Discrimination between stochastic dynamics patterns of ambient noises (Case study for Oni seismic station)

Investigation of complex dynamics of ambient seismic noise remains as an important scientific research challenge. In this work we investigated dynamical features of the ambient noises at Oni seismic station, Georgia. We used stochastic model reconstruction method from measured data sets. Seismic records for different time periods around Oni seismic station have been analysed. It was shown that the dynamics of fluctuations of seismic noise vertical component undergoes essential changes for considered time period from 2005 to 2012. These changes are more noticeable for time periods of preparation and aftershock activity of strong M6.0 earthquake occurred in 2009 in the vicinity of Oni seismic station.     

利用背景噪声进行地震成像的新方法

利用背景地震噪声提取格林函数，即通过一对台站记录的地震噪声进行互相关计算，来近似台站间的格林函数，并进一步通过地震成像获取地下结构的认识，成为最近跨学科研究的热点问题之一.本文首先叙述了其发展背景及过程，然后分别从四个角度，简要介绍了目前对其物理原理的解释.如果该方法能得到进一步的发展，将不再依赖于天然地震以及人工地震，而仅仅利用地震台站记录的地震噪声便可获取高分辨率的地下成像.该方法将会大大促进地震学的发展.     

Relations for local magnitude at seismic station Berggiesshübel

Summary New calibrating functions for the local magnitude M_L based on the maximum ratio of amplitude and period of the S_g - wave, the short-period surface wave and the duration of the whole seismogram and its part starting with the arrival of the S_g -wave recorded by the short-period vertical seismograph at Berggiesshübel (BRG) have been derived. The consistency of these magnitudes was tested for weak earthquakes and industrial blasts in the magnitude range of 1.7–3.5 and epicentral distances up to 80 km. Their differences were 0.2 of a magnitude unit at most, with the exception of the surface wave magnitudes which were systematic by about 0.9 of a magnitude unit greater. The calibrating function for the S_g - wave was compared with the functions for M_L(POT) and M_L(TRI) which were derived by other authors for several Central European seismic stations, including BRG.     

Comparison of ground truth location of earthquake from InSAR and from ambient seismic noise: A case study of the 1998 Zhangbei earthquake

Because ambient seismic noise provides estimated Green’s function (EGF) between two sites with high accuracy, Rayleigh wave propagation along the path connecting the two sites is well resolved. Therefore, earthquakes which are close to one seismic station can be well located with calibration extracting from EGF. We test two algorithms in locating the 1998 Zhangbei earthquake, one algorithm is waveform-based, and the other is traveltime-based. We first compute EGF between station ZHB (a station about 40 km a...     

Comparison of ground truth location of earthquake from InSAR and from ambient seismic noise:A case study of the 1998 Zhangbei earthquake

Because ambient seismic noise provides estimated Greenos function (EGF) between two sites with high accuracy, Rayleigh wave propagation along the path connecting the two sites is well resolved. Therefore, earthquakes which are close to one seismic station can be well located with calibration extracting from EGF. We test two algorithms in locating the 1998 Zhangbei earthquake, one algorithm is waveform-based, and the other is traveltime-based. We first compute EGF between station ZHB (a station about 40 km away from the epicenter) and five IC/IRIS stations. With the waveform-based approach, we calculate 1D synthetic single-force Greenos functions between ZHB and other four stations, and obtain traveltime corrections by correlating synthetic Greenos functions with EGFs in period band of 10-30 s. Then we locate the earthquake by minimizing the differential travel times between observed earthquake waveform and the 1D synthetic earthquake waveforms computed with focal mechanism provided by Global CMT after traveltime correction from EGFs. This waveform-based approach yields a location which error is about 13 km away from the location observed with InSAR. With the traveltime-based approach, we begin with measuring group velocity from EGFs as well as group arrival time on observed earthquake waveforms, and then locate the earthquake by minimizing the difference between observed group arrival time and arrival time measured on EGFs. This traveltime-based approach yields accuracy of 3 km, Therefore it is feasible to achieve GT5 (ground truth location with accuracy 5 km) with ambient seismic noises. The less accuracy of the waveform-based approach was mainly caused by uncertainty of focal mechanism.     

