CRUSTAL SHEAR-WAVE SPLITTING FROM LOCAL EARTHQUAKES IN THE HENGILL TRIPLE JUNCTION, SOUTHWEST ICELAND

The Hengill region in SW Iceland is an unstable ridge-ridge-transform triple junction between an active and a waning segment of the mid-Atla ntic spreading center and a transform that is transgressing southward. The triple junction contains active and extinct spreading segments an d a widespread geothermal area. We evaluated shear-wave birefringence for locally recorded upper-crustal earthquakes using an array of 30 th ree-component digital seismographs. Fast-polarization directions, phi, are mostly NE to NNE, subparallel to the spreading axis and probably caused by fissures and microcracks related to spreading. However, ther e is significant variability in phi throughout the array. The lag from fast to slow S is not proportional to earthquake depth (ray length), being scattered at all depths. The average wave-speed difference betwe en qS1 and qS2 in the upper 2-5 km of the crust is 2-5%. Our results s uggest considerable heterogeneity or strong S scattering.