中国西南区域孕震区电离层TEC变化长时间序列分析

通过中国地壳运动网络提供的GPS观测数据,获取了高精度电离层TEC分布,采用滑动四分位法分析了中国西南区域2008年4—10月(太阳和地磁活动平静时段)6次连续的MW6.0以上地震期间孕震区电离层TEC长时间变化及其异常分布;并在此基础上利用GIM数据对比分析了全球TEC变化特征。鉴于电离层主要受到太阳和地磁等空间天气的影响,将TEC变化与太阳EUV辐射、行星际磁场南向分量IMF Bz以及地磁活动指数Dst和Kp进行了比较。结果发现,该时段内电离层TEC异常扰动与太阳和地磁活动有很好的相关性;除汶川地震外,其他地震前没有发现明显的跟地震相关的TEC异常扰动现象。同时,对比分析了与上述研究区位于同一地磁纬度的"检验区"(30°~50°E,15°~35°N)的GPS TEC随时间变化和异常分布情况,结果显示TEC异常分布的时空特征与研究区域较为一致。由于电离层是一个复杂的系统,其扰动具有多源性,而且地震电离层扰动现象是复杂多变的,因此需要联合地基和天基手段共同观测,并加强其机理研究。     

TEC variations over the Mediterranean before and during the strong earthquake (M = 6.5) of 12th October 2013 in Crete,Greece

In this paper, the total electron content (TEC) data from eight global positioning system (GPS) stations of the EUREF network, provided by IONOLAB (Turkey), were analyzed using discrete Fourier analysis to investigate the TEC variations over the Mediterranean before and during the strong earthquake of 12th October 2013, which occurred west of Crete, Greece. In accordance with the results of similar analyses in the area, the main conclusions of this study are the following: (a) TEC oscillations in a broad range of frequencies occur randomly over an area of several hundred km from the earthquake and (b) high frequency oscillations (f ⩾ 0.0003 Hz, periods T ⩽ 60 m) may point to the location of the earthquake with questionable accuracy. The fractal characteristics of the frequency distribution may point to the locus of the earthquake with higher accuracy. We conclude that the lithosphere–atmosphere–ionosphere coupling (LAIC) mechanism through acoustic or gravity waves could explain this phenomenology.     

Geophysical phenomena during an ionospheric modification experiment at Tromsø, Norway

We present an analysis of phenomena observed by HF distance-diagnostic tools located in St. Petersburg combined with multi-instrument observation at Tromsø in the HF modified ionosphere during a magnetospheric substorm. The observed phenomena that occurred during the Tromsø heating experiment in the nightside auroral E_s region of the ionosphere depend on the phase of substorm. The heating excited small-scale field-aligned irregularities in the E region responsible for field-aligned scattering of diagnostic HF waves. The equipment used in the experiment was sensitive to electron density irregularities with wavelengths 12–15 m across the geomagnetic field lines. Analysis of the Doppler measurement data shows the appearance of quasiperiodic variations with a Doppler frequency shift, f_d and periods about 100–120 s during the heating cycle coinciding in time with the first substorm activation and initiation of the upward field-aligned currents. A relationship between wave variations in f_d and magnetic pulsations in the Y-component of the geomagnetic field at Tromsø was detected. The analysis of the magnetic field variations from the IMAGE magnetometer stations shows that ULF waves occurred, not only at Tromsø, but in the adjacent area bounded by geographical latitudes from 70.5° to 68° and longitudes from 16° to 27°. It is suggested that the ULF observed can result from superposition of the natural and heater-induced ULF waves. During the substorm expansion a strong stimulated electromagnetic emission (SEE) at the third harmonic of the downshifted maximum frequency was found. It is believed that SEE is accompanied by excitation of the VLF waves penetrating into magneto-sphere and stimulating the precipitation of the energetic electrons (10–40 keV) of about 1-min duration. This is due to a cyclotron resonant interaction of natural precipitating electrons (1–10 keV) with heater-induced whistler waves in the magnetosphere. It is reasonable to suppose that a new substorm activation, exactly above Tromsø, was closely connected with the heater-induced precipitation of energetic electrons.     

