Jr. Evans et al., CRUSTAL SHEAR-WAVE SPLITTING FROM LOCAL EARTHQUAKES IN THE HENGILL TRIPLE JUNCTION, SOUTHWEST ICELAND, Geophysical research letters, 23(5), 1996, pp. 455-458
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.