Traveltime dispersion in an isotropic elastic mantle: Strong lower mantle signal in differential-frequency residuals
Résumé
We study wavefield effects of direct P- and S -waves in elastic and isotropic 3-D seismic
structures derived from the temperature field of a high-resolution mantle circulation model.
More specifically, we quantify the dispersion of traveltime residuals caused by diffraction in
structures with dynamically constrained length scales and magnitudes of the lateral variations
in seismic velocities and density. 3-D global wave propagation is simulated using a spectral
element method. Intrinsic attenuation (i.e. dissipation of seismic energy) is deliberately
neglected, so that any variation of traveltimes with frequency can be attributed to structural
effects. Traveltime residuals are measured at 15, 22.5, 34 and 51 s dominant periods by cross-
correlation of 3-D and 1-D synthetic waveforms. Additional simulations are performed for a
model in which 3-D structure is removed in the upper 800 km to isolate the dispersion signal of
the lower mantle. We find that the structural length scales inherent to a vigorously convecting
mantle give rise to significant diffraction-induced body-wave traveltime dispersion. For both P- and S-waves,
the difference between long-period and short-period residuals for a given
source–receiver pair can reach up to several seconds for the period bands considered here. In
general, these ‘differential-frequency’ residuals tend to increase in magnitude with increasing
short-period delay. Furthermore, the long-period signal typically is smaller in magnitude than
the short-period one; that is, wave-front healing is efficient independent of the sign of the
residuals. Unlike the single-frequency residuals, the differential-frequency residuals are sur-
prisingly similar between the ‘lower-mantle’ and the ‘whole-mantle’ model for corresponding
source–receiver pairs. The similarity is more pronounced in case of S -waves and varies between
different combinations of period bands. The traveltime delay acquired in the upper mantle
seems to cancel in these differential signals depending on the associated wavelengths and
the length scales of structure at shallow depth. Differential-frequency residuals may thus prove
useful to precondition tomographic inversions for the lower-mantle structure such as to reduce
the influence of the upper mantle for certain length scales. Overall, standard deviations of the
diffraction-induced traveltime dispersion between the longest (51 s) and the shortest (15 s)
period considered here are 0.6 and 1.0 s for P - and S -waves, respectively. For comparison, the
corresponding standard deviations of the 15 s residuals are 1.0 s and 2.8 s. In the lower-mantle
model, standard deviations are 0.3 and 0.6 s, respectively, which gives an average lower-mantle
contribution to the total dispersion of 50 per cent for P -waves and 60 per cent for S -waves.
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