广州地区低纬电离层TEC扰动与两次强地震的关系研究

利用广州地区GPS-TEC监测网获得的TEC数据,分析了两次7.0级以上地震与低纬电离层TEC扰动存在的可能联系。对TEC的时序分析表明:两次地震发生前TEC的日变幅度均出现下降。但两次地震异常之间存在一定差异。台湾南部邻海7.2级地震前10 d内出现了负异常和正异常,但汶川8.0级地震前8 d和后6d内只出现负异常。此外,还讨论了低层电离层TEC扰动与地磁活动及热层动力学之间可能存在的联系。     

Frequency band of mirroring and bands of increased transmissivity of ULF waves through the ionosphere

Summary Using methods of numerical modelling of the propagation of ULF waves through the ionosphere, the characteristics of the vertical flux of electromagnetic energy are analysed in the ULF range — at the outer boundary of the modelled ionosphere (altitude 1000 km) the reflexibility and penetrability of the energy, at the Earth's surface the transmissivity of the energy. The existence of two frequency bands is proved within the ULF range with different forms of ionospheric wave filtration: a) The band of extremely low frequencies, f<0.1–0.2 Hz (pc3–5 and Pc2 pulsation ranges) with a mirroring effect of the ionosphere-Earth system, but with small absorption; b) the band f>0.2 Hz (the Pc1 range) with increased absorption, but with resonance windows and wave emissions with a very well defined frequency structure.     

Anomalous Variation in GPS TEC,Land and Ocean Parameters Prior to 3 Earthquakes

The present study reports the analysis of GPS TEC prior to 3 earthquakes (M > 6.0). The earthquakes are: (1) Loyalty Island (22°36′S, 170°54′E) on 19 January 2009 (M = 6.6), (2) Samoa Island (15°29′S, 172°5′W) on 30 August 2009 (M = 6.6), and (3) Tohoku (38°19′N, 142°22′E) on 11 March 2011 (M = 9.0). In an effort to search for a precursory signature we analysed the land and ocean parameters prior to the earthquakes, namely SLHF (Land) and SST (Ocean). The GPS TEC data indicate an anomalous behaviour from 1–13 days prior to earthquakes. The main purpose of this study was to explore and demonstrate the possibility of any changes in TEC, SST, and SLHF before, during and after the earthquakes which occurred near or beneath an ocean. This study may lead to better understanding of response of land, ocean, and ionosphere parameters prior to seismic activities.     

中国6.0级以上地震临震电离层扰动时空分布特征研究

基于GPS TEC数据,对1998年11月1日-2010年12月31日中国大陆M_S≥6.0地震临震电离层扰动进行了研究.分析发现,震中上空及周围的东、南、西、北4个方向震前15天到地震当天时序变化趋势较相似,5个方向都观测到了震前3-5天负异常稍高于正异常的现象.按震中±10°、±20°、±30°不同空间尺度分析震前0-15天TEC变化,研究表明临震电离层异常最明显的区域并不位于震中的正上空,而是向磁赤道方向偏移,空间影响尺度大约为±15°左右;正异常在震前14天、10天震中西南方向出现不同程度的高值区,负异常在震前5天震中东南方向较明显.最后,应用静电场假设对这些现象进行了解释.     

Variations of the electromagnetic fields and ionospheric parameters in the Baikal Rift Zone

Variations in the geomagnetic and electric fields and variations of the total electron content (TEC) of the ionosphere recorded in the Baikal Rift Zone (BRZ) during the expeditions in 2009 and 2010 are analyzed. Synchronous bursts in the geomagnetic field on the ground and in the ionosphere, which are caused by propagation of electromagnetic disturbances (spherics) generated by the remote lightning discharges, are revealed. The analysis of the occurrence frequency of the electromagnetic disturbances at an altitude of ∼700 km shows that there is a preferred region of predominant propagation of these disturbances from the Earth-ionosphere waveguide to the upper ionosphere. When the ionospheric penetration point moves through this preferred region, the frequency spectrum of TEC variations changes, and the northern boundary of the region of spectral alteration is located at ∼54°N. The bursts in TEC that map on the zones of the main faults in the Tunka valley are identified. The results probably suggest a relation between the electromagnetic phenomena in the ionosphere and the structures in the lithosphere.     

