First results from the Gauribidanur Radio heliograph     

K. R. Subramanian  R. Ramesh  M. S. Sundara Rajan  Ch. V. Sastry 《Astrophysics and Space Science》1996,243(1):259-262

Observations of the Sun at two frequencies (51 and 77 MHz) using the East-West arm of the Gauribidanur Radio heliograph are presented.  相似文献   

地震前兆含义,科学问题与研究途径的研讨   总被引：5，自引：2，他引：3  

张肇诚  王贵宣 《地震》1997,17(4):429-439

地震前兆的研究已经取得了很大的进展,地震前兆的复杂性引起了广泛的关注,对地震预报、有关科学问题和探索途径的研究现状及其前景有各种评价。文章认为由于一些科学问题没有解决,尽管对中缅边境１９９５年７月连７．３级地震作出了成功预报,经验性预报有一定的局限性,文中讨论了广义地震前兆和狭义地震前兆,广义地震前兆包含二类前兆：场兆和源兆;而狭义前兆仅指后者。大陆震例前兆的系统研究证实了广义地震前兆的存在,然而  相似文献   

A laboratory experiment to monitor the contact state of a fault by transmission waves   总被引：1，自引：0，他引：1  

Naoto Yoshioka  Koji Iwasa 《Tectonophysics》2006,413(3-4):221-238

We performed a series of laboratory experiments in which elastic waves were transmitted across a simulated fault. Two types of experiments were carried out: (1) Normal Stress Holding Test (NSHT): normal stress was kept constant for about 3 h without shear stress and transmission waves were observed. (2) Shear Stress Increasing Test (SSIT): shear stress was gradually increased until a stick-slip event occurred. Transmission waves were continuously observed throughout the process of stress accumulation. We focused on the change in transmission waves during the application of shear stress and especially during precursory slips.It was found in NSHT that the amplitude of transmission waves linearly increased with the logarithm of stationary contact time. The increase amounted to a few percent after about 3 h. Creep at asperity contacts is responsible for this phenomenon. From a theoretical consideration, it was concluded that the real contact area increased with the logarithm of stationary contact time.We observed in SSIT a significant increase in wave amplitude with shear stress application. This phenomenon cannot be attributed to the time effect observed in NSHT. Instead, it can be explained by the mechanism of “junction growth” proposed by Tabor. Junction growth yields an increase in real contact area. It is required for junction growth to occur that the material in contact is already plastic under a purely normal loading condition. A computer simulation confirmed that this requirement was satisfied in our experiments. We also found that the rate at which the amplitude increased was slightly reduced prior to a stick-slip event. The onset time of the reduction well coincides with the onset of precursory slip. The cause of the reduction is attributed to the reset of stationary contact time due to displacement. This interpretation is supported by the result of NSHT. Taking the time of stationary contact in SSIT into account, we may expect the change in wave amplitude to be, at most, only a few percent. The observed slight reduction in increasing rate is, in this sense, reasonable. The static stiffness of the fault also decreases with precursory slip. It was also found that low frequency waves are a better indicator of precursory slip than high frequency waves. This might suggest that low frequency waves with longer wavelength are a better indicator of average behavior of faults. The problem, however, merits a further investigation. The shifts in phase were also found to be a good indicator of the change in contact state of the fault. The changes in both amplitude and phase of transmission waves are unifyingly understood through the theory of transmission coefficient presented by Pyrak-Nolte et al. Rough surfaces have a tendency to give larger stick-slips than smooth surfaces. The amount of precursory slip is larger for rough surfaces than for smooth surfaces. Although it was confirmed by a computer simulation that rough surfaces have larger contact diameters than smooth surfaces, the rigorous relationship between the surface roughness (contact diameter) and the amount of precursory slips was not established.  相似文献   

江西瑞昌1995年ML4.9级地震前后九江地震台倾斜固体潮汐变化分析     

赵爱平  吴敏 《华南地震》2002,22(4):79-84

通过月整点值数据文件和滑动值数据文件这两种数据处理形式。对九江台地倾斜资料进行调和分析计算。分析了M2潮汐因子与瑞昌ML4.9级地震的关系。认为存在震前短临异常或震后异常，并与趋势异常得到相互印证。  相似文献   

Temporal variations of shear-wave splitting in field and laboratory studies in China   总被引：1，自引：0，他引：1  

