Resolution of regional seismic models: Squeezing the Iceland anomaly

Abstract

SUMMARY We present a resolution study of the velocity structure beneath Iceland as constrained by teleseismic traveltime tomography using data from the HOTSPOT seismic network. This temporary PASSCAL network and the tomographic technique that was used to generate the ICEMAN velocity models are typical of regional seismic studies. Therefore, this study also provides a basis for understanding the resolution of other regional seismic experiments. A suite of tests is used to constrain the range of velocity models that satisfy the traveltime observations on Iceland. These include ray-theoretical squeezing experiments, which attempt to force velocity anomalies into specific geometries while still satisfying the data set, and finite-frequency experiments, which use the spectral-element method (SEM) to simulate full waveform propagation through various 3-D velocity models. The use of the SEM allows the verification of the ray-theoretical ICEMAN models without the assumption of ray theory. The tests show that the ICEMAN models represent an end-member of the range of velocity models that satisfy the data set. The 200-km-width Gaussian-shaped upwelling beneath Iceland, imaged in the ICEMAN models, is at the broadest end of the allowed model range; the peak −2 per cent compressional and −4 per cent shear wave perturbations are lower bounds on the amplitude of the velocity model. Such broadening and lowering of velocity anomalies is the product of data coverage, the ray-theory approximation and regularization of the inversion. Comparison of the traveltime delays produced by a 100-km-diameter conduit as measured at short (1 s) and long (∼20 s) periods demonstrate that such a conduit cannot satisfy the observed traveltime delays. Thus the width of the upwelling conduit beneath Iceland must lie in the range of 100 to 200 km. Separate tests on the minimum depth extent of the anomaly show that significant low velocities are required to 350 km depth. Should the true conduit be at the narrower end of the possible range, both compressional and shear wave perturbations greater than 10 per cent would be required to depths of at least 350 km. Mineral physics experiments indicate that such velocity anomalies would in turn require the presence of partial melt or some other fluids to these depths. These bounds on the allowed velocity structure beneath Iceland provide a constraint on geodynamic models for the generation of the Iceland hotspot, whether it is the result of a top-down or bottom-up process.