基于局部优化粒子群算法的背景噪声反演

从背景噪声中提取瑞雷波频散曲线并通过反演获得地下横波速度结构已被广泛应用于大尺度的地下结构探测和小尺度的工程勘探中.基于频散函数的反演目标函数可以有效解决多阶模频散曲线联合反演的模式误判问题, 然而其广泛分布的局部极值导致更为严重的多解性, 在大范围的参数搜索空间下很难获得最优解, 需要搭配全局搜索性能强的优化算法.本文提出局部优化粒子群算法(PSOG), 通过粒子迭代过程中引入局部优化方法提高种群多样性, 避免陷入局部极值并加快收敛速度.为验证新算法的有效性, 结合基于久期函数的目标函数对理论合成数据进行反演, 结果表明, 局部优化粒子群算法比传统算法的稳定性与准确性都有显著提高.处理了上海苏州河地区的背景噪声数据, 成功地对古河道切割造成的软弱层进行成像.PSOG算法与新型反演目标函数的结合在背景噪声勘探的工程应用上具有巨大潜力.

     

新冠疫情防控前后湖北恩施地震台背景噪声变化分析

利用2018年1月至2020年3月恩施地震台站观测的垂直分量连续波形数据,通过计算噪声功率谱密度和概率密度函数,统计其不同频段功率谱密度分布情况,对比分析疫情前后恩施地区噪声水平变化特征.同时,将2018年、2019年和2020年垂直向地震噪声的加速度幅值进行对比,发现春节前几日的加速度幅值变化曲线高度一致,表明这三年...     

基于无监督机器学习的噪声信号聚类分析——以郯庐断裂带潍坊段短周期密集台阵观测为例

非构造活动震源所引起的地面震动通常被看成是地震记录中的噪声信号,此类噪声与微震或非火山震颤等弱构造活动信号往往在时间域或频率域都难以区分,从而会影响利用常规方法对弱构造活动信号的识别与检测.即使利用最新的机器学习方法对微地震信号检测,若缺乏对噪声信号特性的了解,也会对监督模型的训练产生不利的影响.因此,有必要剖析地震噪声信号,理解其特征属性,以及背后可能的物理震源.本研究中,我们利用一个布设于华北东部地区的短周期密集观测台阵,使用K-means算法聚类分析不同类型的地震噪声信号.分析表明密集台阵可以观测到6类噪声信号,噪声来源包括轨道交通、风和附近的电力输送线.

     

甘东南地区宽频带地震台阵背景噪声特征分析

基于甘肃东南地区150个宽频带流动台站2010年的垂直分量连续波形记录,通过计算台站对之间背景噪声的互相关函数并叠加得到5—10s和10—20s两个周期的瑞雷面波信号,并通过信噪比和归一化背景能量流两种方法研究了该地区背景噪声源的时空演化特征.研究结果表明,甘东南地区5—10s和10—20s周期的背景噪声源具有明显的季节变化特征和各自的优势方位.5—10s周期的背景噪声在夏季的能量优势方位为170°—240°,噪声源主要位于印度洋,而冬季为100°—150°,主要位于北太平洋;10—20s周期的背景噪声源则比较复杂,其优势方位受多个大洋的交替影响,夏季噪声源能量优势方位为170°—210°,噪声源主要位于印度洋,冬季为90°—150°和310°—355°,噪声源分别位于北太平洋和北大西洋.由于这两个周期的背景噪声源在甘东南地区存在明显的季节变化,因此在利用背景噪声方法研究该地区介质速度结构时需充分考虑噪声源的非均匀性所产生的影响.      

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-k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.

     

Variations of local seismic response in Benevento (Southern Italy) using earthquakes and ambient noise recordings

The city of Benevento (Southern Italy) has been repeatedly struck by large historical earthquakes. A heterogeneous geologic structure and widespread soft soil conditions make the estimation of site effects crucial for the seismic hazard assessment of the city. From 2000 until 2004, we installed seismic stations to collect earthquake data over zones with different geological conditions. Despite the high level of urban noise, we recorded more than 150 earthquakes at twelve sites. This data set yields the first, well documented experimental evidence for weak to moderate local amplifications. We investigated site effects primarily by the classical spectral ratio technique (CSR) using a rock station placed on the Benevento hill as reference. All sites in the Calore river valley and in the eastern part of the Benevento hill show a moderate high-frequency (f > 4 Hz) amplification peak. Conversely, sites in the Sabato river valley share weak-to-moderate amplification in a wide frequency band (from 1–2 to 7–10 Hz), without evident frequency peaks. Application of no-reference-site techniques to earthquake and noise data confirms the results of the CSRs in the sites of the Calore river valley and of the eastern part of the Benevento hill, but fails in providing indications for site effects in the Sabato river valley, being the H/V ratios nearly flat. One-dimensional modeling indicates that the ground motion amplification can be essentially explained in terms of a vertically varying geologic structure. High-frequency narrow peaks are caused by the strong impedance contrast existing between near-surface soft deposits and stiff cemented conglomerates. Conversely, broad-band amplifications in the Sabato river valley are likely due to a more complex layering with weak impedance contrasts both in the shallow and deep structure of the valley.     

