异相对较小. 流动地震台阵部分台站的环境噪声存在明显的昼夜变化, 个别台站噪声水平明显高于周边台站, 表明这些台站受人类活动干扰较大. 不同台站的噪声水平分析表明, 将台站布设在摆坑内, 能在一定程度上降低高频和低频段的噪声水平. 台站环境噪声特征的研究结果可为流动地震台阵观测数据质量的定量评估, 观测期间的台站优化调整等提供重要依据.      

天津测震台网地震监测能力分析

通过计算地震台站最小监控震级,得到天津测震台网地震监测能力。采用经典谱估计中的Welch算法,计算天津测震台网31个地震台站噪声水平。取噪声水平有效值的30倍作为能检测到最小地震S波的最大振幅,通过近震震级公式,计算各台站地震震级,取第4个能被检测到的台站地震震级作为最小监控震级。     

山东数字测震台站台基背景噪声分析

介绍了山东数字地震台网基本情况,计算了40个测震台站台基背景噪声,利用Welch方法计算噪声功率谱密度（PSD）,进而计算地震台台基1—20 Hz地动噪声均方根值（RMS）和有效动态观测范围。根据计算结果,依照《地震台站观测环境技术要求》,对山东测震数字台网40个参评测震台站进行背景噪声级别分类,并分析不同台站背景噪声水平较低的原因,以期为测震台网的优化建设提供数据支持。     

希尔伯特—黄变换在地电阻率数据处理中的应用

希尔伯特—黄变换用于处理非线性非平稳信号,由经验模态分解和希尔伯特谱分析2部分组成。本文采用希尔伯特—黄变换方法,相继对大同地震台地电阻率月值数据和宝昌地震台地电阻率月值、整点值数据进行处理。结果显示:(1)大同、宝昌地震台地电阻率月值数据对应的Hilbert谱具有较高分辨率,高幅值在归一化频率0.05—0.15区间内呈"余弦"变化形态;(2)希尔伯特—黄变换在提取地电阻率异常变化、高频信息及去除噪声等方面效果较好,在未来地电资料处理中具有广泛的应用前景。     

香港GPS基准站坐标序列特征分析

利用香港GPS连续运行参考站网络2001年1月至2007年8月的观测资料,全面深入地分析了12个基准站坐标序列特征.本文采用主成分空间滤波算法去除公共误差,来提高坐标序列的信噪比,并采用最大似然估计准则定量估计滤波后坐标序列的噪声特性,计算了地球表面质量负荷（包括大气、非潮汐海洋、积雪和土壤水）对香港GPS基准站坐标序列的影响.研究结果表明：香港GPS基准站坐标序列具有高度的空间相关性,其公共误差具有较强的季节性变化特征;地表质量负荷变化引起的香港地壳形变可以解释公共误差序列中约为3mm的垂向周年变化,经过质量负荷改正后的公共误差序列与高阶电离层误差高度相关;滤波后坐标序列的噪声特性可以用可变白噪声加闪烁噪声模型来描述,顾及闪烁噪声所计算的速度误差要比只考虑可变白噪声计算的速度误差大2~6倍;基准站间存在达1.5 mm/yr的相对水平运动,揭示香港地区存在活动断层;部分基准站坐标具有明显的振幅为1～2 mm本地季节性变化,所有测站的残差序列也表现出强烈的季节性变化.     

Mapping the properties of low-frequency microseisms for seismic hazard assessment

The structure of low-frequency seismic noise in the range of periods from 2 min to 500 min is studied from the data of continuous seismic monitoring at 77 seismic stations of the F-net broadband network in Japan from the beginning of 1997 to May 15, 2012. A new statistical characteristic of seismic noise is suggested, namely, the minimal normalized entropy En of the distribution of squared orthogonal wavelet coefficients. This parameter of seismic noise is analyzed in conjunction with the multifractal statistics—the support width of the singularity spectrum, Δα, and the generalized Hurst exponent, α*, which were extensively used by the author in the previous works for analyzing the low-frequency seismic noise. The method for constructing the maps of spatial distribution of Δα, α*, En, and their aggregated normalized value over the time windows with a given length is proposed. The maps are constructed by averaging the succession of the elementary charts, each of which corresponds to a day of observations. It is shown that, for the islands of Japan, the reduction in Δα and α* and the increase in En outline the area of the forthcoming mega earthquake of March 11, 2011, with M = 9 (Tohoku earthquake). According to the analysis of about a year’s worth of data after this event, the region south of Tokyo (Nankai trough) is still dominated by decreased Δα and α* and increased En. This gives grounds to hypothesize that this region remains at a high level of seismic threat since the accumulated stresses were incompletely released by the Tohoku earthquake. Drawing an analogy to the behavior of the coefficient of correlation between Δα and α*, we may suppose that there is an increased probability of a strong earthquake occurring in the second half of 2013 or the first half of 2014. Constructing the averaged maps of the distributions of seismic noise parameters and their aggregated value in a moving time window is suggested as a new method for dynamical assessment of seismic hazards.     

