The variations of the degree of ground resistivity anisotropy during the Tangshan earthquake

Based on the geoelectric observation data of the 1976 Tangshan earthquake of M=7. 8 compiled by the Research Group on the Geoelectricity of the Tangshan Great Earthquake, a dimensionless factor S has been defined as the degree of ground resistivity anisotropy. The S values during the generation process of that earthquake have been calculated and their variations have been analyzed. The result has showed that the variation of the degree of ground resistivity anisotropy existed throughout the process of generation and occurrence of the Tangshan earthquake and the features of its pattern are representative. The S value can therefore be taken as a new precursory factor of earthquakes which can be applied together with other dimensionless factors in the analysis and prediction of earthquakes. A physical explanation of the variation of the S value has also been given.

Dynamic evolution patterns of the degree of ground resistivity anisotropy and the sets mogenic process

Following a new train of thinking, this paper has explored first the potential information in the ground resistivitydata observed by the existing geoelectric observation system, investigated and proposed a new dimensionlessgeoelectric precursor factor, the degree of ground resistivity anisotropy, S, and studied the characteristics of dynamic evolution pattern of S during the seismogenic process. The results show that, during the seismogenic process, the degree of ground resistivity anisotropy (S) displays h process of 'normal' → 'abnormal strengthening(amplitude, range)' → 'abnormal weakening'→ 'earthquake occurrence'→ 'normal'. The earthquake wouldoccur at the time when the S value has entered the late stage of strengthening and turns to weaken and in the gradient belt on the margin ofS anomaly region. The dynamic evolution pattern ofS reflects the changes of the tectonicstress field during the seismogenic process. Therefore, it would be possible to trace the process of earthquake generation and occurrence from the dynamic evolution pattern ofS so as to service eaJ'thquake prediction.

Features and physical proces of the dynamic evolution pattern of ground resistivity precursor front

In order to quantitatively describe the geoelectric precursor anomaly in the short-impending process of earthquakes, a new geoelectric precursor index — (monthly) relative change rate of ground resistivity, R ρ (t) , is designed. Using this index and choosing the internationally accepted ground resistivity data before the Tangshan M =7.8 earthquake of July 28, 1976, the features of dynamic evolution pattern of R ρ(t) are studied. The results show that: ① about 10～9 months before earthquake, the ground resistivity in a certain range around the epicentral region begins to display the anomaly of accelerating descent, and the rate of descent is higher in the epicentral region than in surrounding areas; ② with the shortening of countdown before earthquake, the R ρ(t) value in epicentral region increases gradually (ground resistivity value decreases at an increasing rate); ③ the R ρ(t) value has the epicentral area as a center and its contour lines propagate towards surrounding areas with the shortening of countdown before earthquake; ④after the R ρ(t) value in epicentral region has descended at increasing rate to reach an extremity [ R ρ(t) = (7.0], it turns to descend at decreasing rate (2～3 months) and earthquake occurs when it accelerates again. Meanwhile, earthquake occurs when the contour lines of R ρ(t) stop propagating towards surrounding areas and turn to shrink back (2～3 months later). Its physical process can be explained by the″ swollen hypothesis″ of Prof. Fu and the theory of ″Slip-weakening and rockmass instability″ of Mei, Niu, et al ..

Discussion on Common Problems in Risk Assessment of Lightning Disaster

The problems in calculating parameters of equivalent collection area,earth resistivity and lightning protection category as well as their effects on lightning disaster risk assessment were analyzed,and practical examples proved the effects of those differences on lightning protection identification,intercept efficiency calculation in evaluating lightning disaster risk. In the meantime,several new concepts,such as the height of buildings for lightning protection were defined,and a fixed radius value was set to the ground flash density for calculation,establishing the ground flash density formula to solve the problems in parameter calculation,which would be beneficial to promote the standardization of lightning disaster risk assessment.

Soil resistivity and ground resistance for dry and wet soil

In this paper, soil resistivity and ground resistance at two different sites near an electrical substation are measured using a grounding system grid with and without rods. With the Wenner four-pole equal-method, the soil resistivity is measured at both selected sites, one of which contains wet soil while the other contains dry soil. Cymgrd simulation software is then used to determine the acceptability of these measured resistivity values by finding out the root mean square error between the measured and calculated values for both wet and dry soil sites. These values for wet and dry soil sties were found to be only 0 %and 4.92 %, respectively, and deemed acceptable. The measured soil resistivity values were then used to evaluate the ground resistance values of a grounding grid ‘with rod’for the wet soil site and ‘without rods’ for the dry soil site,and then compared with the simulated ground resistance values. These comparisons were also found to be in good agreement. In addition, ground potential rise, maximum permissible step and touch potentials have also been estimated using the simulation software.