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°E‐N40°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°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.