Variability of the F region of the equatorial ionosphere during quiet geomagnetic conditions prior to strong earthquakes

The deviations in the hourly values of the F-region critical frequency from the monthly median for the ionospheric Huancayo station located near the magnetic equator during periods of quiet geomagnetic conditions are analyzed for 1957–1987. Ionospheric data for five days prior, one day after, and directly for the days of 33 strong (with a magnitude M ≥ 5.5) earthquakes with epicenters in the American longitudinal sector were used. It is revealed that, in 24 cases 1–5 days prior to the considered earthquakes, a decrease in the critical frequencies by more than 20% with a duration from one to six hours was observed mainly in the nighttime. One can assume that these effects (at least, part of them) are related to the processes of earthquake preparation. Disturbances were mainly detected in cases when the radius of the earthquake preparation zone exceeded the distance between the epicenter and the observation station. The need for a further study of the characteristics of different kinds of coupling “from below” on electrodynamical processes in the low-latitude ionosphere for successful recognition of disturbances of the seismogenic nature is noted.     

The ionospheric perturbations prior to the Chi-Chi and Chia-Yi earthquakes

In this paper we analyze the greatest plasma frequency, foF2, named critical frequency, observed by the Chung-Li ionosonde (25.0°N, 121.l°E) during the period of the Chi-Chi (23.87°N, 1 20.75°E) and the Chia-Yi (23.51°E, 120.4°E) earthquakes. The previous 15-day running mean and the associated standard deviation are utilized to construct the upper or lower bound for detecting the seismo-ionospheric perturbations. It is found that the perturbation appeals in 3–4 days prior to the Chi-Chi earthquake as well as 1–3 days prior to the Chia-Yi earthquake.     

Response of tall buildings to weak long distance earthquakes

In the last decade, two tall buildings in Singapore were instrumented with accelerometers and anemometers for the original purpose of identifying the characteristics and effects of wind loading. During the monitoring it became clear that the largest acceleration responses should result from ground motions due to earthquakes having magnitudes between 6 and 8 and epicentres at least 350 km distant. The paper describes the strategy for identifying and capturing the signals from distant tremors, which depends on tracking the RMS response levels in the second vibration mode. Characteristics of some recorded signals are given. While response levels are generally small, the frequency content coincides with the range of fundamental mode frequencies for high rise residential buildings. The validity of using a tall building as a ‘weak‐motion’ seismograph is discussed by considering the mode shape of the building and the measured transfer function between basement and roof responses. Copyright © 2001 John Wiley & Sons, Ltd.     

Deep-focus earthquakes in the Mediterranean region

Summary In Table I the authors give a list of known deep-focus earthquakes in the eastern Mediterranean. Fig. 1 shows that their epicenters generally lie in the zones of active volcanoes, as in the regions surrounding the Pacific Ocean; however, their coincidence with positive gravity anomalies is less evident.

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Zusammenfassung In Tabelle I geben die Verff. eine Liste der ihnen bekannten Tiefenbeben im ?stlichen Mittelmeer. Fig. 1 zeigt, dass deren IIerde im Allgemeinen in den Zonen der t?tigen Vulkane liegen, ?hnlich wie in der Umgebung des Pazifischen Ozeans. Dagegen ist deren Zusammenfallen mit positiven Schwereanomalien weniger doutlich.

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Riassunto Gli AA. hanno raccolto nella Tab. 1 l'elenco dei terremoti profondi a loro noti del bacino mediterraneo orientale. La Fig. 1 mostra che in generale gli epicentri relativi sono distribuiti nelle zone dei vulcani attivi, analogamente a quanto accade ne'le regioni dell'Oceano Pacifico. Per contro, la connessione con le anomalie gravimetriche positive risulta meno evidente.

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Balch Graduate School of the Geological Sciences: Contribution No. 454.     

Climatic characteristics of the ionosphere over Irkutsk. Observations and comparison with the IRI-2001 model

The morphological peculiarities of the behavior of the electron density over Irkutsk (52.3° N, 104.3° E) have been investigated by the current-median method. The observations are compared with the IRI model data at the decay phase of solar activity (2003–2006). Systematic discrepancies between the prediction and observations are discussed.     

Spatial changes in volcanic and seismic activity prior to great earthquakes

Spatial relationship between volcanism and seismicity prior to the occurrence of several great interplate earthquakes in the circum Pacific area has been examined in order to understand the process of underthrusting in detail. The locations of the epicenters of the great earthquakes have been examined in terms of locations of concentrated volcanic and seismic activity in a short time period in order to determine if there is any relationship between these activities, and whether such relationships can be used to refine the underthrusting model. The study shows that in most cases, (1) there is no volcanic activity in the vicinity of the epicenter of the great earthquake and (2) maximum volcanic activity is localized, i.e., volcanoes adjacent to one another exhibit considerable activity in a short time period. Very little systematic spatial relationship between these three parameters is observed although in most cases, there is no volcanic activity at the time of maximum earthquake activity. Locations of active volcanoes and earthquake activity, during the five years prior to the occurrence of the great earthquake, do not appear to be a guide to the epicenter of the great earthquake. This study therefore suggests that although there is temporal relationship between the occurrence of maximum volcanic and seismic activity and the occurrence of great earthquake, there appears to be no systematic spatial relationship between these three parameters.     

