Variations in late syn-rift melt alignment inferred from shear-wave splitting in crustal earthquakes beneath the Ethiopian rift

Abstract

The northern Main Ethiopian rift (MER) marks the transition from continental rifting to incipient seafloor spreading. We constrain anisotropy of the upper-crust in the MER and its uplifted rift flanks using shear-wave splitting from 24 earthquakes located beneath 18 broadband stations. Along the axis of the MER the fast polarization direction is oriented between ∼N and ∼NNE, parallel to Quaternary-Recent faults, aligned cones and maximum horizontal stress. Delay times are highest (0.24 s) where independent seismic studies show evidence of shallow partial melt. We attribute anisotropy along the rift axis to aligned melt-filled micro-cracks and dikes. At stations flanking the rift, the fast polarization direction is oriented ∼NE and delay-times are smaller (0.04-0.14 s). The lower amount of anisotropy is consistent with reduced melt away from the rift axis. These results show melt-induced anisotropy persists into the crust, and magma injection localizes and accommodates strain just prior to continental break-up. Copyright 2005 by the American Geophysical Union.