Asymmetrical pulses, the periodicity and synchronization of low frequency microseisms

Seismic records of eight IRIS broadband stations were analyzed at distances of 70 to 7160 km from the magnitude 8.3 Hokkaido earthquake of September 25, 2003. The stations situated in the subduction zone recorded asymmetrical microseismic pulses lasting 3–10 min a few days before the earthquake. No such pulses were observed in the records of the stations situated outside the subduction zone. Similar pulses were also recorded before the magnitude 7.8 Kronotskii, Kamchatka earthquake of 1997. The pulses are hypothesized to have been caused by creeping movements. Synchronous oscillations of microseismic noise with periods of 1–3 h were recorded as far as 3000 km from the Hokkaido earthquake a few days before it occurred. The noise coherence measure increased for stations closer to the epicenter. The question of the source of this coherence remains open. These effects belong to the class of those occurring in dissipative metastable systems; parts of the terrestrial lithosphere during the precursory periods of seismic catastrophes seem to be such systems.     

玛多7.4级地震前临震微波动现象研究

强震临震微波动是强震临震阶段优势频率集中在11～16 Hz、频谱形态较为整齐的临震异常脉动记录。利用"兰州临震微波动实时监控与跟踪系统"的数据资料,对2021年5月22日玛多7.4级地震震中200 km半径的范围内台站的临震微波动事件进行分析。认为震前距离震中67 km的巴颜喀拉台EW通道存在显著异常,达到杨立明提出的异常跟踪指标;10个测震台站中的8个在本次地震前记录到了临震微波动事件,震中南侧台站记录到的事件多于北侧。     

Some properties of low-frequency seismic noise

The low-frequency seismic noise recorded by the broadband IRIS stations in 1994–2012 is studied in the period range of 40 to 360 s. It is shown that for samples of a few months in length, the power spectra of noise at stations spaced apart a few thousand kilometers and operating in different meteorological and seismotectonic conditions are overall similar, which indicates that the sources of the noise are global. At the same time, the slope of the spectra changes with the increase in the period in the subintervals of 40–90, 120–200, and 200–360 s, which points to the difference of the sources generating the seismic noise. The amplitude of the noise at the stations located a few thousand km apart from the Sumatra earthquake of December 26, 2004, M = 9.2, and from the Tohoku earthquake of March 11, 2011, M = 9.0, increased after these events. This indicates the global character of the aftermath of these seismic catastrophes. After the Kronotskoe earthquake of December 5, 1997, which was weaker (M = 7.9), the noise grew only at the PET station located within 300 km of the epicenter. According to the records at the PET station, this earthquake was preceded by the increased noise level observed in 1994–1997. After 1999, the seismic noise declined and remained low up to the end of the studied interval with a duration of 14 years. Our results show that the low-frequency seismic noise generated by the sources in the atmosphere of the Earth is contributed by the processes taking place in the lithosphere.     

Microseismic anomalies before the Sumatra earthquake of December 26, 2004

Microseismic records from five broadband IRIS stations located at distances of 1000–2000 km from the earthquake source are studied. Unordinary programs are used to extract hidden periodicities, determine signal coherence at different stations, and reveal asymmetry in wave amplitudes. The records obtained at a few stations 60 h before the Sumatra earthquake include periodic oscillations in the range of periods from 20 to 60 min that arose after the McQuary earthquake and continued for about 24 h. Synchronization of waves recorded at all stations commenced 53 h before the Sumatra earthquake and continued up to the time of the earthquake, with the predominant period gradually increasing from a few minutes to tens of minutes.     

唐山7.8级强震前震中周围形变电阻率的下降异常

1976年7月28日唐山7.8级强震前,北京、天津、唐山地区已布设14个形变电阻率观测台站,其中9个台,震前2-3年内记录到形变电阻率长趋势下降异常,这9个台围绕震中形成一个半长轴可达150公里的异常区.位于震中区的两个台还记录到临震前2-3个月内的加速下降异常.根据现场条件下地表浅部岩（土）层受压时,视电阻率下降的实验结果,认为在震中周围较大的区域内,震前浅部岩（土）层被压密,其压缩线应变的数量级约为3×10^-5.这一认识得到唐山震前地面基线实测资料的支持.     

