与地震有关的电离层扰动的甚低频/低频无线电探测

近期人们认识到,电离层对地震影响非常敏感,探测与地震有关的电离层扰动看来对地震短临预测是非常有前途的。我们提出可利用甚低频／低频（3～30kHz／30-300kHz）无线电探测进行地震-电离层扰动的探测。1995年神户地震明显发现电离层扰动现象之后,开始了利用低电离层甚低频／低频传播信号进行地震短临预测的简短历史。在说明先前的甚低频／低频结果之后,我们给出了最新的甚低频／低频发现：一个是电离层扰动与地震在统计上相关,第二个是2004年12月苏门答腊地震的实例研究,说明了这次地震电离层扰动的空间尺度和动力学原理。     

On the correlation between ionospheric perturbations as detected by subionospheric VLF/LF signals and earthquakes as characterized by seismic intensity

The long-term data during seven years from January 2001 to December 2007, as observed by the Pacific VLF/LF network consisting of several Japanese stations and one station in Kamchatka, are extensively utilized to perform a statistical correlation study between the lower ionospheric perturbations as detected by subionospheric propagation and earthquakes (EQs). In this paper, we adopt a physical parameter, the maximum seismicity intensity observed (I) to define the strength of an EQ unlike the previously and conventionally used EQ magnitude and depth, which is a combined effect of EQ magnitude and depth, together with the Earth’s surface information and geological condition around the EQ epicenter. After considering EQs only take place on the land because of the use of seismicity and by using the superimposed epoch analysis, it is found for the larger EQs with I from 5 to 7 (we feel serious trembling and we expect serious damage) that the most important VLF/LF parameter, trend (nighttime average amplitude), shows a definite decrease about 10 days before the EQ by exceeding 2σ (σ: standard deviation) criterion; the dispersion shows a maximum about 10 days before the EQ but not exceeding 2σ line and finally the nighttime fluctuation shows an enhancement about 10 days before the EQ (with exceeding the 2σ level). A definite statistical correlation is confirmed between the ionospheric perturbations and I when I is strong enough in a range from 5 to 7. Whereas, there is no significant correlation between the two when I is in a range from 3 to 4. Finally, together with the corresponding results for EQs in the sea, but close to the land, these results are discussed in the light of lithosphere–ionosphere coupling mechanism.     

甚低频/低频人工源电波信号地震应用研究进展

甚低频/低频人工源电波作为一种主动源发射的通讯导航信号,因其长距离波导传播的特性在地-电离层观测及应用中表现出极大的优势.本文总结了二十世纪末以来基于甚低频/低频(VLF/LF)电波观测技术及其数据分析方法、典型震例及统计研究成果、圈层耦合机理等方面的研究进展,并通过探讨其应用成果的快速积累及前兆扰动起源等研究难点,展...     

强震前ELF/VLF磁场的扰动特征统计研究

本文利用DEMETER卫星记录的变化磁场数据统计研究了2005—2009年北半球7级以上强震前后空间磁场的扰动特征.在震中±10°范围内利用震前2个月至震后1个月的5年同期观测的ELF/VLF[370~897 Hz]频段磁场功率谱密度数据构建了背景场,定义了表征空间磁场在地震时段相对于背景场的扰动幅度指标.统计研究发现26个强震中42%的地震主要表现为震前磁场扰度幅度逐渐上升,超过3倍标准差,随后在扰度幅度下降过程中发震.35%强震在地震前10天内最大扰动幅度超过3倍标准差,在扰度幅度处于最高值期间发震,震后磁场扰动幅度逐渐回落.23%的强震在震前震后无明显规律.随机事件测试发现在空间天气平静且无强震条件下随机地点上空的磁场无明显的时空演化规律,且随机地点上空的空间磁场相对于背景场的平均扰度幅度很小,最大扰度幅度从未超过2倍标准差,并且没有明显的随时间变化特征.     

