A M_S6.6 earthquake occurred at the junction area of Minxian and Zhangxian, Gansu Province, on July 22, 2013. Before the earthquake, the apparent resistivity observed at Tongwei station showed abnormal anisotropic changes. Electrical resistivity is an important physical property for sedimentary rock-soil. The continuous load of compressive stress, by causing crack growth and directional alignment, would tend to increase the connectivity of these crack films. Build-up of strain at the locked fault segment and its vicinity area before an earthquake ought to be accompanied by change in resistivity. Laboratory measurements of resistivity on rock specimens under deformation to failure under uniaxial and triaxial compression show that resistivity of water-bearing rocks declines as the stress exceeds about half of the fracture stress. The decline rate increases considerably near the stage of final fracture. The magnitude of resistivity change in axial direction is usually greater than that in the transverse direction. In-situ experiments taken on field soil using Schlumberger arrays also showed decline change in apparent resistivity under compression stress loading. Monitoring arrays in different directions at the same set of array usually have different magnitudes of change, i.e. anisotropic changes. The array perpendicular to or near perpendicular to the P axis has the maximum magnitude of change, while the magnitude of change is the minimum or even unnoticeable when the array is parallel to or sub-parallel to the P axis.

It can be expected from the above experiment results that absolute stress level is often needed to discuss the relationship between crack variation and stress. However, it is difficult to obtain successive absolute stress-strain measurement at present for a large tectonic region. On the other hand, the general quantitative mathematic relationship between the stress level and micro-crack activity is not clear. One alternative compromise way is to obtain the qualitative spatial distribution characteristic of the stress-strain accumulation required to produce the coseismic slip using the fault virtual dislocation model. In this paper, we use the fault virtual dislocation model to analyze the changes in the apparent resistivity data of Tongwei station before the earthquake. In the model, the coseismic sliding displacements of the earthquake are loaded in the same magnitude but opposite directions, in order to calculate the stress-strain distribution required to generate these coseismic dislocations before the earthquake. The areas of compression enhancement or relative expansion before an earthquake can be displayed. It should be noted that results from the virtual dislocation model are the changes of stress or deformation, not the absolute state of stress-strain. Northeast margin of Tibetan plateau is in compressive tectonics as a whole. The compression areas from the virtual dislocation model can be seen as areas with compression enhancement before the earthquake. However, for the extension areas from the model, we cannot distinguish them between true extension areas and compressive areas. They can be regarded as relative extension areas where the original tensile effect is strengthened or the original compressive effect is released to some extent.

The results show that the Tongwei station is located at the compression stress and strain accumulation area before the occurrence of the earthquake, which coincides with the decreases of the apparent resistivity data. On the other hand, the focal mechanism solution shows that the azimuth of the principal compressive stress of this earthquake is 65°. The angle between the P axis and the N20°W direction of Tongwei station is 85°, and the angle from the EW direction is 25°. Before the earthquake, the decrease amplitude of the N22°W is 1.04%, and the decrease amplitude of the EW' is 0.37%. The anisotropic changes observed in the two directions are consistent with the results given by the experiment results, theoretical models and the summary of earthquake examples. Therefore, it can be considered that there may be a mechanical relationship between the changes in the apparent resistivity of the Tongwei station and the seismogenic process.

