青海茫崖5.0级地震震源区微震检测与序列活动特征分析

以2019年3月28日青海茫崖5.0级地震作为研究震例,选取主震后信噪比较高的9个M_L1.0以上余震作为模板事件,利用模板匹配滤波方法对主震后30天连续波形进行遗漏地震扫描,检测到遗漏地震13个,约为原始地震目录的43%,这对该地区的地震活动性研究,地震危险性评估等后续研究工作提供了比较完备余震序列数据参考。对模板匹配前后b值变化进行对比分析,发现模板匹配后b值略有下降,佐证了强震后短时间内b值明显降低的普遍现象。最后对扩充后的地震目录进行双差重定位,发现地震主要集中分布在茫崖—油沙山断裂北侧,呈现出成团成片的丛集状分布特点。     

台阵处理技术和模板匹配滤波技术在微弱地震信号检测中的应用

与大地震短时间内释放巨大能量不同,微小地震每次释放的能量较小,容易被忽略。然而,某一地区一段时间内微小地震的频繁发生,可能预示着该地区地下介质达到某种临界状态,大地震发生的风险提升。微地震监测不仅可以应用于地下结构成像、地震预测等天然地震学领域,还可用于探测页岩气开采过程中产生的微裂隙位置,指导水压致裂等石油生产工作。系统探测微地震活动,也是研究灌注废水和水库诱发地震活动的重要前提。在实际观测中,由于微地震的信号较弱,背景噪声水平较高,观测困难,我们需要通过一些方法,提高波形信噪比,提高微弱地震信号检测的能力。为了进行微弱地震信号检测,我们重点研究了以下两种方法:一种是用于探测微弱信号的台阵处理技术,该方法基于密集分布的台阵数据,通过一定时间偏移后的波形叠加,增强一致的事件信号,压制不一致的噪声信号。另一种是用于检测遗漏地震的模板匹配滤波技术,该方法通过波形扫描,识别出与模板事件相似度高的地震事件,在低信噪比情况下也有良好的检测效果。对于台阵处理技术,我们以2013年2月15日发生的俄罗斯陨石坠落事件为例,研究2011—2013年布设在云南及其周边地区的ChinArray一期台阵数据。在陨石坠落事件前20分钟左右,一次5.8级地震事件在汤加地区发生,该事件信号传至ChinArray的时间与俄罗斯陨石坠落事件大致相同,为我们提取信号带来难度。运用台阵处理技术中的Vespagram和FK分析,我们从中分辨出来自俄罗斯陨石坠落事件的面波震相,对应后方位角329.7°,慢度34.73s/deg。我们同样运用F-K分析对日本的F-net宽频带台阵数据进行处理,得到后方位角为316.61°的面波震相。利用ChinArray和F-net的不同后方位角,我们将俄罗斯陨石坠落事件定位在58.80°N,58.72°E,和USGS的定位结果(55.15°N,61.41°E)相差约438km。较大的定位偏差可能是由于地下介质的不均匀性,导致面波在传播过程中偏离大圆路径。尽管如此,我们的结果证明,运用恰当的台阵处理技术,可以在ChinArray上识别出远程微弱信号。在第二部分工作中,我们运用模板匹配滤波技术,检测2015年7.8级尼泊尔地震发生前1.6天至之后4.4天内,藏南地区的微地震活动。结果显示,匹配滤波扫描后,我们在藏南检测到5倍于中国地震台网(CENC)目录上的地震。尼泊尔主震发生后,藏南地区的地震活动性明显增加,其中包括主震发生后数小时内发生的5.8级定日地震和5.3级聂拉木地震。两次地震为正断层型,与当地拉张型构造应力状态一致。此外,通过计算由尼泊尔主震导致的静态库仑应力改变和动态应力改变,我们发现,虽然静态库仑应力改变和地震活动性的相关性略高于动态应力的改变,但动态应力改变的绝对大小(~2.2 MPa)比静态库仑应力的改变(~10kPa)大两个量级。通过模板匹配滤波扫描,我们可以检测到更多微地震活动,有助于大地震的触发作用研究。除了近场触发地震外,模板匹配滤波技术还可用于远场动态触发地震活动的探测。以2012年8.6级印度洋地震为例,运用CENC目录地震活动性分析和频谱分析等方法,我们没有在云南地区发现大范围动态触发现象。我们以腾冲地区的17个CENC目录事件作为模板,运用模板匹配滤波技术,扫描印度洋地震前1.7天至之后3.3天的连续波形数据,得到新检测地震事件12个。扫描前后,地震活动性都没有明显变化,说明腾冲地区虽然地处容易触发地震的火山区,但是在8.6级印度洋地震后3.3天内,并没有动态触发现象产生。最后通过与2004年9.1级苏门答腊地震在云南地区明显触发现象的对比,认为两次地震产生的动态应力大小或背景活动性的不同,是影响触发作用的主要因素。综上所述,基于波形的相似性,台阵处理技术和模板匹配滤波技术通过叠加或互相关扫描的方式,增强事件信号,压制噪声信号,提高波形信噪比。运用这两种技术,微弱地震信号可以被有效检测,并更好地用于解释近场和远场的地震相互作用。     

