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Crustal shear-wave splitting from local earthquakes in the Hengill triple junction, southwest Iceland

J.R. Evans·G.R. Foulger·B.R. Julian·A.D. Miller
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Abstract

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, ϕ, 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 ϕ 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.

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J.R. Evans, G.R. Foulger, B.R. Julian, A.D. Miller. 1996. Crustal shear-wave splitting from local earthquakes in the Hengill triple junction, southwest Iceland. https://doi.org/10.1029/96gl00261

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