Influence of shear wave velocity heterogeneity on SH-wave reverberation imaging of the mantle transition zone

Abstract

Summary Long-period (T > 10 s) shear-wave reflections between the surface and reflecting boundaries below seismic stations are useful for studying phase transitions in the mantle transition zone (MTZ) but shear-velocity heterogeneity and finite-frequency effects complicate the interpretation of waveform stacks. We follow up on a recent study by Shearer & Buehler (2019) (SB19) of the top-side shear-wave reflection Ssds as a probe for mapping the depths of the 410-km and 660-km discontinuities beneath the USArray. Like SB19, we observe that the recorded Ss410s-S and Ss660s-S traveltime differences are longer at stations in the western US than in the central-eastern US. The 410-km and 660-km discontinuities are about 40–50 km deeper beneath the western US than the central-eastern US if Ss410s-S and Ss660s-S traveltime differences are transformed to depth using a common-reflection point (CRP) mapping approach based on a 1-D seismic model (PREM in our case). However, the east-to-west deepening of the MTZ disappears in the CRP image if we account for 3-D shear-wave velocity variations in the mantle according to global tomography. In addition, from spectral-element method synthetics, we find that ray theory overpredicts the traveltime delays of the reverberations. Undulations of the 410-km and 660-km discontinuities are underestimated when their wavelengths are smaller than the Fresnel zones of the wave reverberations in the MTZ. Therefore, modeling of layering in the upper mantle must be based on 3-D reference structures and accurate calculations of reverberation traveltimes.

The Upper‐Mantle Structure Beneath Alaska Imaged by Teleseismic S‐Wave Reverberations

“…The absolute depths of MTZ discontinuities resolved with our method depend strongly on the chosen 3D upper shear‐velocity structures (M. Liu et al., 2022). The MS18 and J18 models used in this study show a high‐velocity NE‐SW‐striking slab from central Alaska to the Alaska Peninsula, which extends from the lithosphere to at least 300‐km depth and is surrounded by low‐velocity anomalies.…”

“…Based on the Fresnel‐zone sizes at MTZ depths given by Shearer and Buehler (2019) and the resolution tests performed by M. Liu et al. (2022), we smooth the results using a radially symmetric cos 2 taper with a 3° radius and treat only structures with scales greater than 3° as reliably resolved.…”

“…We trace the topside reflection rays using the 1D iasp91 model (Kennett & Engdahl, 1991) to determine their reflection points for assumed horizontal discontinuities at depths from 350 to 800 km in 5‐km increments for the 10‐s data set and from 5 to 71 km in 2‐km increments for the 5‐s data set. Since the time‐to‐depth conversion is affected by 3D shear‐velocity structure (M. Liu et al., 2022), we correct the Ssds ‐ S differential times using 3D velocity models. At each reflected depth, we assume the 1D ray path, compute the average velocity deviation with respect to the iasp91 model along the two‐way reverberation path of Ssds on the receiver side, and then calculate the deviation of differential time.…”

“…We note that this synthetic test is only a measure of our ability to separate source-and receiver-side structure, not the true resolution of our imaging approach. Based on the Fresnel-zone sizes at MTZ depths given by Shearer and Buehler (2019) and the resolution tests performed by M. Liu et al (2022), we smooth the results using a radially symmetric cos 2 taper with a 3° radius and treat only structures with scales greater than 3° as reliably resolved.…”

“…Van Stiphout et al (2019) applied appropriate velocity corrections and found little topography on the 660-km discontinuity (hereinafter "660"), suggesting that the slab probably has not reached the lower mantle. The discrepancy in "410" and "660" topography between different PRF studies could be partly due to the 3D velocity corrections, which significantly affect the discontinuity depths (M. Liu et al, 2022) yet have considerable uncertainty given the aforementioned disagreement between different tomography models.…”