青海祁连MS5.2地震微震检测与目录完备性研究

利用模板匹配方法对2015年11月23日青海省祁连县M_S5.2地震进行遗漏地震检测研究,由于主震后短时间内目录中遗漏事件较多,故对主震后1天的连续波形进行检测。主震后1天内青海测震台网记录到的余震个数(包括单台)共62个,选取主震后M_L1.0以上余震30个作为模板事件,通过匹配滤波的方式扫描出遗漏地震31个,约为台网目录给出的0.5倍。基于包络差峰值振幅与震级的线性关系估测检测事件的震级参数,最后将检测后的余震目录与台网余震目录在主震后1天内的最小完备震级进行对比分析,结果发现检测后最小完备震级从M_L1.2降到了M_L0.7,得到青海测震台网在祁连地区最小完整性震级为M_L0.7。     

GPU并行加速下的弱模板匹配及其在川滇地区的应用

针对模板识别类方法计算耗时太长这一关键问题,将研究组之前开发的弱模板匹配方法中的互相关、归一化、拓宽峰3个主要运算部分进行了GPU并行加速,有效地提高了运算效率;进一步讨论了在离散采样下,滤波频段对于互相关计算结果的影响,并据此选取了效果好的滤波频段用于弱模板匹配;并将该方法运用到川滇地区,以双差地震层析成像法精定位后的地震作为模板,扫描了2019年6月17日四川长宁地震前后各1个月共60天的连续数据,检测出81 704个地震;根据地震发生频次与互相关值间的关系,选择适当的阈值,筛选出7 618个地震作为最终结果,是深度学习自动拾取算法检测到地震数目的 3倍。弱模板匹配方法得到的微小地震构建了更为完备的地震序列,反映出地震在时间上更多的丛聚性,结合其空间位置信息,可以用于断层形态、地震活动性及其变化方面的研究。     

2013年3月涿鹿微震群遗漏地震事件检测和发震构造分析

震群活动时,短时间发生大量地震,不同地震事件的记录波形相互交叠影响,易造成地震目录的遗漏,对震群发震构造分析等研究带来不利的影响.本文针对2013年3月3日至5日在河北涿鹿发生的微震震群,利用匹配滤波技术,以地震台网观测目录所记录地震事件的波形为模板,在连续波形记录中搜索与模板相似的信号,从而检测台网目录遗漏的地震.利用波形互相关标定新检测到地震事件的P波和S波到时,进而对其震中位置和震级做出估计.计算结果显示,通过互相关扫描检测到52个地震台网常规分析遗漏的地震,约为地震目录给出的45个事件的1.16倍.检测到的遗漏地震震级估算为M_L0.1~0.9,通过震级-频次统计分析,加入遗漏地震后地震目录的完整性在M_L0.3~0.8范围内有较明显的改善.根据地震事件精定位结果,推测此次震群的发震构造为北西走向倾角较大的断层,施庄断裂为发震构造的可能性较大.     

