Strength asperities along oceanic transform faults: a model for the origin of extensional earthquakes on the Eltanin transform system

We use finite element simulations of the state of stress in the vicinity of the Eltanin transform system to investigate the possible origin of earthquakes occurring on, or in the immediate vicinity of, transform segments, and featuring normal faulting mechanisms with tension axes oriented approximately 45° away from the azimuth of plate motion, in principle unexpected along transform faults. In addition to solutions previously described in the literature, we report on seven new such mechanisms, including a large event with magnitude >6 in 2001. We model the transform segments as either weak, i.e., featuring the same elastic properties as the ridge, or strong, i.e., featuring the same properties as the surrounding cooled oceanic lithosphere. In addition, we allow irregularities in the geometry of the transform contact, in the form of either bends in the plate boundary (‘jogs’) or asperities, i.e., strong elements in an otherwise weak transform system. We also consider the effect of oblique ridge push on the Pacific plate. We conclude that the seismic properties of the Eltanin transform system are best described by the model of a weak transform, in which a few strong asperities are embedded. We speculate that their origin may be linked to the complex and recent history of readjustments in rates and directions of spreading, notably to the south of the Eltanin system.