基于人工源的甚低频电磁波空间传播特征统计分析

利用2005年1月至2010年11月DEMETER卫星记录的NWC发射站的VLF电场功率谱数据,采用指数拟合的方法,分析了VLF电磁波在卫星高度激发的电场空间分布和衰减特征.研究结果表明: (1) VLF电场在发射站上空及其磁共轭区有着很强的对应关系,存在南、北2个强电场中心涡; (2) 相对于发射站的位置,VLF电场中心点具有经度和纬度偏移,日侧地磁经度偏移均值大于夜侧,而地磁纬度偏移均值则小于夜侧; (3) 日侧VLF电场强度呈现出周期性的年变化; (4) 在VLF电场中心10°范围内,电场强度随距离快速衰减,衰减常数b在长达6年的时间内保持稳定.在以上研究结果基础上初步构建的卫星高度人工源电磁波空间分布特征,将为研究地表-电离层电磁波传播机理提供基础技术支撑.     

有限元方法研究精细地-电离层波导VLF波传播特性

基于频域有限元方法仿真了精细地-电离层波导结构甚低频(VLF, Very Low Frequency)电磁波幅度和相位传播特性.首先给出了如何确定VLF波发射点和接收点传播路径所在大圆的方法, 接着分析了获取波导结构中不同阶波导模式传播系数的耦合模理论, 数值讨论了不同阶波导模式的衰减率和相对相速度随模式数和工作频率的变化关系, 给出了波导不同结构参数对一阶模衰减率和相对相速度的影响.与文献结果对比验证了本文有限元方法模拟VLF波传播的正确性和高效性.最后利用实验和数值仿真结果, 通过对电子密度国际电离层参考模型进行修正, 讨论了澳大利亚—青岛台VLF波幅度和相位日变化的规律.本文工作为研究地-电离层波导结构VLF波电磁特性以及提高陆基超远程甚低频导航系统的精度, 提供了新的思路和方法.

     

利用中国GPS站网对地震波引发的大尺度电离层扰动的观测

地震瑞利波在沿地表传输时衰减很慢,其能量在远离震中的区域仍然能够激发大气和电离层扰动.本文利用中国境内的GPS接收机网络观测电离层总电子含量（total electron content,TEC）,分析了2011年日本地震后在中国区域上空产生的电离层扰动.研究发现,瑞利波的能量从地面经大气耦合传输到电离层高度,导致在中国区域上空电离层出现与瑞利波传播同步的TEC扰动.利用中东部的稠密接收机网络,还揭示了扰动的大尺度二维空间结构：瑞利波经过后产生的TEC扰动呈条带状,在中纬度地区沿西北-东南方向排列,而在低纬度大致为东西方向.条带的转向可能与地磁场作用下的中性-离子耦合过程有关,大气波动导致的等离子扰动倾向于沿磁力线方向（向南）传播,从而形成垂直磁力线方向（东西）的波前结构.这是首次在远离震中的区域使用GPS站网研究地震波耦合电离层扰动的大尺度二维空间结构.

     

Subionospheric VLF/LF monitoring of ionospheric perturbations for the 2004 Mid-Niigata earthquake and their structure and dynamics

This paper reports on the detailed spatial structure and temporal dynamics of the ionospheric perturbations associated with the huge 2004 Mid-Niigata earthquake in Japan. The terminator time method in VLF/LF subionospheric propagation has been used to deduce the presence and dynamics of the seismo-ionospheric perturbations by making full use of our Japanese VLF/LF network observations. It is found that significant shifts were observed in terminator time for some selected paths, a few to several days before the earthquake, and we obtained the anisotropic shape of the inferred ionospheric perturbation likely elongated along the fault lines there. The temporal dynamics of inhomogeneity of the perturbations is suggested on the basis of a comparison of the observed terminator time shift and theoretical full-wave computations (2D FDTD and 3D full-wave scattering).     

玉树地震前电离层VLF信号信噪比变化

2010年4月13日23点49分38秒(世界时), 青海省玉树县发生了M_S7.1地震. 根据法国DEMETER卫星电场探测仪(ICE)实验数据, 研究震区上空半径500 km范围内电离层10—20 kHz甚低频(VLF)电场频谱信噪比发现, 玉树地震前3个不同频率的地面VLF发射站信号对应的信噪比均出现相同的变化特征, 即震前DEMETER卫星1个重访周期内的平均信噪比明显减弱, 而2009年相同时段和区域的平均信噪比并未出现相似变化趋势. 分析认为, 玉树地震孕震期信噪比衰减现象很可能是由地震-电离层耦合所致.     

