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 si x independent parameters of the tectonic stress tenser. Using the Coul omb-Navier failure criterion as an additional constraint, it is possib le to estimate the fifth parameter, characterized by the normalized cr itical 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 unifor m, faults with different orientations and at different depths have the same normalized critical stress difference. On this basis, a new meth od is proposed to estimate the average friction coefficient and the no rmalized critical stress difference from inversion of a population of faults of measured orientations and slip directions. This method is ap plicable both to newly formed faults and to reactivated faults. This m ethod is applied to four data sets. In three cases, an average frictio n coefficient <(mu)over bar>(0) = 0.64, 0.70 and 0.88 is obtained. One case shows a relatively low average friction coefficient <(mu)over ba r>(0) = 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 <(mu)over bar> = 0.75 deriv ed from laboratory experiments.