Reinterpretation of the RRISP-77 Iceland Shear Wave Profiles

W. Menke, B. Brandsdottir, P. Einarsson and I. Bjarnason

Geophys. J. Int., 1996

Abstract

Two shear wave profiles, E and G, collected during the 1977 Reykjanes Ridge Iceland Seismic Experiment have played an important role in models of the Icelandic crust. They were originally interpreted as indicating very low shear wave velocities and abnormally low shear wave quality factors in the 10-15 km depth range. These attributes, which are indicative of near-solidus temperatures, were used to support the hypothesis that the crust of Iceland is relatively thin (10-15 km) and underlain by partially molten material. More recent seismic data, however, contradict this hypothesis and suggest that the crust is thicker (20-30 km), and cooler. A reexamination of the RRISP-77 data indicates that the low shear wave velocities are artifacts arising from source static anomalies (in the case of Profile G) and misidentification of a secondary shear phase, SmS, as S (in the case of the E Profile). Furthermore, the attenuation occurs at ranges when rays from the shots pass near the Askja (profile E), and Katla and Oraefajokull (Profile G) volcanoes. It may therefore have a localized source, and not be diagnostic of Icelandic crust as a whole. This new interpretation of the RRISP-77 shear wave data is consistent with models having a thick, cold crust.

(Top) RRISP shear wave profile, showing interpretation of shear arrivals as S and SmS waves. (Bottom) Synthetic seismograms computed for 30 km thick crust. near Katla volcano recorded on the USGS/U. Durham array at Hengill.

Text

Text (PostScript version) (PDF version).

Figures

Fig. 1 Map of Iceland showing locations of datasets used in paper. (PostScript version).
Fig. 2 RRISP shear wave profiles. (PostScript version).
Fig. 3 SIST shear wave profiles showing P wave traveltimes scaled by factor of 1.79. (PostScript version).
Fig. 4 RRISP P wave profile from shot G, showing traveltimes of nearby Iceland Array stations. (PostScript version).
Fig. 5 >Record sections showing P, PmP, S and SmS waves from an earthquake near Katla volcano recorded on the USGS/U. Durham array at Hengill. (PostScript version).
Fig. 6 Record sections showing Pn, P and S waves from an earthquake near Bardarbunga volcano recorded on the USGS/U. Durham array at Hengill.(PostScript version).
Fig. 7 Traveltime curve for 30 km thick crust. (PostScript version).
Fig. 8 RRISP shear wave profile compared to syntehtic seismogram. (PostScript version).
Fig. 9 SmS/S wave sprectral ratios. (PostScript version).