2017年9月临城震群遗漏地震检测及发震构造初探

以河北测震台网记录的连续波形数据为基础,采用模板匹配滤波方法,检测2017年9月4日—6日河北临城震群遗漏地震。利用波形互相关扫描,检测到19次被测震台网常规分析遗漏的地震,震级范围为M_(L )0.0—1.2,并标定新检测地震事件的P波和S波到时,估计其震中位置及震级。通过地震精定位和最大地震震源机制解,认为此次临城地震序列的主要发震构造应为近EW向活动断层。     

以虚拟地震的理论格林函数为模板搜寻小地震

搜寻小地震而得到更加完备的地震目录是地震学的基本课题.传统的匹配滤波方法用已知地震的波形与连续观测做互相关,可有效识别小地震.然而,一些地区缺乏早期观测或地震活动性低而无真实地震可做模板,造成传统匹配滤波法难以施展.使用虚拟地震的理论波形做模板可解决该问题.若使用的虚拟地震包含所有可能的震源机制,即离散地覆盖整个震源机制解空间,虚拟地震的数量将大量增加,导致计算量剧增.本研究借鉴裁剪-粘贴法(CAP)中对滑动互相关的处理方式,发展了以虚拟地震的理论格林函数为模板的匹配滤波方法(Green′s function-based matched filter,简称为GFMF),在不改变计算结果的前提下通过减少滑动互相关的次数,节省计算时间.本文将该方法应用到加州一地震序列,凭借对震源位置和震源机制的网格搜索,得到了该序列的时空分布变化特征和该区域全部中等地震的震源机制.研究结果显示,虚拟地震可以用作模板来检测小地震以解决真实模板地震不足的问题.若不对虚拟地震的震源机制进行遍历,得到的地震数量将减少70%以上.这表明本研究对震源机制遍历的相关优化是重要的.     

波形模板匹配定位方法在北京地区的应用

在地震学研究中，高效的微震检测方法是既重要又具有挑战性的问题。本文对波形模板匹配检测方法、匹配定位技术、波形自相关检测技术进行详细介绍和对比，对国内外应用实例进行总结，并展望微震检测方法应用前景和发展趋势。利用基于图形处理器加速的匹配定位技术和双差地震定位法，对北京地区19个台站记录的2015年连续地震资料进行分析。基于中国地震台网中心提供的地震目录，筛选出245个地震事件作为模板事件，检测得到1229个地震事件，约为地震台网原始地震目录数量的5倍。精定位结果可显示小震沿黄庄-高丽营断裂周边小断裂分布形态特征，本文微震检测和定位结果可为研究北京地区地震活动性与发震断层深部构造提供基础数据支撑。     

一种基于模板匹配的震相关联和提取技术及其初步应用

本文提出一套有别于常规模板匹配算法的流程,不仅能够提供被识别地震的时间、空间、强度信息,还可根据震相关联提取每个地震对应的震相到时资料供后续研究使用。该流程基本原理为:地震事件发生会使得震相记录在时间上趋于丛集,因此可利用密度聚类方法对震相信息进行提取。具体实现步骤为:(1)对采用相同降采样和滤波处理的模板事件和连续波形进行互相关系数计算,挑选相关性超过阈值的到时信息形成离散时间序列;(2)对整个序列进行DBSCAN聚类分析并对每个聚类簇开展自洽性检验,获取单个地震的震相关联,进而提取震相到时;(3)通过去重复事件、震级测算与定位获得最终的结果。将该方法应用到龙门山断裂带南段地震空区,结果显示,2013年5月至2016年12月,35个模板共获得了93个确切的地震信息,是所使用模板的2.65倍,包含390条P波震相以及791条S波震相。绝大部分模板并没有识别出自身的重复/相似地震,而被识别出的地震是由少量模板匹配成功的,主要分布于双河和安顺附近。精定位结果显示,双石-大川断裂倾向NW,同时在安顺以西可能存在某条不知名的、浅部直立、深部倾向NW的"铲状"断裂,震源深度与芦山地震序列类似,存在自SW至NE变深的现象,可能反映了龙门山断裂带南段不同段落具有类似的构造特征。测试结果表明,本文设计的流程可提供包括震源参数和每个地震震相到时信息在内的完整震相报告,在模板识别领域可作为一套计算机自主处理的可行方案。     