漾濞县MS6.4、玛多县MS7.4地震前地电场变化分析

2021年5月21日云南漾濞县发生M_S6.4地震、5月22日青海玛多县发生M_S7.4地震,分别分析漾濞震中500 km内12个地电场台数据、玛多震中500 km内8个地电场台数据,获知:(1)漾濞地震周围罗茨等8台优势方位角在震前出现了异常变化,弥渡等4台看不出明显异常变化;盐源等7台相关系数在震前2～6个月出现了大幅度下降,元谋、苴林等5台相关系数看不出明显异常变化。(2)玛多地震周围门源等4台方位角在震前出现了异常变化,都兰等4台方位角看不出明显异常变化;门源等5台在玛多地震前2～6个月相关系数出现了大幅下降、变化范围明显变窄等现象,都兰、白水河、金银滩等3台优势方位角和相关系数都看不出明显异常变化。(3)两次地震分析中地电场优势方位角和相关系数异常变化在时间上皆具有同步或准同步性。     

1976年松潘7.2级地震前几种异常波形的记录特征及其在大震短临预报中的意义

本文研究了１９７６年松潘７．２级地震前，几个台站记录到的异常地震波形的形态特征及时空分布特征。异常波形可分为４种形态；震中距小于８０ｋｍ的台站都不同程度地记录到了异常波形；临近地震时，记录中出现的波形数量增多，振幅增大；临震前１０天波形密集，呈现明显的突跳变化。根据异常波形的特征，本文还对异常波形的物理机制进行了初步的研究，认为异常波形的出现是地震前孕震区岩石产生微破裂的结果。     

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.     

2011年缅甸7.2级地震前云南地下流体异常特征分析

对2011年3月24日缅甸M_S7.2地震前云南地区地下流体前兆异常进行了系统研究。 结果表明, 云南距震中540 km范围内共有14测点17个测项异常; 异常时空特征表现出异常演化具有明显的阶段性, 地震前5个月开始异常逐月增多, 并似有向震中收缩发展的迹象。     

汶川8.0级地震前定点形变异常特征

对汶川8.0级地震震中周围800 km范围内近50个定点形变台站资料作小波变换处理,发现从震前3个月开始,30个台站资料处理结果显示有异常出现.距震中200 km以内的台站,其异常信息主要集中在小波分解的细节部分第6层;距震中200-550 km范围内的台站,其异常主要集中在第7层或第8层;距震中550-650 km范围内的台站,其异常主要集中在第8层或第9层.表明异常信息的频率随着各台站距离震中的远近呈现由低到高的特征.     

利用地电阻率短临异常预报天祝—景泰6.2级地震的可能性

景泰6.2级地震前,震中300km范围内虽有9个地电台,但有短临异常的台站仅三个,且异常时空分布规律性差。由于这些特点,尽管震前已发现了这些异常,但终因不认识其复杂图象而未作出短临预报。本文利用地电异常与断裂展布、发震应力之间的关系研究结果,解释了景泰地震前地电短临异常的时空分布特征,认为这种异常图象是特定构造格局下的真实反映。在此基础上,探讨了震前作出地震三要素预报的可能性。     

Migration of pulsations after earthquakes

The analysis of the records from IRIS broadband seismic stations revealed wave trains with a period of ~129.5 min emerging after the strong earthquakes. These wave trains are simultaneously observed at the stations shifted by 180° in longitude in the form of the oscillations with opposite polarity. The occurrence time of the wave trains at the different stations testify to their westward migration by 2.5° per hour. The wave trains are earlier recorded at the stations that are close to the earthquake’s epicenter and to its antipode; they become less manifest with the increase in the distance from the station to the line connecting the epicenter with its antipode. The amplitude of the oscillations in the wave trains is estimated at a few mm and μGal. The wave trains presumably reflect the emergence of an area of free postseismic flexural oscillations of the lithosphere.     

7级以上强震前水温月超差个数异常分析

选取近年来中国大陆及周边3次7级以上地震,对周边地震一定范围内“十五”数字化水温台站的月超差个数进行统计分析。结果表明,2010年玉树7．1级地震前,周边700 km 以内77％的水温台站出现月超差个数的向上突跳异常。2011年缅甸7．2级地震前,未发现此项异常。2013年芦山地震前,周边700 km 内仅有13％的水温台站出现此项异常。这3次7级以上地震,震前不同的月超差个数异常特征,应与每次地震不同的发震构造和孕震过程有关。目前,此项异常震例较少,且仅在青藏高原有明显的前兆意义。     

2020年7月12日唐山MS5.1地震预警处理能力分析

2020年7月12日,河北唐山发生M_S5.1地震,河北省地震预警系统成功地处理并产出了这次地震预警各种结果数据,本文借助此次地震对河北地震预警网内震中距200km范围内台站产出质量以及地震预警前5次处理结果进行详细分析。此次地震发生在河北地震预警网内,平均台间距为10km,首台触发后3s、震后6s发布首次处理结果,与编目结果相比,震级偏差为-1.3,震中位置偏差为2.6km,盲区半径为18km。随着参与定位台站数量增多,震级与位置偏差越来越小,但震级仍整体偏小。河北地震预警网台站产出质量整体较高,其中烈度台作为地震预警最重要的组成部分,是决定预警效果的关键因素。本次地震震中距200km范围内,烈度台平均信噪比为48,震中距50km范围内平均信噪比为112,符合预警系统对信噪比的要求。本次地震预警结果表明,河北地震预警网内台站布局基本合理,波形质量较高,地震预警系统处理软件在本次地震中预警产出效果较好,已经具备了一定的地震预警能力。     

