The thin hot plume beneath Iceland

We present the results of a seismological investigation of the frequency-de pendent amplitude variations across Iceland using data from the HOTSPOT arr ay currently deployed there. The array is composed of 30 broad-band PASSCAL instruments. We use the parameter t*, defined in the usual manner from spe ctral ratios (Halderman & Davis 1991), to compare observed S-wave amplitude variations with those predicted due to both anelastic attenuation and diff raction effects. Four teleseismic events at a range of azimuths are used to measure t*. A 2-D vertical cylindrical plume model with a Gaussian-shaped velocity anomaly is used to model the variations. That part of t* caused by attenuation was estimated by tracing a ray through IASP91, then superimpos ing our plume model velocity anomaly and calculating the path integral of 1 /vQ, That part of t* caused by diffraction was estimated using a 2-D finite difference code to generate synthetic seismograms. The same spectral ratio technique used for the data was then used to extract a predicted tk. The t * variations caused by anelastic attenuation are unable to account for the variations we observe, but those caused by diffraction do. We calculate the t* variations caused by diffraction for different plume models and obtain our best-fit plume, which exhibits good agreement between the observed and measured t*. The best-fit plume model has a maximum S-velocity anomaly of - 12 per cent and falls to 1/e of its maximum at 100 km from the plume centre . This is narrower than previous estimates from seismic tomography, which a re broadened and damped by the methods of tomography. This velocity model w ould suggest greater ray theoretical traveltime delays than observed. Howev er, we find that for such a plume, wave-front healing effects at frequencie s of 0.03-0.175 Hz (the frequency range used to pick S-wave arrivals) cause s a 40 per cent reduction in traveltime delay, reducing the ray theoretical delay to that observed.