Abstract
Two nearby earthquakes seaward of the Tonga trench, one normal faulting and one thrust faulting, were studied in detail by using comparative event, surface wave techniques. The focal depth for the thrust‐faulting event was found to be 49 km below sea level, consistent with the focal depth estimated from the pP‐P delay time. The normal‐faulting event had a depth of 14 km. These are the first precise depth determinations for earthquakes which are associated with the bending of the oceanic lithosphere before subduction. The pattern of horizontal deviatoric tension near the surface and compression within the interior of the lithosphere is consistent with the stresses predicted by an elastic plate model, although the depth of the thrust event suggests that either the elastic layer begins near the base of the crust or the rheological model should be modified to include a plastic layer of finite strength.
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