Abstract We introduce and discuss an automated, analytical method for determination of shear-wave splitting. To determine the polarization direction of the first-arriving shear wave (fast wave), we evaluate the linearity of the particle motion of shear waves by maximizing as a function of azimuth the aspect ratio, i.e., the ratio of the projections of the particle displacement on orthogonal axes in the horizontal plane. In a time window that contains only the first shear-wave arrival, the azimuth at which the maximum aspect ratio occurs is taken as the polarization direction of the fast shear wave. Then the time separation between the split shear waves is determined by time-shifting the component with the slow shear-wave arrival toward the fast shear-wave arrival until maximum linearity in particle motion is observed. The polarization direction of the fast shear wave can be accurately predicted for seismograms having good signal-to-noise ratios, as are typical of local earthquake recordings. Similar procedures are used to evaluate particle motion in a vertical plane in the time interval including the first shear-wave arrival to examine the apparent incident angle in order to reveal the likelihood of interfering S-P conversions.