Theoretical resistivity variations along stressed strike-slip faults

Abstract

A simple model of a strike-slip fault is analyzed to see if observable resistivity variations would be produced by strain' accumulation. Fault displacement is found to produce only pure shear loading.Estimates of the effect of this stress system on rock resistivity are made by using existing resistivity data measured during isotropic and triaxial loading. For a model of the San Andreas fault, 50 cm of fault displacement are found to produce apparent resistivity variations of about 1%. The model considered requires long periods of time (>20 years) to produce 1% resistivity variations. This is in 6ontrast to reported observations of large resistivity changes (>20%) developing in a period of several months before an earthquake. If more observational data confirm preearthquake resistivity changes, very different models must be considered to explain the observations. Several investigators [Barsukov, 1972; Mazzela and Morriso n, 1974] have reported variations 0f electrical resistivity prior to earthquakes. These observations have been explained by the dramati c changes in the electrical properties of rock which occur prior to failure in the laboratory [Brace and Orange, 1968a]. While the mechanism which explains the laboratory resistivity changes may not be the only one acting in the field situation, it is of interest to see how'large a resistivity variation could be produced by microcrack generation due t ø strain accumulation prior to an earthquake. In this paper a simple tectonic model of a strike-slip fault will be combined with resistivity-stress behavior deduced from laboratory measurements to see how large a strain-induced resistivity anomaly can be produced and whether it can be detected by surface resistivity measurements. The results will be compared with reported field observations from along the San Andreas fault. RESISTIVITY BEHAVIOR OF STRESSED ROCK At ,moderate temperature (<300øC) the electrical resistivity of rocks is controlled primarily by the following factors: (1) the pore fluid resistivity, (2) the rock porosity, (3) the degree of saturation, (4) the size of interconnected pores, and (5) the interconnectivity of the pores. Applied stresses will affect resistivity by modifying the pore-size distribution, i.e., the porosity, the pore sizes, and the interconnectivity of the pores. Experiments conducted by Brace et al. [1965], and Brace and Orange [1968a, b], illustrate these points. In Figure 1 is shown the resisitivity of a granite subjec.ted to isotropic effective confining stress •c. The effective confining stress is the confining stress •c less the pore pressure P. Resistivity is seen to

Detecting DC Electrical Resistivity Changes in Seismic Active Areas: State-of-the-Art and Future Directions

“…In this context, one of the most challenging research problems still debated in the geoscience community is the study of subsurface resistivity changes before, during, and after earthquakes. In a seminal paper, Fitterman [13] introduced the theoretical basis underlying possible variations in the spatio-temporal patterns of earth resistivity close to active fault systems. Rikitake [14] and Gokhberg et al [15] considered this geophysical parameter as a possible earthquake precursor.…”

Experimental Studies on the Changes in Resistivity and Its Anisotropy Using Electrical Resistivity Tomography

“…The striking changes in electrical resistivity and its anisotropy were widely observed before earthquake, mine earthquake, volcanic activities, and geologic motions [1][2][3][4][5][6][7][8][9][10][11][12]. In order to address the change characteristics of resistivity and its anisotropy, many investigators conducted a plenty of experiments on rock samples under compression and during fracture, shear, and frictional sliding in lab [13][14][15][16][17][18][19][20][21][22][23][24][25] and theoretic studies [26][27][28][29][30]. Experimental results reported by Brace et al [13] and Brace and Orange [14][15][16] in lab showed that striking changes in electrical resistivity were observed for a variety of igneous and sedimentary crystalline rocks when water-saturated crystalline rocks were stressed to fracture.…”