REINTERPRETATION OF THE RRISP-77 ICELAND SHEAR-WAVE PROFILES

Two shear-wave profiles, E and G, collected during the 1977 Reykjanes Ridge Iceland Seismic Experiment have played an important role in mode ls of the Icelandic crust. They were originally interpreted as indicat ing very low shear-wave velocities and abnormally low shear-wave quali ty factors in the 10-15 km depth range. These attributes, which are in dicative of near-solidus temperatures, were used to support the hypoth esis that the crust of Iceland is relatively thin (10-15 km) and under lain by partially molten material. More recent seismic data, however, contradict this hypothesis and suggest that the crust is thicker (20-3 0 km) and cooler. A re-examination of the RRISP-77 data indicates that the low shear-wave velocities are artefacts arising from source stati c anomalies (in the case of profile G) and misidentification of a seco ndary shear phase, SmS, as S (in the case of profile E). Furthermore, the attenuation occurs at ranges when rays from the shots pass near th e Askja (profile E) and Katla and Oraefajokull (profile G) volcanoes. It may therefore have a localized source, and not be diagnostic of Ice landic crust as a whole. This new interpretation of the RRISP-77 shear -wave data is consistent with models having a thick, cold crust.