Evidence for fault-related directionality and localized site effects from strong motion recordings of the 2003 Boumerdes (Algeria) earthquake: Consequences on damage distribution and the Algerian seismic code

The Algiers–Boumerdes region has been struck by a destructive magnitude 6.8 (M_w) earthquake on May 21, 2003. The study presented in this paper is based on main shock strong motions from 13 stations of the Algerian accelerograph network. A maximum 0.58g peak ground acceleration (PGA) has been recorded at 20 km from the epicenter, only about 150 m away from a PGA of 0.34g, with both a central frequency around 5 Hz, explained by a strong very localized site effect, confirmed by receiver function technique results showing peaks at 5 Hz with amplitudes changing by a factor of 2. Soil amplifications are also evidenced at stations located in the quaternary Mitidja basin, explaining the higher PGA values recorded at these stations than at stations located on firm soil at similar distances from the epicenter. A fault-related directionality effect observed on the strong motion records and confirmed by the study of the seismic movement anisotropy, in agreement with the N65 fault plan direction, explains the SW–NE orientation of the main damage zone. In the near field, strong motions present a high-frequency content starting at 3 Hz with a central frequency around 8 Hz, while in the far field their central frequency is around 3 Hz, explaining the high level of damage in the 3- to 4-story buildings in the epicentral zone. The design spectra overestimate the recorded mean response spectra, and its high corner frequency is less than the recorded one, leading to a re-examination of the seismic design code that should definitively integrate site-related coefficient, to account for the up to now neglected site amplification, as well as a re-modeling of the actual design spectra. Finally, both the proposed Algerian attenuation law and the worldwide laws usually used in Algeria underestimate the recorded accelerations of the 6.8 (M_w) Boumerdes earthquake, clearly showing that it is not possible to extrapolate the proposed Algerian law to major earthquakes.     

Dynamics of the large-scale ULF electromagnetic wave structures in the ionosphere

The present article displays the results of theoretical investigation of the planetary ultra-low-frequency (ULF) electromagnetic wave structure, generation and propagation dynamics in the dissipative ionosphere. These waves are stipulated by a spatial inhomogeneous geomagnetic field. The waves propagate in different ionospheric layers along the parallels to the east as well as to the west and their frequencies vary in the range of (10–10⁻⁶) s⁻¹ with a wavelength of order 10³ km. The fast disturbances are associated with oscillations of the ionospheric electrons frozen in the geomagnetic field. The large-scale waves are weakly damped. They generate the geomagnetic field adding up to several tens of nanotesla (nT) near the Earth's surface. It is prescribed that the planetary ULF electromagnetic waves preceding their nonlinear interaction with the local shear winds can self-localize in the form of nonlinear long-living solitary vortices, moving along the latitude circles westward as well as eastward with a velocity different from the phase velocity of the corresponding linear waves. The vortex structures transfer the trapped particles of medium, as well as energy and heat. That is why such nonlinear vortex structures can be the structural elements of the ionospheric strong macro-turbulences.     

Earthquake-related Electric Field Changes Observed in the Ionosphere and Ground

The changes of the ionospheric electric field before and after four huge earthquakes, which include the Ms 8.7 earthquake of 2004 and the Ms 8.5 earthquake of 2005 in Sumatra of Indonesia, the Ms 8.0 Wenchuan earthquake of 2008 in China, the Ms 8.8 earthquake of 2010 in Chile, and their strong aftershocks are studied in this paper. The significant results revealed that the power spectral density of low-frequency electric field below 20 Hz in the ionosphere, a kind of electromagnetic radiation phenomena, increased abnormally before and after the earthquakes and partially corresponded to the increased power spectral density of the low-frequency geoelectric field in time. This research preliminarily indicates that the low-frequency electromagnetic radiation during the imminent stages before such earthquakes could be detected by the observation of the ionospheric electric field. However, the spatial, temporal, and intensive complexities of the electric field anomalies in the ionosphere before earthquakes have come in sight also.     