The quiescence of earthquakes with ML34.0 as an important precursory characteristic prior to strong shocks in North China region

Before a strong earthquake, there is often a process of quiescence or 2absence2 of regional seismic activity, which is a phenomenon that has attracted the attention of many scholars in China and other countries (LU, et al, 1985; Wesson, 1973). In the past ten-odd years, some Chinese seismologists made, earlier or later, special studies on the quiescence anomaly of seismicity in some specified areas from different angles. Taking the converted annual frequency of M=5.0 earthquakes in North China region as a characteristic quantity, JIANG and FENG (1989) analyzed the quiescence anomaly of seismic activity by fuzzy recognition.     

大地震红外辐射异常信息时空特征分析

寻找和确定大地震的临震信息是预测地震的关键.本文针对2008年5月12日中国四川汶川8.0级、2010年1月12日海地太子港7.3级、2月27日智利8.8级以及4月14日中国青海玉树7.1级等四次灾难性地震事件,分析了大震发生前震中附近地区卫星遥感射出长波辐射(OLR)信息的时空场分布及变化特征.发现在四次大震发生前1...     

Diurnal periodicity of weak earthquakes in Central Asia

Data on the time moments and numbers of weak earthquakes (M ≥ 0) are analyzed in five regions of Central Asia where long-term extensive instrumental observations of seismicity were conducted. Spectra of time moments of seismic events were calculated in the range of periods from 1 to 48 h with a step of half an hour. The time variations in the numbers of these earthquakes were also calculated in windows of 4 to 48 h wide with a summation step of 1 h. The presence of significant high-quality spectral extrema at periods of 24 and 12 h is established for all catalogs used, and less significant extrema at a period of 8 h are established for some catalogs. These periodicities are observed in the analysis of weak earthquakes with M < 2.2 (K < 8) and are virtually absent for stronger seismic events. It is shown that lunisolar tides have no appreciable gravitational effect on the observed variations in seismic emission. Daily variations in the actual sensitivity of the observation network due to the daytime increase in the noise level as a factor responsible for the observed diurnal periodicity raise doubts.     

A model of the generation of electromagnetic emissions detected prior to earthquakes

Recent satellite and ground-based observations prove that during the formative period of earthquakes VLF/LF and ULF electromagnetic emissions are observed in seismogenic areas. This work offers an original model of self-generated electromagnetic oscillations of local segments of the lithospheric origins of the emissions. In the paper, the seismogenic area is considered to be an oscillatory-distributed system. This model simplifies physical analyses of the nonlinear effects and qualitatively explains the mechanisms that generate very low frequency electromagnetic waves in the period prior to an earthquake.     

Problematic historical earthquakes in the Baikal region

Macroseismic data on strong historical earthquakes of the 18th–19th centuries are presented that are unknown to a wide range of seismologists and therefore missing in earthquake catalogs. The data are related to such earthquakes as the 1715 (1716) Nerchinsk and 1727 Kirensk, and ones occurred in the southern Baikal region in 1809 and in the region near cape of Svyatoi Nos in 1862. Additional information about the earthquake of 1771 is given that considerably refines the parameters of this event. The earthquakes of 1725, 1742, 1772, and 1885 and interpretations of evidence doubted by various researchers are also considered.     

拉萨地区及其周边的地震活动性研究

印度和欧亚陆-陆板块碰撞造就的青藏高原地震活动十分频繁.但由于地理因素限制,青藏高原内部地震台站分布稀疏,因此全球地震台网很难监测青藏高原内部发生的小型地震活动.本文通过对北京大学布设的流动台阵自2007年1月至2008年3月地震记录的分析,精确定位了拉萨周边地区的79个区域小震,发现了两个可能是受到岩浆或地下水活动影响而形成的震群,以及拉萨附近一条近期比较活跃的断层,并给出了全球台网对于该地区地震定位误差的估计.     

C、D、Mf值综合异常交汇区在华东地区中强地震前的中期异常

从大震前中小地震活动有个增强的过程和大地震常常发生在异常区外围的现象出发,提出用描述地震分布时、空、强基本特点的空间集中度Ｃ、地震危险度Ｄ和地震强度因子Ｍｆ进行交汇预测未来震中位置。应用在华东地区的震例一般预测位置可达１°～１．５°半径,并能较好地避免单值预报的虚报、漏报。     

Quantitative observation and study on rhythmic abnormalities of activities in animals prior to earthquakes