Yuan Gao  Stuart Crampin   《Journal of Applied Geophysics》2003,54(3-4):279

Observations of shear-wave splitting at seismic stations above a swarm of small earthquakes on Hainan Island, China, and other examples world-wide, suggest that the time-delays of split shear-waves monitor the build up of stress before earthquakes and the stress release as earthquakes occur. Rock physics experiments on marble specimens also show variations of shear-wave time-delays with uniaxial pressure analogous to the field observations. The rock experiments show an abrupt decrease in time-delays immediately before fracturing occurs. Similar precursory behaviour has been observed before earthquakes elsewhere, and is believed to be important for two reasons. Precursory changes in shear-wave splitting could be used for short-term forecasting, but of greater importance may be the information such behaviour provides about the source processes in earthquake preparation zones.  相似文献   

华北地区大震前区域地震活动的中短期变化          下载免费PDF全文

林怀存  刘希强 《地震地质》1997,19(1):38-42

对发生在华北地区３１个Ｍ≥５地震震前区域地震活动的时间序列进行的研究得出：（１）华北地区发生Ｍ≥５地震前，区域地震活动的短期平静现象与大震发生的相关性很强。３１个Ｍ≥５地震前的平静时间主要集中在２０～８７ｄ，１２０～２１０ｄ和大于２１０ｄ。同时还得出大震前的短期平静时间与震级大小无关的结论；（２）区域地震活动在短时间内出现增强趋势，与发生大震的相关性很弱，相关的仅占总数的６．４％  相似文献   

Very quiet intervals of geomagnetic activity and the solar wind structure     

Jaroslava Hruska  Antonin Hruska 《Pure and Applied Geophysics》1989,131(3):343-355

Extended periods of very low geomagnetic activity as described by very quiet intervals (VQI's) occur only at those times when the solar wind velocityV has a generally decreasing trend, i.e., they mainly occur either after the velocity peak of a high speed solar stream has passed the Earth, or at times when the Earth is immersed in a low speed solar plasma provided that the daily mean value ofdV/dt is negative. The VQI's most frequently start whendV/dt<0 anddB _Z/dt>0 (B _Z is the geocentric solar magnetrospheric-GSMZ-component of the IMF) and end most likely whendV/dt>0 anddB _Z/dt<0. The temporal trends of the solar wind (SW) velocity affect the variation of thea _p index only when the level of geomagnetic activity is generally low.It is suggested that a gradual expansion or contraction of the magnetosphere, associated with a slow variation of the SW pressure, plays a role in the modification of the reconnection-driven magnetohydrodynamic (MHD) fluctuations in the magnetosphere.  相似文献   

AbnormalSq behaviour     

E. C. Butcher 《Pure and Applied Geophysics》1989,131(3):463-483

It is well-known that the amplitude and phase of theSq(H) variation show considerable variability from day to day. In this paper we consider one aspect of the phase variability—that associated with AQDs. AQDs (or abnormal quiet days) are defined as magnetically quiet days where the maximum excursion ofH at a mid-latitude station on the poleward side of the focus occurs outside the normal time range 0830–1330 LST. Such days exhibit properties, many of which appear quite distinct from the properties of the normalSq(H) variation. The properties of AQDs, and the proposals that have been made to explain them, are considered in detail. The consequences of these proposals and some problems which need to be addressed in order to obtain a fuller understanding of the dynamics of the ionosphere on AQDs are also discussed.  相似文献   

我国西南地区地面和低层大气臭氧的观测分析   总被引：6，自引：2，他引：6       下载免费PDF全文

肖辉  沈志来  黄美元 《大气科学》1993,17(5):621-628

本文介绍了1987—1989年在以重庆、成都和贵阳三地区为代表的西南地区进行了地面和低层大气臭氧的观测结果,给出了该地区大气臭氧时空分布的一些特征.分析表明,成都、重庆和贵阳等地区O_3浓度的时空分布及变化规律不尽相同,但都与前体污染物特征以及日照强度、大气层结稳定度.天气过程等密切相关.  相似文献   

灰色预测系统在唐山地震前兆数据处理中的应用     

吴子泉  金安忠 《内陆地震》1991,5(2):149-154

本文应用灰色预测系统GM(1,1)模型,对宝坻等四个台唐山地震前地电、水化、水位、形变等手段的月均值资料进行了处理。结果表明:宝坻地电、管庄水氡、宁河俵口水位及牛口峪定点水准都出现了不同程度的震前异常变化,从而证明了灰色预测系统在地震前兆数据的处理中有着一定的应用价值。  相似文献