Verification of the concept of seismoionospheric coupling under quiet heliogeomagnetic conditions,using the Wenchuan (China) earthquake of May 12, 2008, as an example

The variability of the ionosphere during April–May, 2008, has been analyzed in detail in order to reveal anomalous variations related to seismic activity, initiated by the strongest Wenchuan earthquake (M = 7.9) in the Sichuan province on May 12, 2008. Information about the total electron content (TEC) from the network of GPS receivers in the earthquake region, the global IONEX TEC maps, and the reconstructed vertical profiles of electron density according to the data of GPS receivers were used as a data source. The spatial and time localization of the observed anomalies, their morphological features, and the absence of geomagnetic disturbances during the observation period undoubtedly demonstrate that the observed variations were caused by seismic activity.     

Characteristics of the ionospheric correction in ULF wave diagnostics of the magnetosphere

Summary Amplitude and energy correction character istics of the vertical propagation of ULF wave from the magnetosphere through the ionosphere to the Earth's surface, necessary for micropulsation wave diagnostics of the magnetosphere by means of ground-based observations, are introduced on the basis of matrices ofR–T coefficients [1–3]. The coefficients of vertical reflexibility, penetrability, transmissibility (or limpidity) and the absorption of the electromagnetic energy flux are defined, as well as analogous coefficients in the dimensions of the magnetic amplitude of the ULF wave, propagating through the given layer of the ionosphere. An examplary model of the ionosphere is used to demonstrate the frequency variations of these characteristics in the ULF wave range.     

A statistical analysis of seismo-ionospheric TEC anomalies before 63 <Emphasis Type="Italic">M</Emphasis><Subscript>w</Subscript> ≥ 5.0 earthquakes in Turkey during 2003–2016

In this study, pre-seismic and post-seismic total electron content (TEC) anomalies of 63 M_w?≥?5.0 earthquakes in Turkey (36°–42°N, 26°–45°E) were statistically investigated. The largest earthquake that occurred in Turkey during 2003–2016 is the M_w 7.1 Van earthquake on October 23, 2011. The TEC data of epicenters is obtained from CODE-GIM using a simple 4-point bivariate interpolation. The anomalies of TEC variations were determined by using a quartile-based running median process. In order to validate GIM results, we used the GPS-TEC data of available four IGS stations within the size of the Van earthquake preparation area. The anomalies that are detected by GIM and GPS-TEC show a similar pattern. Accordingly, the results obtained with CODE-GIM are reliable. The statistical results show that there are not prominent earthquake precursors for M_w?≤?6.0 earthquakes in Turkey.     

Statistical analysis of ionospheric TEC anomalies before global <Emphasis Type="Italic">M</Emphasis><Subscript>w</Subscript> ≥ 7.0 earthquakes using data of CODE GIM

Based on Center of Orbit Determination in Europe (CODE) global ionospheric map (GIM) data, a statistical analysis of local total electron content (TEC) anomalies before 121 low-depth (D ≤ 100 km) strong (M _w ≥ 7.0) earthquakes has been made using the sliding median differential calculation method combining with a new approach of image processing technique. The results show that significant local TEC anomalies could be observed 0–6 days before 80 earthquakes, about 66.1% out of the total. The positive anomalies occur more often than negative ones. For 26 cases, both positive and negative anomalies are observed before the shock. The pre-earthquake TEC anomalies show local time recurrence for 38 earthquakes, which occur around the same local time on different days. The local time distribution of the pre-earthquake TEC anomalies mainly concentrates between 19 and 06 LT, roughly from the sunset to sunrise. Most of the pre-earthquake TEC anomalies do not locate above the epicenter but shift to the south. The pre-earthquake TEC anomalies could be extracted near the magnetic conjugate point of the epicenter for 40 events, which is 50% out of the total 80 cases with significant local TEC anomalies. In general, the signs of the anomalies around epicenter and its conjugate point are the same, but the abnormal magnitude and lasting time are not.     

