Wavefront healing renders deep plumes seismically invisible

Abstract

S U M M A R YSince W. J. Morgan proposed that intraplate volcanism at some Pacific hotspots is caused by hot plumes rising from the lower mantle, geophysicists have been actively pursuing physical evidence for mantle plumes. Several seismic studies have mapped low-velocity anomalies below a number of hotspots. However, the association of low-velocity structures with plume tails has remained controversial given the debate on whether lower-mantle plumes impart observable traveltime or amplitude perturbations on seismic waves. Using high-resolution numerical simulations of plume ascent through the mantle and their effects on waveforms, we demonstrate that the delay of shear waves by plume tails at depths larger than 1000 km are immeasurably small (<0.2 s) at seismic periods commonly used in waveform analysis. Therefore, we conclude that narrow lower mantle plumes are not detectable.

Multiscale Seismic Tomography

“…(4) In some cases, the surface location of a hotspot may not be correct (e.g., Louisville in Fig. (7) Narrow lower-mantle plumes may not be detected due to the wavefront healing effect (Hwang et al 2011). It is also possible that some of the "hotspots" adopted here are simply not a real hotspot.…”

Evaluating the Resolution of Deep Mantle Plumes in Teleseismic Traveltime Tomography

“…Plumes may be responsible for the emplacement of large igneous provinces, continental breakup, and midplate volcanism (see Ballmer, van Keken, & Ito, 2015, for a review). Second, the delays of seismic waves after propagating through narrow plume conduits in the lower mantle may be immeasurably small (e.g., Hwang et al, 2011;Maguire et al, 2016;Rickers et al, 2012). Seismic tomography in particular is a powerful technique to illuminate plumes in the deep mantle and their interactions with large-scale flow and physical boundaries in the mantle transition zone.…”

Theory and Observations - Seismic Tomography and Inverse Methods

“…Wave front healing is related to the finite-frequency nature of a seismic wave and is ignored in ray theory. Wave front healing can be significant for small (compared to the wavelength) and deep anomalies (Hwang et al, 2011;Malcolm and Trampert, 2011). Going back to our example from Iceland, it is possible that a narrow (e.g., plume-stem) seismic anomaly is present below a depth of 700 km, which has imparted a travel time delay to a throughgoing wave that is still not observable at the surface.…”