关键词: 岷县-漳县地震, 视电阻率, 通渭地震台, 异常变化, 断层虚位错模式

Abstract:

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.

Key words: Minxian-Zhangxian earthquake, apparent resistivity, Tongwei station, anomaly, fault virtual dislocation model