Crustal medium has varying degrees of electrical conductivity. Electrical resistivity is an important physical property for geomaterials. Electrical resistivity is commonly used in geophysics to investigate the deformation of the crust. Resistivity of sedimentary rock and soil is a combination of resistivity of both solid matrix and crack/pore fluid, and also depends on the degree of fluid saturation, crack ratio or porosity, and crack shape. Since the electrical resistivity is related to mechanical properties, build-up of strain ought to be accompanied by resistivity changes, which might warn of an impending earthquake. Many studies on electrical resistivity changes of rock and soil have been carried out in an attempt to find a physical basis for earthquake prediction. Rock resistivity changes under deformation up to failure in uniaxial and triaxial experiments have been measured in laboratory. For water-bearing rocks, resistivity decreases or increases at low stress and decreases greatly at high stress, just before failure. A series of midpoint Schlumberger arrays were placed on rock samples. Apparent resistivity of the perpendicular array has the maximum decrease magnitude, while the parallel array has the minimum magnitude. The decrease magnitude of the oblique array falls within the upper and lower bounds of the other two arrays. In-situ experiments show the similar anisotropic changes in apparent resistivity. Apparent resistivity has been continuously monitored at fixed stations in China for more than 50 years, using Schlumberger arrays. Apparent resistivity has been monitored the same pattern of anisotropic changes before great earthquakes as the results from experiments. Surface DC apparent resistivity observation using a Schlumberger array has a depth detection range approximately equal to the electrode spacing of AB. There may also be lateral electrical inhomogeneity under the survey area, i.e. the resistivity of the medium varies in the horizontal direction within the same depth range. Lateral inhomogeneity within the detection range will cause anisotropy in apparent resistivity, but whether it will cause anisotropic changes is still uncertain. The depth range of stratum resistivity affected by tectonic stress before earthquakes has not been fully discussed. In this paper, we use finite element method to calculate the anisotropic changes in apparent resistivity caused by resistivity changes from different depth range. Lateral homogeneity and lateral inhomogeneity models are taken into account, respectively. The results show that for Schlumberger array with spacing of AB=1 000m, anisotropic changes in apparent resistivity caused by resistivity changes of stratum below 300m is inconsistent with the observed results in experiments and seismic examples. Under this situation, apparent resistivity of the perpendicular array has the minimum decrease magnitude, while the parallel array has the maximum magnitude. On the other hand, the apparent resistivity decreases in a small range, and the difference between the two monitoring directions is not significant. The depth of stratum where resistivity changes take place needs to rise to a range of tens of meters from the surface. Then the magnitude and feature of anisotropic changes are consistent with the experiments and field observations. For stratum with lateral inhomogeneity, apparent resistivity dose not display anisotropic changes when resistivity of different stratum units under the same depth has the same magnitude of isotropic variation. Anisotropic changes in apparent resistivity take place only when stratum resistivity shows anisotropic variations. However, lateral heterogeneity has little effect on anisotropic changes of apparent resistivity.

Parts of the consecutive apparent resistivity monitoring stations of China have recorded clear diurnal variations. The relative amplitudes of diurnal variations at these stations range from 1.3‰ to 5.8‰. The daily accuracies of apparent resistivity observation are better than 1‰, because the background electromagnetic noise is rather low at these stations. Therefore, the diurnal variations of apparent resistivity recorded at these stations are real phenomena. The diurnal variation shapes can be divided into two opposite types according to their characteristics. One type is that the apparent resistivity data decreases during the daytime but increases during the nighttime(Type 1). The other type is the apparent resistivity data increases during the daytime but decreases during the nighttime(Type 2). There is a correspondence between the diurnal and annual variation patterns of apparent resistivity. For the monitoring direction with diurnal variation of Type 1, the apparent resistivity decreases in summer and increases in winter. However, for the monitoring direction with diurnal variation of Type 2, the apparent resistivity increases in summer and decreases in winter.
We take an analysis on the mechanism of apparent resistivity diurnal variation, combining the influence factors of water-bearing medium's resistivity, the electric structure of stations, and the apparent resistivity sensitivity coefficient(SC)theory. Intuitively, diurnal variation of apparent resistivity is caused by diurnal variation of medium resistivity in the measured area. The diurnal variation of medium resistivity will inevitably be caused by the factors with diurnal variation. Among the possible factors, there is diural variation in earth tide and temperature.
Our analysis displays that apparent resistivity diurnal variation is not caused by the usually-believed earth tide, but by the ground temperature difference between daytime and nighttime. The earth tide strain is too small to cause remarkable effects on the apparent resistivity data. On the other hand, the daily tide strain has two peak-valley variations, and its phase and amplitude has a period of approximate 28 days. However, the apparent resistivity data do not show these corresponding features to earth tide. Furthermore, the detection range of current apparent resistivity stations is within a depth of several hundred meters. Within this depth range, the medium deformation caused by solid tide can be regarded as uniform change. Therefore, all monitoring directions and all stations will have the same pattern of diurnal variation.
In general, the temperature increases in the daytime but decreases in the nighttime. For most water-bearing rock and soil medium, its resistivity decreases as temperature increases and increases as temperature decreases. Diurnal temperature difference affects about 0.4m of soil depth. Therefore, resistivity of this surface thin soil layer decreases in the daytime while increases in the nighttime. Under layered medium model, SC of each layer represents its contribution to the apparent resistivity. For the stations with positive SC of surface layer, apparent resistivity decreases in the daytime but increases in the nighttime. While for the stations with negative SC of surface layer, apparent resistivity diurnal variations display the opposite shape.

