On the strength of oceanic fracture zones and their influence on the intraplate stress field

Abstract

We use the locations and source mechanisms of oceanic intraplate earthquakes to test the hypothesis that the strength of oceanic fracture zones is less than that of normal oceanic lithosphere. If fracture zones are weaker than the surrounding lithosphere, they should be sites of concentrated intraplate seismicity, and the lithospheric stress field should be perturbed in their vicinity. The 77 earthquakes selected for the study have well‐determined focal mechanisms and epicenters in regions where fracture zones are well mapped. We have searched for dependence of faulting style, fault orientation, or principal stress direction on the distance from the nearest fracture zone. If fracture zones were generally weaker than the surrounding lithosphere, one of the principal horizontal stresses would be oriented nearly perpendicular to the fracture zone; we find no evidence that principal stresses near fracture zones are oriented preferentially in this manner. There is a slight tendency for earthquakes to occur near fracture zones, and patterns of fault orientation and sense of slip support the view that differential cooling and horizontal contraction on fracture zones may contribute seismogenic stress. These patterns are not demonstrable at high significance, however. If oceanic transform fault zones are weak relative to the surrounding lithosphere, these results suggest that the strength of the fracture zones increases within a few million years after cessation of transform motion to levels comparable with that of normal oceanic lithosphere.