1976年唐山地震前土层水平最大剪应变的时空分布

土层应变仪观测网的观测结果表明:土层水平最大剪应变在唐山地震前有异常,异常后便发生了主震;异常时间等值线从震中向外围传播,其传播速度为每天0.1—6.6公里,从震中区向外围加快,以沿着通过唐山主震震中区的北东和北西向断裂带的传播速度为最快;最高等值线圈闭震中或在震中附近     

唐山地震前近震中区地电阻率的震前突变现象

1976年唐山地震前,在距震中250公里范围內已经建立了16个地电台站。震前10天内有几个台站地电阻率在较平静的趋势背景值上出现大幅度突变异常,尽管这些台站占少数,但却集中分布在发震的沧东断裂附近。 根据唐山地震震源机制解结果,以及发震构造和地下水位变化的实际资料说明,地电阻率突变异常的空间分布与唐山地震断层面是一致的,与震前地下水位变化异常带也是吻合的,地下水震前变化在时间上与地电阻率突变的出现是同步的。      

Characteristics of Far-field Precursory Anomalies Before the M_S8.1 Earthquake in the West of Kunlun Mountains Pass

In this study, a number of typical precursory anomalies recorded by stations in Qinghai, Gansu, Sichuan, Xinjiang, Ningxia, Hebei and Shaanxi provinces and autonomous regions before the M_S8.1 earthquake in the west of Kunlun Mountains Pass are collected and checked. According to the standards of earthquake cases in China, the criteria of the precursory anomalies are determined, and 53 distinguished. The characteristics of these anomalies before the M_S8.1 earthquake are analyzed, with results showing a very large earthquake affected area. The precursory anomalies recorded by instruments were 2900 km away from the epicenter, and according to the study in this paper, reached 2100 km away. The results also show that the anomalies present characteristics of long duration, multi-measurement items and large-amplitude variation. The authors believe that in large earthquake monitoring, attention should be paid to the variation of data over a large area, ranging up to thousands kilometers, with much denser earthquake observation networks.     

基于断层虚位错模式讨论2008年汶川MS8.0地震前视电阻率变化

2008年汶川M_S8.0地震前,距震中450km范围内有7个视电阻率台站运行,震前记录到了不同形态和幅值的变化。本文采用断层虚位错模式,在模型中将汶川地震同震滑动位移按大小相等但方向相反的方式进行加载,计算震前产生这些同震位错所需积累的应力应变分布。计算结果表明：在100m深度,震前正应力和剪切应力主要积累区域位于龙门山断裂带两侧约100km范围内,最大应力变化量为0.5MPa;体应变积累区域与应力积累区域基本一致,最大应变值为10^-5,显示这些区域在震前存在较高的挤压应力应变积累,而在这些区域外,应力应变积累程度较低。成都台和江油台位于震前主要的应力应变挤压积累集中区域,视电阻率观测数据均为震前下降-准同震阶跃-震后回升,与震前应力应变积累-震后释放的变化形式一致,这2个台站震前的异常变化与汶川地震关系密切;而甘孜台、武都台、冕宁台、小庙台和天水台位于应力应变积累较弱的区域,其视电阻率变化与汶川地震的关联性较弱。     

岷县-漳县MS6.6地震前后甘肃前兆异常及响应特征

分析总结了2013年7月22日甘肃岷县—漳县MS6.6地震前后甘肃前兆台网观测资料变化情况,发现该地震前有7个台站、4种观测手段、14个测项出现了明显的异常,既有长期、中长期异常,也有短期和短临异常,说明对资料的分析既要看短期变化,还要看中期和长期变化。出现异常测点的震中距多数在200 km范围之内。另外部分水位、流量和水温测点的资料记录到不同程度的同震效应;形变观测多数测点记录到比较明显的同震响应,记录同震响应的测点与震中位置、方向、距离无明显关系。     

新疆伽师地震的近场观测和对它的初步认识

1998年5月～7月中国地震局物探中心在伽师震区布置了一个40×50km2的三维地震观测台网,布置了19台轻便地震仪,在二个月的时间里接收到近700个地震事件,并接收了十一个炮点的人工地震资料.对天然地震资料进行了精确定位,震级大于26的地震计算了震源机制解,并进行了讨论.最后对伽师10次6级以上地震的震中位置及能量的释放过程以及与坷坪断裂的关系进行了初步探讨