An investigation of seismic anisotropy due to cracks in the upper oceanic crust at 45 N, Mid-Atlantic Ridge

Abstract

Summary A detailed seismic refraction survey using three 3-component ocean bottom seismographs over young (1.1-3.4 Ma old) Mid-Atlantic Ridge at 45°N recorded compressional wave velocity anisotropy in the upper crust of about ±5 per cent and associated particle motion deviations of up to ±8°. The magnitudes and azimuths of these anisotropic variations suggest that they are generated by sets of subparallel near-vertical water-filled cracks in layer 2. The directions of the cracks cannot be determined uniquely but they lie between the seafloor spreading and the ridge axis directions, in the range ±(30–60)° with respect to the ridge axis. We explain this unexpected crack orientation by the occurrence of cracking and fracturing induced in the upper crust as it moves away from the spreading axis. The principal horizontal stresses responsible for this fracturing are parallel and perpendicular to the ridge axis. The apparently negligible seismic influence exerted by ridge-parallel sets of (shallow?) cracks observed at the ridge axis is explained by their infilling by sediment and hydrothermal precipitates and by their closure under ridge-normal compression. We find a normal oceanic crustal velocity structure that can be explained by decreasing numbers of open cracks with increasing depth through the upper 2 km of the crust. Lateral variability across the 25 km square survey is limited in magnitude (typically ±0.05 s variations in delay times), and the observed travel times are consistent with a model of the crust in which the isovelocity surfaces at depth are a subdued copy of the basement topography.