Composition of high frequency ambient noise from cross-correlation:A case study using a small aperture array

Long-time cross correlation of ambient noise has been proved as a powerful tool to extract Green's function between two receivers.The study of composition of ambient noise is important for a better understanding of this method.Previous studies confirm that ambient noise in the long period (3 s and longer) mostly consists of surface wave,and 0.25-2.5 s noise consists more of body waves.In this paper,we perform cross correlation processing at much higher frequency (30-70 Hz) using ambient noise recorded by a small aperture array.No surface waves emerge from noise correlation function (NCF),but weak P waves emerge.The absence of surface wave in NCF is not due to high attenuation since surface waves are strong from active source,therefore probably the high ambient noise mostly consists of body wave and lacks surface wave.Origin of such high frequency body waves in ambient noise remains to be studied.     

Analysis of broadband seismic noise at the German Regional Seismic Network and search for improved alternative station sites

The German Regional Seismic Network (GRSN) comprizes now 16 digital broadband stations equipped with Wieland-Streckeisen STS-2 seismometers, 24-bit dataloggers and a seismological data center at Erlangen. It covers the whole territory of Germany with station-spacings between 80 km to 240 km. The stations are sited in very different environments ranging from near shore at the Baltic Sea coast up to distances of about 700 km from the coast, both within cities and up to about 10 km away from any major settlement, industry or traffic roads. The underground varies from outcropping hard rocks in Hercynian mountain areas, sedimentary rocks in areas of Mesozoic platform cover to up to 1.5 km unconsolidated Quarternary and Tertiary subsoil. Accordingly, seismic background noise varies in a wide range between the upper and lower bounds of the new global noise model. The noise conditions at the GRSN have been investigated systematically by means of displacement power spectral analysis within the frequency range 10^-2 5 for RUE and > 10 for BSEG have been confirmed for frequencies between about 0.6 Hz 3 Hz. Strong lateral velocity and impedance contrasts between the outcropping Triassic/Permian sedimentary rocks and the surrounding unconsolidated Quarternary/Tertiary sediments are shown to be the main cause for the strong noise reduction and signal-to-noise ratio improvement at RUE and can account for about 50% of the noise reduction at BSEG.     

利用背景噪声数据提取地震台站间面波的可靠性分析——以中国大陆中东部地区的宽频带台站为例

利用中国大陆中东部地区以国家台网为主的100个分布均匀的宽频带地震台记录到的21个月的连续波形数据,经过单台数据处理和互相关叠加计算后,由时频分析法提取了研究区各台站对间瑞雷波的格林函数.为了检验经验格林函数的可靠性和稳定性,对沿部分路径的经验格林函数和频散曲线进行了质量评估.检测结果表明,自21个月叠加的台站对间背景噪声中提取的经验格林函数与实际的地震面波一致,提取的格林函数可靠.此外,统计了使用从3 21个月不同长度数据叠加后,经验格林函数信噪比大于10的频散曲线数目.结果表明,至少要使用12个月的数据才能提取到信噪比足够大,数目足够多,可用于反演面波速度结构的经验格林函数;12个月的叠加时长,可以保证30 s以下周期的频散曲线在时间上稳定.     

利用背景噪声数据提取地震台站间 面波的可靠性分析 以中国大陆中东部地区的宽频带台站为例

利用中国大陆中东部地区以国家台网为主的100个分布均匀的宽频带地震台记录到的21个月的连续波形数据, 经过单台数据处理和互相关叠加计算后, 由时频分析法提取了研究区各台站对间瑞雷波的格林函数. 为了检验经验格林函数的可靠性和稳定性, 对沿部分路径的经验格林函数和频散曲线进行了质量评估. 检测结果表明, 自21个月叠加的台站对间背景噪声中提取的经验格林函数与实际的地震面波一致, 提取的格林函数可靠. 此外, 统计了使用从3—21个月不同长度数据叠加后, 经验格林函数信噪比大于10的频散曲线数目. 结果表明, 至少要使用12个月的数据才能提取到信噪比足够大, 数目足够多, 可用于反演面波速度结构的经验格林函数; 12个月的叠加时长, 可以保证30 s以下周期的频散曲线在时间上稳定.     

