Seismological Constraints on Stress in the Oceanic Lithosphere [and Discussion]

Studies of earthquakes provide some of the basic constraints on the stress within the oceanic lithosphere. The focal mechanisms of earthquakes indicate the directions of the principal stress axes. These directions can be compared with the stresses predicted by plate driving-force models, given the caveat that they can be biased by the locations of pre-existing weak zones and local processes. One basic result, that lithosphere older than about 35 Ma is in deviatoric compression, can constrain the basal drag on plates and hence mantle viscosity. A second result, the absence of a broad zone of ridge-normal extension in younger lithosphere, suggests that the ridge axis is rheologically weak. The focal depths of earthquakes, which indicate the depth to which the lithosphere is strong enough to support seismicity, can be combined with thermal models and laboratory rock mechanics to constrain the rheology of the lithosphere, both in plate interiors and at plate boundaries. An additional data type, which could in principle yield inferences about the magnitudes of stress, consists of the seismic moments, magnitudes, and source time functions of the earthquakes. Some initial results suggest that transform fault earthquakes release seismic energy more slowly than ridge events, perhaps due to lower stress drops or source geometry.