On the resolution of plumes by seismic tomography

Seismic tomography indicates that the shear velocity in the upper mantle beneath hotspots such as Afar, Hawaii, and Iceland is relatively low. However, the continuation of low shear-velocity anomalies into the lower mantle, as expected for deep mantle plumes, is in most cases not evident. We speculate that, due to wave front healing, seismic waves traversing through narrow low-velocity structures in the lower mantle are recorded without a measurable delay. To investigate the significance of wave front healing on traveltimes, we compute the delay of S waves that propagated through hypothetical plume structures. We use thermochemical plume models similar to Lin and Van Keken (2005) but updated with compressible convection with consistent equation of state. The velocity structure of the plume is conversed from temperature using Goes et al. (2002). Since our models have an axial symmetry, we use the SHaxi method (Jahnke et al., 2008) to compute 3D waveforms for periods as short as 2.5 s. By waveform cross-correlation we determine the traveltime delay of teleseismic shear waves as a function of seismic period and epicentral distance for a range of realistic plume models. Using these computations we discuss the resolution of plumes via seismic tomography.