2017年九寨沟MS7.0地震序列活动特征分析

中强地震序列的主震发生后,短时间内受台站距震中较远、尾波干扰和波形重叠等因素的影响,往往会遗漏大量的地震,而地震目录的完整性会直接影响到震后趋势判定和余震序列特征分析的科学性和可靠性。本文利用基于GPU加速的模板匹配方法对2017年8月1～12日的连续波形进行扫描计算,检测九寨沟M_S7.0地震前后遗漏的地震事件,选取台网目录中信噪比较高的1033个地震事件作为模板,在主震前7天至震后5天期间识别出4854个检测地震事件,为台网可定位目录的3.3倍,除去对台网单台地震事件的修正外,还检测到1797个遗漏地震事件,将完备震级从1.6级降低到1.4级。基于补充了遗漏地震的完整地震目录,对2017年8月8日九寨沟M_S7.0地震序列活动特征进行分析。结果表明,前震序列在主震前短时间内出现了地震活动的密集增强,b值也显示为低值状态,可能是深部断层发生破裂之前的加速蠕动的结果。随着时间的推移,余震序列的完备震级逐渐下降并趋于稳定,b值存在缓慢升高的趋势,未来较长时期内余震序列仍将处于持续衰减的状态。     

A 15-year-Long catalog of seismicity in the Eastern Tennessee Seismic Zone (ETSZ) using matched filter detection

We present a detailed catalog of 13 671 earthquakes in the Eastern Tennessee Seismic Zone (ETSZ) that spans January 1, 2005 to July 31, 2020. We apply a matched filter detection technique on over 15 years of continuous data, resulting in arguably the most complete catalog of seismicity in the ETSZ yet. The magnitudes of newly detected events are determined by computing the amplitude ratio between the detections and templates using a principal component fit. We also compute the b-value for the new catalog and comparatively relocate a subset of newly detected events using XCORLOC and hypoDD, which shows a more defined structure at depth. We find the greatest concentration along and to the east of the New York-Alabama Lineament, as defined by the magnetic anomaly, supporting the argument that this feature likely is related to the generation of seismicity in the ETSZ. We examine seismicity in the vicinity of the Watts Bar Reservoir, which is located about 5 ?km from the epicenter of the M_W 4.4 December 12, 2018 Decatur, Tennessee earthquake, and find possible evidence for reservoir modulated seismicity in this region. We also examine seismicity in the entire ETSZ to search for a correlation between shallow earthquakes and seasonal hydrologic changes. Our results show limited evidence for hydrologically-driven shallow seismicity due to seasonal groundwater levels in the ETSZ, which contradicts previous studies hypothesizing that most intraplate earthquakes are associated with the dynamics of hydrologic cycles.     

On applicability of the RTL prognostic algorithms and energy estimation to Sakhalin seismicity

Premonitory phases (seismic quiescence and foreshock activity) have been retrospectively identified before the Neftegorsk and Uglegorsk earthquakes using the RTL technique. The probabilities that these phases were accidental are less than 1 and 2%, respectively. This allows an optimistic assessment of the possibility of applying this technique to seismicity at Sakhalin. The estimates of the time and energy class for the two earthquakes, using a model of self-organized seismic criticality, proved to be unconvincing because obvious acceleration of the seismic process prior to these seismic events did not occur. The applicability of this approach to the seismicity at Sakhalin should be tested for future large earthquakes. The regional Sakhalin catalog for 1980–2000, with a lowest completely reported energy class of K = 8 (lent by the Geophysical Service, Russian Academy of Sciences) was used as the database for this study.     

A double seismic antenna experiment at teide Volcano: existence of local seismicity and lack of evidences of Volcanic tremor

Data analyzed in the present work correspond to a 40 days field experiment carried out in Teide Volcano (Canary Islands, Spain) with two short-period small-aperture dense seismic antennas in 1994. The objective of this experiment was to detect, analyze and locate the local seismicity. We analyzed also the background seismic noise to investigate the possible presence of volcanic tremor. From a set of 76 events, we selected 21 of them in base of their good signal-to-noise ratio and their possibility to locate their seismic source by using the seismic antennas. A visual classification based on the S–P time and seismogram shape has permitted to establish three groups of events: local seismicity (S–P time between 3 and 5 s), very local earthquakes (S–P time smaller than 3 s) and artificial explosions. These earthquakes have been located by applying the Zero Lag Cross-Correlation technique and the inverse ray-tracing procedure. Those earthquakes that were recorded simultaneously by both seismic antennas were also located by intersecting both back-azimuths. The analysis of the seismicity has revealed that the amount of seismicity in Teide Volcano is moderate. This seismicity could be distributed in three main areas: inside the Caldera Edifice (below the Teide–Pico Viejo complex), in the eastern border of the Caldera Edifice and offshore of the island. At present, this activity is the only indicator of the volcano dynamics. The analysis of the back-ground seismic noise has revealed that at frequencies lower than 2 Hz, the Oceanic Load signal is predominant over other signals, even over local earthquakes with a magnitude of 2.0. Due to this, although if in the Teide area were present a weak volcanic tremor, or other volcanic signals with predominant peaks below 2 Hz, to observe them would be a very difficult task.     

