Wave diffraction and resolution of mantle transition zone discontinuities in receiver function imaging

The structure of seismic discontinuities in the mantle transition zone at depths of about 400 and 670 km provides important constraints on mantle convection as the associated mineral phase transformations are sensitive to thermal perturbations. Teleseismic P-to-S receiver functions have been widely used to map the depths of the two discontinuities. In this study, we investigate the resolution of receiver functions in imaging topographic variations of the 400-km and 670-km discontinuities based on wave propagation simulations using a Spectral Element Method (SEM). We investigate wave diffraction effects on direct P waves as well as P-to-S converted waves by varying the length scale of topography of the two discontinuities. We observe strong wave diffractional effects in both P waves and teleseismic receiver functions at periods of similar to 10 to 20 s. Ray theory overpredicts traveltime anomalies by a factor of 2-5 when the topography length scale is about 400 km. In addition, ray-theoretical predictions are out of phase with measurements which indicates that locations of small-scale topographic variations can not be resolved using ray theory. The observed traveltime anomalies further reduce to 10-20 per cent of ray-theoretical predictions when the topography length scale reduces to about 200 km. We calculate 2-D boundary sensitivity kernels for direct P waves as well as receiver functions. In general, calculations based Born sensitivity kernels fit the 'ground-truth' SEM measurements very well. They account for wave diffraction effects as well as phase interactions such as P and pP waves arriving in P-wave coda. 3-D wavespeed structure in the upper mantle beneath seismic stations may introduce significant traveltime anomalies on P waves and transition zone receiver functions. We show that traveltime corrections at periods of about 10 to 20 s are frequency dependent when the size of the anomalies becomes less than 500 km.