P- and S-wave delays caused by thermal plumes

Many studies have sought to seismically image plumes rising from the deep mantle in order tosettle the debate about their presence and role in mantle dynamics, yet the predicted seismicsignature of realistic plumes remains poorly understood. By combining numerical simulationsof flow, mineral-physics constraints on the relationships between thermal anomalies and wavespeeds, and spectral-element method based computations of seismograms, we estimate thedelay times of teleseismic S and P waves caused by thermal plumes. Wave front healing isincomplete for seismic periods ranging from 10 s (relevant in traveltime tomography) to 40 s(relevant in waveform tomography). We estimate P-wave delays to be immeasurably small(<0.3 s). S-wave delays are larger than 0.4 s even for S waves crossing the conduits of thethinnest thermal plumes in our geodynamic models. At longer periods (>20 s), measurementsof instantaneous phase misfit may be more useful in resolving narrowplume conduits. To detectS-wave delays of 0.4-0.8 s and the diagnostic frequency dependence imparted by plumes, itis key to minimize the influence of the heterogeneous crust and upper mantle. We argue thatseismic imaging of plumes will advance significantly if data from wide-aperture ocean-bottomnetworks were available since, compared to continents, the oceanic crust and upper mantle arerelatively simple.