A qualitative study of seismic-ionospheric precursor phenomena monitored by a very close to the epicenter VLF and LF receiver

This work investigates the occurrence of disturbances across a wide range of VLF and LF frequencies received prior to a seismic event (Mw = 4), that took place on May 12th 2012, the epicenter of which was very close (14 km) to the VLF/LF station. The signals analyzed were emitted from five VLF and five LF European transmitters. This seismic event produced precursory ionospheric disturbances, identified as spectral distortion, three days before its occurrence, providing a distinct pattern open to further investigation. Although the basis of the ionosphere interaction with seismic phenomena has been well documented in previous studies, the close proximity of the receiver to the seismic event provides a new perspective to this study. The monitored signals have undergone normalization and then they have been processed by means of the Hilbert-Huang Transform. Diagrams of the signals relevant to the phenomena are presented and the disturbances that are present in the raw data are accentuated through further processing.     

磁暴二倍法预报大震的物理机制讨论

从组合模式，磁暴对蠕滑幕的整步和蠕滑幕周期谱的角度讨论了磁暴二倍法预报地层的机制。     

Comparative analysis of the characteristics of global seismic wave propagation excited by the Mw9.0 Tohoku earthquake using numerical simulation

Giant earthquakes generate rich signals that can be used to explore the characteristics of the hierarchical structure of the Earth’s interior associated with the eigenfrequencies of the Earth.We employ the spectral element method,incorporated with large-scale parallel computing technology,to investigate the characteristics of global seismic wave propagation excited by the2011 Mw9.0 Tohoku earthquake.The transversely isotropic PREM model is employed as a prototype of our numerical global Earth model.Topographic data and the effect of the oceans are taken into consideration.Wave propagation processes are simulated by solving three-dimensional elastic wave governing equations with the seismic moment tensor obtained from the Global Centroid Moment Tensor Catalog.Three-dimensional visualization of our computing results displays the nature of the global seismic wave propagation.Comparative analysis of our calculations with observations obtained from the Incorporated Research Institutions for Seismology demonstrates the reliability and feasibility of our numerical results.We compare synthetic seismograms with incorporated and unincorporated ocean models.First results show that the oceans have obvious effects on the characteristics of seismic wave propagation.The peak displacement and peak velocity of P waves become relatively small under the effect of the ocean.However,the effect of the ocean on S-waves is complex.The displacement and velocity of S waves decrease rapidly over time using an unincorporated ocean model.Therefore,the effects of the ocean should be incorporated when undertaking quantitative earthquake hazard assessments on coastal areas.In addition,we undertake comparative analysis on the characteristics of the Earth’s oscillation excited by the 2004 Sumatra-Andaman,2008 Wenchuan,and 2011Tohoku earthquakes that incorporate the effect of the Earth’s gravitational potential.A comparison of the amplitude spectra of the numerical records indicates that energy released by the three big earthquakes is different.Our comparative analysis realizes that the computing results can accurately reproduce some eigenfrequencies of the Earth,such as toroidal modes 0T2 to 0T13and spheroidal modes 0S7 to 0S31.These results demonstrate that numerical simulations can be successfully used to investigate the Earth’s oscillations.We propose that numerical simulations can be used as one of the major tools to further reveal how the Earth’s lateral heterogeneities affect the Earth’s oscillations.     

Study of the characteristics of earthquake precursory anomalies of He and H2 Beijing No. 2 Cotton Mill well and their relation with earthquakes

This paper systematically summarized the relation of the abnormal change of He and H₂ dissolved gasses in deep hot-water well at Beijing No. 2 Cotton Mill and their relation with seismicity of the region. An empirical formula for the relation of the precursory anomaly of He and H₂ gasses with the magnitude and occurrence time of earthquake has been derived. Two events of magnitude greater than 5 in North China had been successfully predicted by using this formula. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,15, 490–497, 1993.     

