Mechanism of the Rat Island earthquake sequence of February 4, 1965, with relation to island arcs and sea-floor spreading

Mechanisms of the main shock and of fourteen larger aftershocks of the Rat Island earthquake of February 4, 1965, indicate two classes of foci. The first, including the main shock and nine aftershocks all located within the island arc proper, corresponds either to an overthrust of the island block relative to the oceanic block or to reverse faulting oblique to the trend of the arc. The second, consisting of foci under the trench, represents fractures arising from a horizontal tensional stress. The entire motion is conformable to an oceanic plate moving under the island arc. The sequence is terminated by a transform fault at the eastern end of the active zone.