1. A computer-implemented method of determining a stress state associated with a geological fault, comprising: storing a data set representing fault displacement values, or proxy displacement values, associated with the fault; storing an error function that calculates error as a function of normalized fault displacement and normalized slip tendency; selecting principal stress values for a candidate stress state; calculating a slip tendency for the candidate stress state for each value in the data set; calculating an error value for the data set, using the error function, the slip tendency, and the displacement or the proxy displacement values; wherein the error function is conditionally calculated depending on whether the displacement value or proxy displacement value is greater than the slip tendency; repeating the selecting and calculating steps for a number of candidate stress states; and determining the stress state that minimizes the error value.