Current leakage,metallic conductor,and local anomalous resistivity body are main disturbance sources which affect the successive observation of apparent resistivity in stations,besides the observing system failure.We construct a finite element model using a 3-layered horizontal medium to discuss the dynamic characteristics of disturbances caused by metal conductor and local anomalous resistivity body in the measuring filed.The numerical results show that low resistivity source which is located in areas where the sensitivity coefficient is positive will cause decline on apparent resistivity observation.While low resistivity source will cause increase when it is located in areas where the sensitivity coefficient is negative.Disturbance caused by high resistivity source is opposite to the one from low resistivity source.The general dynamic feature of disturbance is that the disturbance amplitude increases as the resistivity of shallow layer decreases,while the amplitude declines when the shallow layer's resistivity increases.For the measuring direction which has normal annual variation form,low resistivity source which is located in area where the sensitivity coefficient is positive will increase the annual variation amplitude,while it will reduce annual amplitude when it is in a negative sensitivity coefficient area.Annual amplitude changes caused by high resistivity source are opposite to the changes caused by low resistivity source.For the measuring direction which has abnormal annual variation form,dynamic annual feature is opposite to the one in direction of normal annual variation form.If the dynamic feature is opposite to the annual variation and disturbance amplitude is also greater than annual amplitude,the annual variation will change direction.Disturbance amplitude from metallic conductor is affected by the resistivity and cross-section area,the lower of the resistivity and the larger of the cross-section area,the greater of the disturbance amplitude.

We calculate three-dimensional sensitivity coefficients distribution of apparent resistivity observation when Schlumberger array is used by using finite element method. Analysis results suggest that for the situation of one-dimensional positive or minus coefficient of surface medium, three-dimensional sensitivity coefficients distribution at surface shows similar patterns, and sensitivity coefficients distributions of different layered electric structures are also similar. There are two approximate ellipses at the two-dimensional surface plane between current electrodes and potential electrodes, where sensitivity coefficients are minus, and sensitivity coefficients at other areas are positive. Sensitivity coefficients at two approximate ellipses between current electrodes and potential electrodes are minus at the vertical section along monitoring line, while others are positive. From the three-dimensional view, minus sensitivity coefficients are at the two approximate half ellipsoids between current electrodes and potential electrodes when arrays are applied at surface. And coefficients near the electrodes are much greater than other areas. When resistivity of local areas at surface changes, we can qualitatively analyze the disturbing effects caused by the areas using three dimensional sensitivity coefficients distribution, and the analysis result can serve as reference for further experiment and numerical model quantitative analysis.

Lushan M7.0 earthquake occurred in Lushan County, Ya'an City, Sichuan Province of China, on 20 April 2013, causing 196 deaths, 23 people missing and more than 12 thousands of people injured. In order to analyze the possible seismic brightness temperature anomalies which might be associated with Lushan earthquake, daily brightness temperature data are collected from Chinese geostationary meteorological satellite FY-2E, for the period from 20 April 2011 to 19 April 2013 and the geographical extent of 25°～35°N latitude and 98°～108°E longitude. Continuous wavelet transform method is used to analyze the power spectrum of brightness temperature data, for its good resolution both in time and frequency domains. The results show that relative wavelet power spectrum(RWPS)anomalies appeared since 15 January 2013 and still lasted on 19 April. Anomalies firstly appeared at the middle part of Longmenshan Fault zone. Then, they gradually spread towards southwestern part of Longmenshan Fault. Anomalies also appeared along the Xianshuihe Fault since about 1 March. Eventually, anomalies gathered at the intersection zone of Longmenshan and Xianshuihe Faults. The anomalous area and RWPS amplitude increased since the appearance of anomalies and reached maximum in late March. Anomalies attenuated with earthquake approaching, and eventually the earthquake occurred at the southeastern edge of anomalous area. Lushan earthquake was the only obvious geological event within the anomalous area during the time period, so the anomalous changes of RWPS are possibly associated to the earthquake.