Estimation of the frictional strength of faults from inversion of fault-slip data: a new method

The conventional stress inversion methods estimate only four of the six independent parameters of the tectonic stress tensor. Using the Coulomb-Navier failure criterion as an additional constraint, it is possible to estimate the fifth parameter, characterized by the normalized critical stress difference, i.e. the critical stress difference divided by the effective overburden pressure. This parameter is related to the average friction coefficient of faults. If the stress field is uniform, faults with different orientations and at different depths have the same normalized critical stress difference. On this basis, a new method is proposed to estimate the average friction coefficient and the normalized critical stress difference from inversion of a population of faults of measured orientations and slip directions. This method is applicable both to newly formed faults and to reactivated faults. This method is applied to four data sets. In three cases, an average friction coefficient \̄gm0 = 0.64, 0.70 and 0.88 is obtained. One case shows a relatively low average friction coefficient \̄gm0 = 0.22, but this value is of poor quality due to the effect of a possibly nonuniform stress field. These results are in agreement with the average value of friction coefficient \̄gm0 = 0.75 derived from laboratory experiments.