地磁转换函数在上海及邻近地区地震预报中的应用

地磁转换函数的变化已被广泛用于中短期地震预报。本文利用佘山地震台１９７７￣１９９６年和崇明地震台１９８３￣１９９６年的地磁短周期资料处理所得的转换函数变化,监视跟踪上海及邻近地区地震活动。通过几次中强地震震例的总结,提出用地磁转换函数方法来预报上海及其邻近地区地震的一些判断。用此方法,曾较好地预报过１９９０年常熟５．１级地震和１９９６年长江口以东海中６．１级地震。     

北京磁测预报地震效能分析

北京磁测预报地震是以压磁理论与感应磁效应为物理基础的,应用差值比较、图象演变、快谱幅比、统计参量等方法,分析地磁资料,研究震磁前兆信息,开展地震监测预报工作。以１９９２￣１９９７年期间书面的预报意见为依据,分析了地震预报效能,结果表明,北京磁测的地震监测预报有一定的实际效果与积极作用,今后应当加强震磁前兆的观测研究。     

Observed Response of the Ionospheric F2–Layer and Lower Thermosphere to Geomagnetic Storms during High Solar Activity

The response of the critical frequency of the ionosphere F2–layer, described by its main Fourier components (daily constant, diurnal and semidiurnal waves) and the lower thermosphere dynamics to the geomagnetic storms in July 1991 and February 1992 is studied. The daily constant displays a negative response, however, the magnitude of reaction depends on the season and latitude. The amplitudes of diurnal and semidiurnal waves increase during a geomagnetic storm, as this enhancement is very strong at high latitudes in winter. The prevailing neutral wind, especially the zonal wind, shows an inclination to decrease during the geomagnetic storm (the effect is more distinct in summer). The amplitudes of diurnal and semidiurnal tides also demonstrate a tendency toward reduction during high geomagnetic activity.     

中生代地球系统与核—幔边界动力学研究进展

本文介绍了９０年以来地球物理学在地球磁场方面的研究成果,揭示了２００－１００Ｍａ全球低的地球磁场古强度;１２４－８３Ｍａ低的极性反转频率以及１７０－１１０Ｍａ近乎停滞的真极移。     

DK_1应力孔前兆异常的特点

本文以地震对应法分析了莆田地震台DK_1应力孔的前兆异常特点。     

The relation between magnetic range values and spectral power

Spectral power is shown to be proportional to the square of the range for variables with a normal distribution. Plots of log power versus log range for 3  hr intervals of data from Canadian magnetic observatories show a close fit to a straight line with a slope of 2. The same results are obtained from all sites in the Canadian magnetic observatory network, which extends from the polar cap to auroral and sub-auroral latitudes. This indicates that a square-law relation between spectral power and range is a general property of magnetic field variations.     

CrustalP-wave velocity and velocity-ratio study in northeast India by a microearthquake survey

The upper crustal (20 km)P-wave velocity beneath the Shillong Plateau and Nowgong area has been studied by the time-distance plot method. TheP-arrival data of the shallow (20 km) microearthquakes from three temporary networks are used, and the average velocity is found to be 5.55 km/s. The velocity ratio (V _p /V _s ) for the upper crust (0–20 km) as well as for the lower crust (21–40 km) are determined by the Wadati-plot method and station-by-station method. The average value obtained by the two methods is compatible; theV _p /V _s ranges between 1.74 to 1.76. A generalized seismic velocity model of the area is suggested by this study, which has been very useful for microearthquake location.     

EMRIDGE: The electromagnetic investigation of the Juan de Fuca Ridge

From July to November 1988, a major electromagnetic (EM) experiment, known as EMRIDGE, took place over the southern end of the Juan de Fuca Ridge in the northeast Pacific. It was designed to complement the previous EMSLAB experiment which covered the entire Juan de Fuca Plate, from the spreading ridge to subduction zone. The principal objective of EMRIDGE was to use natural sources of EM induction to investigate the processes of ridge accretion. Magnetotelluric (MT) sounding and Geomagnetic Depth Sounding (GDS) are well suited to the study of the migration and accumulation of melt, hydrothermal circulation, and the thermal evolution of dry lithosphere. Eleven magnetometers and two electrometers were deployed on the seafloor for a period of three months. Simultaneous land-based data were made available from the Victoria Magnetic Observatory, B.C., Canada and from a magnetometer sited in Oregon, U.S.A.Changes in seafloor bathymetry have a major influence on seafloor EM observations as shown by the orientation of the real GDS induction arrows away from the ridge axis and towards the deep ocean. Three-dimensional (3D) modelling, using a thin-sheet algorithm, shows that the observed EM signature of the Juan de Fuca Ridge and Blanco Fracture Zone is primarily due to nonuniform EM induction within the ocean, associated with changes in ocean depth. Furthermore, if the influence of the bathymetry is removed from the observations, then no significant conductivity anomaly is required at the ridge axis. The lack of a major anomaly is significant in the light of evidence for almost continuous hydrothermal venting along the neo-volcanic zone of the southern Juan de Fuca Ridge: such magmatic activity may be expected to have a distinct electrical conductivity signature, from high temperatures, hydrothermal fluids and possible melt accumulation in the crust.Estimates of seafloor electrical conductivity are made by the MT method, using electric field records at a site 35 km east of the ridge axis, on lithosphere of age 1.2 Ma, and magnetic field records at other seafloor sites. On rotating the MT impedance tensor to the principal axis orientation, significant anisotropy between the major (TE) and minor (TM) apparent resistivities is evident. Phase angles also differ between the principal axis polarisations, and TM phase are greater than 90° at short periods. Thin-sheet modelling suggests that bathymetric changes accounts for some of the observed 3D induction, but two-dimensional (2D) electrical conductivity structure in the crust and upper mantle, aligned with the ridge axis, may also be present. A one-dimensional (1D) inversion of the MT data suggests that the top 50 km of Earth is electrically resistive, and that there is a rise in conductivity at approximately 300 km. A high conductivity layer at 100 km depth is also a feature of the 1D inversion, but its presence is less well constrained.