1. A computer-implemented method of determining the stress state associated with a geological fault, comprising: storing a dataset 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 stress state; calculating the slip tendency for that stress state for each measurement 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; repeating the selecting and calculating steps for a number of stress states; and determining the stress state that minimizes the error value.