新冠肺炎疫情对佘山与大洋山地震台背景噪声的影响

人类的生产生活产生的振动会以高频地震波的形式被地震台站所记录。2020年1月,新冠肺炎疫情爆发,为了应对此次疫情,各地政府分别启动应急响应,国内地震记录出现最长、最突出的人为地震降噪期。各地震台站背景噪声显著下降,在人口稠密及工业发达地区尤为明显。同时,静噪期为探测地下地震源的微弱信号提供了可能。静噪期内,佘山地震台2 Hz频点背景噪声功率谱密度值比平时降低10 dB,而大洋山地震台10 Hz频点背景噪声功率谱密度值较平时降低约5 dB;佘山地震台2—10 Hz以及大洋山地震台10 Hz以上频率的背景噪声受静噪期影响明显。结合地震台站所处位置分析,疫情期间佘山地震台附近人口流出较多,2—10 Hz频率的背景噪声变化明显;大洋山地震台背景噪声则受工厂、轮渡、高速汽车等影响较大,f ≥10 Hz的背景噪声变化显著,而频率在2—10 Hz则无明显变化,表明该台人口总数趋于平稳。地震噪声和人类活动之间的相关性表明,地震学研究可以提供实时人口动态估计。     

利用背景噪声互相关研究汶川地震震源区地震波速度变化

利用2007年3月至2009年3月四川数字地震台网的宽频带连续波形资料,通过计算地震背景噪声互相关提取台站对间的经验格林函数,在0.1～0.5 Hz频带下测量每天经验格林函数与参考经验格林函数的走时偏移,进而得到各台站对在该时段内的相对地震波速度变化.结果表明,2008年5月12日汶川Ms8.0级地震造成了震源区地震波速度的急剧降低,最大降幅达0.4%;大致以安县为界,余震带西南部地区在汶川主震后波速降即达到最大值,而东北部地区的最大波速降一般出现在主震后的1~4个月,相对地震波速度变化的这种分段特性与地震序列的时空分布特征有较好的对应关系;在震源区外围的四川盆地也观测到了震后波速降低,而川西高原内部则没有出现显著的波速变化.进一步的分析和计算结果表明主震的静态应力变化和强地面运动引起的地表破坏都不能很好地解释震后波速的急剧降低,地震导致的断层区内部结构破坏和周边介质应力状态改变可能是波速变化的主要原因.     

From ambient noise recordings to site effect assessment: The case study of Barcelona microzonation

As local site effects have a drastic influence on seismic hazard, it is a major issue to characterize them in vulnerable areas such as highly urbanized zones, like Barcelona city. The aim of this work is to improve the knowledge of geophysical characteristics of Barcelona in the perspective of a seismic microzonation that takes into account site effect. The first step was to gather the existing data from geological, geotechnical, geophysical, and seismological investigations, bringing us to keep the four zones proposed by previous work as a base of zonation. The second step was to characterize each zone by time-averaged shear-wave velocity and fundamental resonance frequency, with ambient noise techniques over 17 sites, providing new knowledge about the soil of Barcelona. The third step was to propose an amplification function between an average soil for each zone and a standard reference rock site, using empirically based propositions and to compare them to previous numerical approaches.     

线缆式海底地震观测技术——以浯屿岛海底地震观测台为例

地球科学研究的新型观测平台——线缆式海底地震观测台（网）,是一种基于海底光电复合缆的光电传输技术,将传统的陆地地震观测台站向海区延伸、弥补海区地震观测台站的不足,实现实时、长期连续观测地球内部过程的新技术,目前已成为海洋科学的创新性研究平台,相关的技术方法也有了突飞猛进的进步.2018年,我国研发建设的"福建近海海底地震观测台",在前人探索研究的基础上,突破了线缆式海底地震观测台（网）关键技术,实现数据实时传输功能,具备进入地震行业网的能力.经过超过三个月的试运行检验,证实了系统的稳定性和可靠性.本文详细介绍了福建浯屿岛海底地震台（网）研发的关键技术与建设流程、关键技术环节相关规范和检测结果.地震采集数据结果表明,浯屿岛海底地震观测台实现了数据实时流服务,并可接入中国地震局JOPENS系统,数据实时传输正常,并自动存储实时波形数据;试运行期间以实际99%以上的高运行率,达到了国家测震台网管理规定的优秀标准.研发结果可为我国今后的海底地震观测台（网）建设提供重要参考指标与经验借鉴.