Seismic Anisotropy in the Crust at the Mid-Atlantic Plate Boundary in South-West Iceland

Seismograms of microearthquakes recorded on stations of the SIL network in south-western Iceland exhibit strong shear-wave splitting, and are consistent with being caused by aligned parallel cracks in the uppermost crust. Splitting times of 0.1-0.3 s are observed, with the larger values (0.2-0.3 s) occurring beneath stations in the highly lineated Western Volcanic Zone, and the smaller values (0.1 s) in the younger, less fractured South Iceland Seismic Zone. Five of the stations have a fast shear-wave polarization azimuth of N30 degrees E-N40 degrees E, parallel to the axis of the Western Volcanic Zone (i.e. the Mid-Atlantic Ridge in Iceland) and in the direction of maximum horizontal compression defined by fault, dyke and fissure strikes and microearthquake focal mechanisms. Station GYG, in the north-western part of the network, has the significantly different azimuth of N70 degrees E. This is parallel to one of the local strike-slip fault trends, and is probably due to a lineated rock fabric caused by those faults.