Seismic Source Classification by Means of a Sonogram- Correlation Approach: Application to Data of the RSM Seismic Network (Central Italy)

—?In this paper, we apply a new sonogram correlation approach for seismic source classification. In order to test the technique, we use the waveforms recorded by 6 among 16 of the stations of the Rete Sismometrica Marchigiana (RSM) network, relevant to the vertical component of 365 earthquakes that occurred in the analyzed area from April 1996 until September 1999. All the sonograms of each station are cross-correlated in a single station procedure and the calculated degrees of resemblance are used to classify the events by means of a bridging technique. The results obtained with the single-station approach are integrated into a network classification. We identified three families in the investigated area. Once the efficiency of the method is tested, we analyzed the families obtained for the selected subset of events. Considering the temporal and the spatial distribution of the events forming each family, we classified them as quarry blasts. This classification, even if relevant to a subset of earthquakes occurring in Central Italy, modifies the present knowledge of the seismicity distribution in the area and allows estimation of the errors affecting the hypocenter locations provided by the RSM network. An extension of this new technique to the whole Central Italy earthquake catalog promises an interesting review of the seismicity in the area.     

地震活动图像特征与强震危险区经验性预测依据的初步研究

在研究近期（10年）强震危险性判定和总结四川地区1972--2002年强震预测经验的基础上，清理了中国大陆1920--2002年47次浅源大震事件前地震活动图像特征（M≥4．7），提炼出10个方面12项经验性预测依据：主体活动区、多发时段、关联序列、大陆及地区地震异常图像、地区（带）-地段（震源区）地震增强图像（含信号震、诱发地震、地震条带）、相关地震、窗口地震、复发间隔、大陆及地区缺震、地段缺震与地段强震一缺震转折等。它们在47次震例中的综合出现率≥0．58（即依据比7／12以上）占42例；≥0．66（即8／12以上）占34例。因此，强震（M≥4．7）活动图像经验性预测依据可以作为预测有较大可能发生大地震危险区的依据之一。     

2014年鲁甸MS6.5地震前后地震活动性

2014年 8月3日在云南省鲁甸县发生M_S6.5地震,造成了巨大的人员伤亡和财产损失.关于鲁甸地震的起因,特别是鲁甸地震与溪洛渡水库之间的关系存在一些争议.分析中强地震前后地震活动性的变化可以为判断主震起源提供一定的依据.然而,由于鲁甸地震震中附近固定台站数目少、台站距离震中远及主震后尾波干扰等因素的影响,会造成地震目录的缺失,从而可能影响地震活动性的可靠分析.为了获得更加完备的地震目录,本文采用模板匹配方法对鲁甸地震前后小地震进行了搜索和识别.以台网目录中的541个事件作为模板,在主震之前33天到主震后5天的连续波形中识别出991个未在目录中列出的地震,使余震的完备震级从1.8降低到1.3.根据检测目录得到主震前33天内的b值为1.04±0.18,余震的b值在震后5天内由0.5左右逐渐上升至0.9附近并保持稳定;根据2009年到主震发生前的台网目录得到该区域的背景b值为0.93±0.04.即鲁甸地震前后区域地震活动b值均与背景b值接近,这与典型水库诱发的中强震具有的b值特征不相符.同时通过进一步分析台网目录,未发现溪洛渡水库蓄水前后研究区域内的地震活动性存在明显变化,这也与典型水库诱发地震区域的地震活动性特征不相符.综合以上结果,我们认为2014年鲁甸M_S6.5地震不具有典型水库诱发地震的特征.     

