Skewed orientation groups in scatter plots of earthquake fault plane solutions: Implications for extensional geometry at oceanic spreading centers

Systematic analysis of earthquake focal solutions derived from centroid moment tensors shows well-defined orientation groups in scatterplots of fault plane normals and associated slip line vectors. Consideration of the geometry implied by these orientation groups can allow resolution of the ambiguity inherent in the choice as to which of the two conjugate fault plane solutions should apply, and in many cases, the same classification can be applied to the entire orientation group. Examining scatter plots of data from normal fault earthquakes on spreading ridges typically shows orthogonal relations but there are also many cases where there is a skew with respect to the great circles defined by faults on adjacent transform faults. This can be explained by finite rock strength in the adjacent transforms, requiring resolved shear stress to allow movement, thus requiring rotation of the trajectories of the deviatoric stress axes: anticlockwise for right-lateral transforms and clockwise for left-lateral transforms. This asymmetry also requires formation of tilt block geometries reminiscent of Basin and Range style continental extension.