祁连山地震带中强地震前热红外异常研究

对祁连山地震带1991—2004年间发生的13次中强地震,采用NOAA卫星热红外遥感资料数据库进行地面温度的时空网格化扫描,获得多年连续的热红外时序,研究地震前后的温度演化过程,提取震前地表温度的异常特征。研究结果表明,有38%的地震在震前出现了热红外异常现象,有的表现为温度降低,有的则表现为温度升高,体现了地震诱发因素的复杂性;温度异常幅度均为2~3℃,异常出现的时间均在2个月内;震级越大,异常越明显,但并非所有6级以上地震都比5~6级地震的热红外异常明显。     

Interpretation in terms of gyrotropic waves of Schumann-resonance-like line emissions observed at Nakatsugawa in possible association with nearby Japanese earthquakes

The observation of ULF/ELF electromagnetic waves in the frequency range below 50 Hz has been continued at Nakatsugawa (in the Gifu prefecture), Japan since 1998. This paper summarizes anomalous Schumann resonance (SR) phenomena and SR-like line emissions observed at Nakatsugawa in possible association with recent nearby earthquakes (EQs) (the 2004 Mid-Niigata prefecture and the 2007 Noto-Hanto (peninsula) EQs), which have been already described in detail by Ohta et al. (2009). The intensity of particular modes of SR increased before these large EQs and the excitation of other anomalous SR-like line emissions also existed at the frequency shifted by about 2 Hz from the typical SR modes. Since temporal changes of the anomalous SR modes and line emissions are synchronous in time, there might be a possibility that the line emission is a consequence of the anomalous SR. In this paper we propose an interpretation of those anomalous phenomena in terms of excitation of gyrotropic waves due to input wave from below with a band from 15 to 20 Hz as an exciter. The theoretical computational results seem to be generally consistent with the observational finding.     

Anomalous Variations of Ionosphere Associated with the Strong Earthquake at Pakistan-Iran Border at a Low Latitude Station Agra,India

In this paper, we analyze the TEC data for April 2013 observed at Agra station, India (geogr. lat. 27.2° N, long. 78° E) to examine the effect of earthquake of magnitude M = 7.8 which occurred on 16 April 2013 at Pakistan–Iran border region. We process the TEC data using the s statistical criterion to find out anomalous variation in TEC data. We also study the VLF propagation signal from NPM, Hawaii (21.42° N, 158° W), which is monitored at the same station (Agra station) in the light of this earthquake as well as solar flares. The nighttime fluctuation method is used to analyze the VLF data for the period of ±5 days from the day of earthquake (11–21 April 2013). The anomalous enhancements and depletions are found in TEC data on 1–9 days before the occurrence of event.     

Ultra low frequency electromagnetic wave anomaly and its spectrum characteristics before earthquakes

Many observations and researches show that abundant and intense ultra low frequency (ULF) electromagnetic anomalies appeare before local earthquakes. In order to understand this phenomenon systematically, the transmission property and the formation of the seismic-electric dipole are discussed in this paper. The results show that the vertical component of the ULF electric anomaly before earthquake has the properties of better penetration, less disturbance and high signal-to-noise ratio. The signal can be easily extracted and received. The anomaly should be an important way to seek for reliable short-term and imminent earthquake precursors. After the spectrum analysis of the (ULF) electric field anomalies, we found that the periods of the anomaly related to local earthquakes are from a few minutes to a dozens minutes. There is a relation between the spectrum of the anomaly and the epicentral distance. The ULF electric field anomalies strongly rely on the orientation of the seismogenic zone. The percolation electric field anomaly and its spectrum amplitude do not have obvious relation with the size of local earthquakes. Contribution No. 94A0097, Institute of Geophysics, SSB, China. The English version is improved by Prof. Guo-Hua FAN, Institute of Geophysics, State Seismological Bureau.     

Ultra low frequeney electromagnetic wave anomaly and its spectrum characteristics before earthquakes

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