In this paper, the normal daily activities and abnormal activities related to earthquake premonitory information are given by a quantitative observation and analysis of activities in the sparrow (SR,Passer montanus), budgerigar (BG,Melopsittacus undulatus) and rat (RT,Rattus norvegicus). The results show that the quantitative observation of habitual abnormalities in animals may provide some cues for the short-term earthquake prediction. The normal activity rhythms for the SR and BG are similar, and both present M mode. The high activities occurs during 07^h – 10^h and 15^h – 16^h, respectively, the low activities occurs during 12^h – 13^h, and at night both birds are basically silent. For the RT, the normal rhythmic activity has the middle magnitude during 07^h – 10^h and 17^h – 18^h, the low and high magnitudes occur during 11^h – 16^h and from 19^h to 06^h at the next day. For the SR, BG and RT, observable abnormal changes of the normal activity rhythm were found before earthquakes. The night activities of the SR and BG were increased noticeably. For the RT the activities during the low magnitude of activities at the day time were also increased. They both are about 300 times greater than the normal activity value. Moreover, the total activity values per day were increased, and were about 2 times of the normal value. Thex ²-test shows that the abnormalities of the daily activity rhythm and following increase of the daily activity events are significantly correlated with earthquakes of magnitude over 4.3 in Tangshan seismic area within the region of 200 km distance from the observation station. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,14, 111–118, 1992.     

河北省唐山地区小地震目录完整性分析

采用基于G—R关系的交互式分析方法,研究最小完整性震级Mc的分布特征,由此确定河北省唐山老震区不同震级下限完整目录的起始时间,发现唐山老震区1970年以来ML 2.7以上地震目录较为完备。     

The pattern of diurnal periodicity of weak earthquakes in Iran

The catalogue of earthquakes recorded in Iran during 2006–2010 by a dense network of digital telemetric seismic stations is analyzed. The spectrum of the time series of these earthquakes contains a sharp maximum at 24 h. The corresponding curve of diurnal periodicity constructed by the superposed epoch method has a clear double-peak maximum near noon(11 a.m.-5 p.m. local time), which exceeds the level of seismic activity observed during the rest time of the day by a factor of 3-3.5. In the same time interval, the average hourly magnitudes of seismic events sharply drop from M = 2.15 to M = 1.95. The ratio of the normalized number of earthquakes in the daytime to those at other times of the day, which was determined within a moving window half a square degree in size, has seven distinct compact spatially isolated maxima whose magnitudes attain several dozens to a hundred units. These maxima are probably caused by industrial activity, such as road building and quarry explosions. We also note the presence of the weekend effect when the daytime maximum in the weekly curve of diurnal variations in seismic activity almost completely disappears on Friday, which is the weekend in Muslim countries. At the same time, elimination of the supposed noise component from the catalogue by the approved technique changes nothing for the daytime maximum in the daily pattern of earthquakes in Iran. In order to account for this inconsistency, we suggest invoking additional information on the technogenic seismicity and considering weak earthquakes induced by quarry explosions and vibrations of industrial machines, in particular, power units of numerous hydroelectric power stations distributed over the territory of Iran.     

Three-dimensional assimilation model of the ionosphere for the European region

We consider a three-dimensional assimilation model of the ionosphere. We discuss the governing equations of the physical model and the data assimilation technique. We provide examples of the model results: plots of the calculated ionospheric parameters, such as the density and temperature of electrons and ions. We compare the model results with independent sources of data on the state of the ionosphere. We make some conclusions regarding the model accuracy and possible areas of its practical application.     

华北地区强震前的信号震及其预测意义

根据对华北地区1970年以来M_S≥6地震以前中小地震活动空间图像变化特征的分析, 研究了“信号震”发生的时空特征及其地震活动背景, 由“场-源”关系特征对一般地震进行严格的筛选识别, 从而得出信号震的有关预测指标。 信号震一般发生在强震前的2年之内, 多数发生在15个月内; 信号震与强震的距离不超过200 km, 多数在100 km之内; 震级强度一般为M_L4.0～5.3。 信号震一般发生在局部的M_L≥4.0地震平静区内, 一般发生在中小地震条带上或条带附近, 在其周围或附近存在中小地震空区。 检验结果表明, 信号震发生后的9个月之内, 其预测概率P_t即超过0.5, R_t值达到0.27; 预测区域半径在距信号震震中100 km之内时, 其发生概率P_d可以达到0.73; 预测强震震级一般为M_S≥6.0。 研究表明, 信号震的环境应力值τ₀明显高于其他地震, 显示了高应力背景的异常显著性, 它所辐射的地震波中很可能含有未来强震孕震区的大量的本质性信息。     

北京地区小震活跃与首都圈地区中等地震活动关系研究

通过研究北京地区的小震活跃与首都圈地区中等地震活动的关系，发现1999年到2003年间频度异常与首都圈地区Ms4．0以上地震具有一定对应关系，但频度异常对未来地震地点判定的意义不大，需要将北京地区小震频度异常和首都圈及附近地区小震活动的有序性特征有机结合起来，才能更好地把握中等地震发生的时间和地点，最后指出，频度异常指标具有一定的时间性特征。