Intraplate seismicity and stresses in young oceanic lithosphere

Abstract

We have conducted a systematic study of oceanic intraplate seismicity in the period 1964–1983 occurring in lithosphere younger than 35 million years. Focal mechanisms determined for 26 events using first motions, P wave modeling and surface wave amplitude radiation patterns, along with 13 previously determined mechanisms, indicate a diversity of faulting styles and stress orientations in young oceanic lithosphere. There is no evidence for a worldwide transition from extension to compression at a specific lithospheric age. Tensional axes of normal faulting events are oriented oblique to the spreading direction and do not show extension in the spreading direction. Congressional axes of thrust events show a weak preferred orientation in the spreading direction, presumably due to ridge push stresses. However, stress axes of thrust events in very young lithosphere (age <9 Ma) do not show a correlation with the spreading direction, suggesting that the ridge push force is small near the ridge. These observations indicate that tensional stress in the spreading direction is concentrated in a very narrow zone at the ridge axis, in agreement with models which assume that 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. Alternatively, the depths of the normal faulting events may be a result of the higher thermal gradients at depth in young lithosphere, if thermoelastic stresses are important in generating this seismicity. Differential thermal contraction along fracture zones does not seem to be a significant source of near‐ridge intraplate seismicity. High seismicity rates in young lithosphere, noted previously on a global scale, seem characteristic of all three major ocean basins. Additionally, most of the total oceanic intraplate moment release takes place in young lithosphere. However, most of the events and more than 96% of the total moment release of the near‐ridge intraplate seismicity are contained in five areas. Two zones in the Indian Ocean, one near Chagos Bank and the other in the Amsterdam‐St. Paul region, indicate tension parallel to the trend of the plate boundary. A region of high seismicity near the East Pacific Rise between 3° and 7°N shows both normal and thrust faulting, while another region, on the Cocos plate west of the Panama Fracture Zone, is characterized by strike‐slip faulting. Thrust, strike‐slip, and normal faulting events are found in a small seismically active zone of the South‐Central Pacific. The extremely uneven distribution of seismicity, if not a result of the relatively short time period studied (20 years), indicates local perturbations strongly affect near‐ridge seismicity.