基于GPS观测研究2010年青海玉树MS7.1地震震后地壳形变特征及其机制

2010年4月14日青海玉树M_S7.1地震发生在青藏高原东南部甘孜-玉树地震带,在震后7~10天内,我们快速建立了由15个GPS测站组成的跨地震破裂带观测剖面,包括1个连续站,3个半连续站和11个流动站,对所有站进行了240多天的观测,获取了该次地震的震后形变时空特征.采用欧拉矢量和位错模型解算了背景速度场,并从GPS观测的形变场中扣除该分量.采用分层黏弹性位错模型计算余震引起的地表形变,结果表明余震对部分测站的位移造成不可忽视的影响.采用对数模型拟合位移时间序列,表明特征衰减时间为6.7±1.2天.利用最速下降法反演震后余滑时空分布,反演结果表明震后断层活动以左旋滑动为主,断层南盘具有少量的抬升.在空间分布上,余滑主要位于同震破裂区的两侧,西北侧的余滑几乎达到地表,而东南区的余滑基本在同震破裂区的下方,余滑最大的区域位于结古镇东南下方10~20 km的深度范围.随着震后离逝时间的增加,2个余滑区在空间上保持不变,余滑区的面积逐渐扩大.余滑的矩释放为（1.5~5.1）×10¹⁸Nm,相当于1个M_W6.1~6.4地震释放的能量.分层岩石圈黏弹性模型计算的地壳孔隙弹性反弹形变与地表观测值相差较大,不能解释观测到的震后变形.采用麦克斯维尔流变体模型计算下地壳和上地幔松弛引起的地表形变,显示出其对地表形变的贡献较小.GPS观测得到的震后形变所具有的快速衰减特征,以及余滑模型能够较好地拟合GPS地表形变,表明2010年玉树M_S7.1地震后早期阶段的地壳形变主要是由余滑机制决定的.     

2008汶川Ms8.0地震发生的深层过程和动力学响应

汶川M_s8.0强烈地震发生在一条现今并不活动的龙门山构造带上,造成了以汶川、映秀为中心及其周边地域的严重破坏和人员的重大伤亡.然而强烈震发生前却未见有可能的确切征兆或浅表层异常活动,即浅层过程与地震发生的深层过程并不匹配.为此对这次强烈地震“孕育”、发生和发展的深层过程进行了分析和探讨,初步研究表明：①在印度洋板块与欧亚板块陆—陆碰撞、挤压作用下,喜马拉雅造山带东构造结向NNE方向顶挤、楔入青藏高原东北缘,迫使高原深部物质向东流展,在受到以龙门山为西北边界的四川盆地阻隔下,一部分物质则转而向东南侧向运移;②龙门山地带在地形上差达3500±500 m左右,而地壳厚度在龙门山西北部为60±5 km左右,四川盆地为40±2 km左右,而龙门山地带与其东、西两侧相比则为地壳厚度变化幅度达15~20 km的突变地域,即为应力作用的耦合地带;③中、下地壳和地幔盖层物质以地壳低速层、低阻层（深20~25 km）为第一滑移面,以上地幔软流层顶面为第二滑移面,且在四川盆地深部“刚性”物质阻 隔下,深部壳、幔物质以高角度在龙门山构造带和四川盆地的耦合地带向上运移（或称逆冲）,且在龙门山地表三条断裂构成的断裂系向下延伸到20 km左右深处汇聚,二者强烈碰撞、挤压、震源介质破裂;在物质与能量的强烈交换下,应力得到释放,故形成了这次M_s.0强烈地震.为此从深部初步揭示了这次强烈地震“孕育”、发生和发展的深层动力过程.     

Distribution and concentrations of petroleum hydrocarbons associated with the BP/Deepwater Horizon Oil Spill,Gulf of Mexico

We examined the geographic extent of petroleum hydrocarbon contamination in sediment, seawater, biota, and seafood during/after the BP/Deepwater Horizon Oil Spill (April 20–July 15, 2010; 28.736667°N, −88.386944°W). TPH, PAHs, and 12 compound classes were examined, particularly C1-benzo(a)anthracenes/chrysenes, C-2-/C-4-phenanthrenes/anthracenes, and C3-naphthalenes. Sediment TPH, PAHs, and all classes peaked near Pensacola, Florida, and Galveston, Texas. Seawater TPH peaked off Pensacola; all of the above classes peaked off the Mississippi River, Louisiana and Galveston. Biota TPH and PAHs peaked near the Mississippi River; C-3 napthalenes peaked near the spill site. Seafood TPH peaked near the spill site, with PAHs and all classes peaking near Pensacola. We recommend that oil concentrations continued to be monitored in these media well after the spill has ceased to assist in defining re-opening dates for fisheries; closures should be maintained until hydrocarbon levels are deemed within appropriate limits.