| The strength of the oceanic lithosphere is constrained by the maximum depth of oceanic intraplate seismicity, which increases with lithospheric age and appears to be bounded by a 700(DEGREES)-800(DEGREES)C isotherm. This limiting depth is approximately equal to the flexural elastic thickness of the lithosphere and is consistent with experimental olivine rheologies which predict rapid weakening at high temperatures. Because intraplate earthquake focal mechanisms indicate that older oceanic lithosphere is in compression, models of the stresses produced by a combination of ridge push and basal drag forces require the magnitude of the drag be less than a few bars for rapidly moving plates. A two dimensional return flow model of mantle convection with a viscosity of 10('22) poise predicts higher basal drag values unless a thin asthenosphere with viscosity less than 10('19) to 10('20) poise decouples the plate from the underlying mantle.;Tests with synthetic data indicate that a deconvolution method, developed in this study, is useful for determining the depth, mechanism, source time function and rupture characteristics of an earthquake from P and SH waveforms. This technique is used to determine the source parameters of three large intraplate earthquakes.;Focal mechanisms of intraplate earthquakes in young oceanic lithosphere indicate a diversity of faulting styles. Tensional axes of normal faulting events are oriented oblique to the spreading direction, whereas compressional axes of thrust events show a weak preferred orientation in the spreading direction. This indicates that tensional stress in the spreading direction is concentrated in a very narrow zone at the ridge axis, suggesting the axial region has very little tensile strength. Normal faulting events occur at greater depths and temperatures than thrust events, possibly because rocks under tensional stress have strength maxima at higher temperatures or, alternatively, because tensional thermoelastic stress is concentrated at depth as a result of higher thermal gradients. The intraplate seismicity rate is highest in young lithosphere, indicating that stress sources concentrated in young lithosphere (such as thermoelastic stresses) are significant. However, most of the near-ridge seismicity occurs in five areas, suggesting local perturbations strongly affect near-ridge seismicity. |
