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

摘要

The Hengill region in SW Iceland is an unstable ridge-ridge-transform triple junction between an active and a waning segment of the mid-Atlantic spreading center and a transform that is transgressing southward. The triple junction contains active and extinct spreading segments and a widespread geothermal area. We evaluated shear-wave birefringence for locally recorded upper-crustal earthquakes using an array of 30 three-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, there 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 between qS1 and qS2 in the upper 2-5 km of the crust is 2-5%. Our results suggest considerable heterogeneity or strong S scattering. [References: 12]