Estimation of Background Noise for International Monitoring System Seismic Stations

v--vThe prototype International Data Centre (IDC) in Arlington, Virginia has been acquiring data from seismic stations at locations designated in the Comprehensive Test-Ban Treaty for the International Monitoring System (IMS) since the start of 1995. A key characteristic of these stations is their background noise levels and their seasonal and diurnal variability. Since June 1997 an automated sample selection effort has collected over 700,000 individual noise sample spectra from 39 primary and 57 auxiliary stations. Monthly median and 5 and 95 percentile estimates have been calculated for each channel of every station. Compatibility of median spectra obtained for the same station and channel in the same month for two different years confirms the consistency of the noise-sampling algorithm used. A preliminary analysis of the results shows strong (more than a factor of two) seasonal variation at a quarter of all stations. Strong diurnal variations at half of the sites indicate that many of the selected sites are poorly located with respect to cultural noise sources. The results of this study are already being used to evaluate station quality, improve those processes that require background noise values, such as automatic association and requesting auxiliary station data, and to improve the estimation of station and network detection and location thresholds.     

一种自动扫描地震漏检事件方法的研究与实现

地震事件的自动处理过程中会发生遗漏.台站数量越多,地震分析员在手动扫描遗漏事件时所花费的时间就越长.本文讨论了一种自动扫描漏检事件的方法,该方法利用分析员的分析经验,结合台站分布状况和地球物理学规则,重新关联那些具有很可能来自特定震源区域而未被分析员关联的信号,以形成一个待验证的事件,供分析员审定.该方法大大减少了分析员的工作量,提高了地震公报的质量.     

镜泊湖火山测震台网监测效能评估

2019年镜泊湖火山测震台网新增2个测震台后,共有5个测震观测子台对火山区进行地震监测.利用其观测数据,进行噪声功率谱和监测能力计算,绘制地震监测能力图,并与原测震台网监测能力进行对比分析.研究表明,镜泊湖火山测震台网监测能力得到很大提升,可满足火山地震监测需要.     

安徽数字测震台网新参评台站背景噪声分析

计算并分析安徽数字测震台网9个新参评台站数字化记录背景噪声,得到各台址背景噪声均方根RMS值、有效测量动态范围、噪声功率概率密度谱,按照地噪声水平规定,对各台进行台基噪声分类,数据表明,9个新参评台站有4个Ⅰ类台址、5个Ⅱ类台址。对于9个新参评台站噪声功率概率密度谱及各频点噪声干扰源,分析认为,大多数台站在频率10 Hz附近存在2组概率较高的背景噪声,与白天和夜晚的不同噪声水平相对应。     

利用背景噪声估计华北地区场地放大效应

用噪声互相关函数聚束方法得到中国华北地区噪声源的极值方位和慢度,然后将噪声傅里叶谱在该方位和慢度点进行聚束,并计算得到噪声的频率波数功率谱。将该功率谱作为输入,单台站噪声的自功率谱作为响应,二者的比值可用于计算华北地区相对场地放大因子。计算结果显示:平原地区放大效应明显大于山地地区,而平原地区的场地放大效应与渤海湾盆地沉积层厚度有直接关联。本研究可为计算场地放大效应提供一种可行的途径。     

中国中东部地震台站噪声互相关函数中面波前驱信号的分析研究

利用中国中东部地震台网中430个宽频带台站2008和2009两年的垂直分量记录,我们计算了台站对之间的噪声互相关函数(Noise Cross-correlation Function,NCF).在相当多的NCF中,沿大圆路径传播的瑞利面波信号之前存在一个较强的前驱信号,该信号持续时间约50 s,频率范围为0.07~0.12 Hz.此信号在同一台站对的NCF中稳定存在、到时相同,而在不同台站对的NCF中到时不一致,这表明该信号可能源于一个固定的较小区域内持续存在的噪声源.基于格点搜索方法的定位结果显示该噪声源位于日本九州岛附近,其激发出的信号传播速度约2.7 km/s.假定该信号由一点产生,我们计算了合成地震图,合成地震图与观测到的前驱信号基本一致,验证了定位结果的可靠性.此噪声源会对特定路径上的面波信号产生干扰,在相关研究中需要采取一定措施避免.该噪声源的物理机制有待于进一步的研究.     

基于CWQL的山西测震台网准实时背景噪声计算分析

以山西省测震台网为例,利用CWQL软件调取JOPENS5.2系统台站实时波形数据与对应台站参数,计算台基环境地噪声水平.分析认为,CWQL软件可用于日常测震观测系统数据质量检测,得到各测震台站各分向PSD功率密度函数分布及RM S值.研究结果实现了对山西数字测震台网连续波形数据质量、JOPENS系统台站配置参数的准实时...