2008年汶川8．0级地震前川青块体及邻区地震活动性分析

本文简要分析了2008年5月12日汶川8．0级地震前川青块体及邻区的地震活动性特征。结果表明,汶川地震发生的龙门山断裂本身历史地震活动水平不高,其周边的鲜水河断裂、岷江断裂、虎牙断裂等历史上强震多发;汶川8．0级地震前龙门山断裂带3级以上中小地震活动增强或平静异常现象不明显,地震活动参数值均在正常变化区间内;4级以上地震异常平静,形成了地震空区;2007年底到2008年初位于龙门山断裂带及其附近台站的0．1级以上小震月频度增强明显。     

Pattern Dynamics and Forecast Methods in Seismically Active Regions

—Large, extended fault systems such as those in California demonstrate complex space-time seismicity patterns, which include repetitive events, precursory activity and quiescence, and aftershock sequences. Although the characteristics of these patterns can be qualitatively described, a systematic quantitative analysis remains elusive. Our research suggests that a new pattern dynamics methodology can be used to define a unique, finite set of seismicity patterns for a given fault system. In addition, while a long-sought goal of earthquake research has been the reliable forecasting of these events, very little progress has been made in developing a successful, consistent methodology. In this report, we document the discovery of systematic space-time variations in seismicity from southern California using a new technique. Here we present examples of this analysis technique on data obtained prior to events in seismically active areas that show coherent regions associated with the future occurrence of major earthquakes in the same areas. These results strongly support the hypothesis that seismic activity is highly correlated across many space and time scales within large volumes of the earth's crust.     

Temporal change of characteristics of shallow volcano-tectonic earthquakes associated with increase in volcanic activity at Kuchinoerabujima volcano, Japan

Shallow volcano-tectonic (VT) earthquakes recorded at the Kuchinoerabujima island volcano in southwest Japan are analyzed in order to clarify the role of hydrothermal activity in the development of volcanic seismicity. From analysis of shallow VT earthquakes in 2006, two specific episodes of elevated seismicity are observed in April and November 2006. The VT earthquakes have hypocenters at depths of 0–0.4 km beneath the summit crater, and normal fault focal mechanisms with WNW–ESE extension consistent with the tensional stress field indicated by the alignment of craters and fissures. Although the hypocenters and focal mechanisms are found to be largely invariant during these episodes, the corner frequencies of the VT earthquakes underwent a pronounced increase and decrease accompanying the changes in seismicity rates. The corner frequencies increased to 20–25 Hz approximately one month prior to the onset of elevated seismicity, and then decreased to 10–15 Hz in the period of peak seismicity. The rupture length also decreased at the onset of seismicity, thereafter increasing as the seismicity continued. The peak seismicity in terms of the daily number of VT events was accompanied by inflation around the crater, suggestive of a pressure increase in the volcanic system. It is inferred that the increase in shallow VT seismicity and rupture length is related to the development of a fractured zone. The pressure increase in the volcanic system is attributed to the intrusion of hydrothermal fluids, which is supported by an observed increase in fumarolic temperature and activity. The preceding monochromatic events are thus considered to be generated by the effect of fluid-filled cracks. The shortening of rupture length is then inferred to be related to the closing of non-fluid-filled cracks in the fracture zone under the increasing pressure field, leading to a transition from monochromatic events to low-frequency and shallow VT seismicity.     

Shallow seismicity at Montagne Pelée volcano,Martinique, Lesser Antilles

Continuous seismic monitoring at Martinique since the 1902 eruption of the Montagne Pelée volcano did not detect local earthquakes for the first 70 years. For the only eruption which occurred in this time span in 1929 the seismograph was 20 km away and of a standard type, not particularly suited for the detection of small-scale local seismicity. Improvement of the monitoring array over the last 15 years with the installation of sensors on the volcano itself allowed the detection of signals of local origin which were interpreted as being due to surface sources, such as rockfalls and landslides. Since December 1985 seismic sources in the volcano itself, i.e. small earthquakes at shallow depth, were identified and located with the aid of a temporary upgrading of the array close to these weak sources. Such an onset of local seismicity could not have been detected with previous seismic equipment; such episodes of seismicity in the volcano might have occurred in the past, apparently quiescent history of the volcano as the reinterpretation of seismograms of some events in 1976 would indicate, without evolving to more important volcanic phenomena. For seismographs on volcanoes the constant upgrading of observation capabilities is certainly perferred to a strict continuity of standard observations.