Seismological constraints on stress in the oceanic lithosphere

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. 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. 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. -from Authors