The polarization of the S wave at stations distributed azimuthally about the source is examined for each of twenty-five Aleutian Island earthquakes. A combination of data from the first motion of P and from the polarization of S is then used to study the focal mechanisms of the earthquakes. This combination of P and S wave data is found to make possible a good determination of the focal mechanism in cases where data from the first motion of P alone do not suffice.
The earthquakes are divided into three groups according to three basic patterns of S wave polarization. The first group (fourteen earthquakes) corresponds to a double couple. The second group (five earthquakes) and the third group (six earthquakes) are conformable to conjugate shears and may therefore be explained by single couple sources of opposite moment, respectively. It is shown that a uniform principal stress system predominates in the region and that the axis of greatest compressive stress is normal to the trend of the island arc.