SUMMARY The Earth's mantle transition zone (MTZ) plays a key role in the thermal and compositional interactions between the upper and lower mantle. Seismic anisotropy provides useful information about mantle deformation and dynamics across the MTZ. However, seismic anisotropy in the MTZ is difficult to constrain from surface wave or shear wave splitting measurements. Here, we investigate the sensitivity to anisotropy of a body wave method, SS precursors, through 3-D synthetic modelling and apply it to real data. Our study shows that the SS precursors can distinguish the anisotropy originating from three depths: shallow upper mantle (80–220 km), deep upper mantle above 410 km, and MTZ (410–660 km). Synthetic resolution tests indicate that SS precursors can resolve $$\ge $$3 per cent azimuthal anisotropy where data have an average signal-to-noise ratio (SNR = 7) and sufficient azimuthal coverage. To investigate regional sensitivity, we apply the stacking and inversion methods to two densely sampled areas: the Japan subduction zone and a central Pacific region around the Hawaiian hotspot. We find evidence for significant VS anisotropy (15.3 ± 9.2 per cent) with a trench-perpendicular fast direction (93° ± 5°) in the MTZ near the Japan subduction zone. We attribute the azimuthal anisotropy to the grain-scale shape-preferred orientation of basaltic materials induced by the shear deformation within the subducting slab beneath NE China. In the central Pacific study region, there is a non-detection of MTZ anisotropy, although modelling suggests the data coverage should allow us to resolve at least 3 per cent anisotropy. Therefore, the Hawaiian mantle plume has not produced detectable azimuthal anisotropy in the MTZ.
more »
« less
Constraining the 410-km discontinuity and slab structure in the Kuril subduction zone with triplication waveforms
SUMMARY The detailed structure near the 410-km discontinuity provides key constraints of the dynamic interactions between the upper mantle and the lower mantle through the mantle transition zone (MTZ) via mass and heat exchange. Meanwhile, the temperature of the subducting slab, which can be derived from its fast wave speed perturbation, is critical for understanding the mantle dynamics in subduction zones where the slab enters the MTZ. Multipathing, i.e. triplicated, body waves that bottom near the MTZ carry rich information of the 410-km discontinuity structure and can be used to constrain the discontinuity depth and radial variations of wave speeds across it. In this study, we systematically analysed the trade-off between model parameters in triplication studies using synthetic examples. Specifically, we illustrated the necessity of using array-normalized amplitude. Two 1-D depth profiles of the wave speed below the Tatar Strait of Russia in the Kuril subduction zone are obtained. We have observed triplications due to both the 410-km discontinuity and the slab upper surface. And, seismic structures for these two interfaces are simultaneously inverted. Our derived 410-km discontinuity depths for the northern and southern regions are at 420$$\pm $$15 and 425$$\pm $$15 km, respectively, with no observable uplift. The slab upper surface is inverted to be located about 50–70 km below the 410-km discontinuity. This location is between the depths of the 1 and 2 per cent P-wave speed perturbation contours of a regional 3-D full-waveform inversion (FWI) model, but we found twice the wave speed perturbation amplitude. A wave speed increase of 3.9–4.6 per cent within the slab, compared to 2.0–2.4 per cent from the 3-D FWI model, is necessary to fit the waveforms with the shortest period of 2 s, indicating that high-frequency waves are required to accurately resolve the detailed structures near the MTZ.
more »
« less
- Award ID(s):
- 1802247
- PAR ID:
- 10324350
- Date Published:
- Journal Name:
- Geophysical Journal International
- Volume:
- 228
- Issue:
- 2
- ISSN:
- 0956-540X
- Page Range / eLocation ID:
- 729 to 743
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract The upper boundary of the mantle transition zone, known as the “410-km discontinuity”, is attributed to the phase transformation of the mineral olivine (α) to wadsleyite (β olivine). Here we present observations of triplicated P-waves from dense seismic arrays that constrain the structure of the subducting Pacific slab near the 410-km discontinuity beneath the northern Sea of Japan. Our analysis of P-wave travel times and waveforms at periods as short as 2 s indicates the presence of an ultra-low-velocity layer within the cold slab, with a P-wave velocity that is at least ≈20% lower than in the ambient mantle and an apparent thickness of ≈20 km along the wave path. This ultra-low-velocity layer could contain unstable material (e.g., poirierite) with reduced grain size where diffusionless transformations are favored.more » « less
-
Abstract Alaska is a tectonically active region with a long history of subduction and terrane accretion, but knowledge of its deep seismic structure is limited by a relatively sparse station distribution. By combining data from the EarthScope Transportable Array and other regional seismic networks, we obtain a high‐resolution state‐wide map of the Moho and upper‐mantle discontinuities beneath Alaska using teleseismic SH‐wave reverberations. Crustal thickness is generally correlated with elevation and the deepest Moho is in the region with basal accretion of the subducted Yakutat plate, consistent with its higher density due to a more mafic composition. The crustal thickness in the Brooks Range agrees with the prediction based on Airy isostasy and the weak free‐air gravity anomaly, suggesting that this region probably does not have significant density anomalies. We also resolve the 410, 520, and 660 discontinuities in most regions, with a thickened mantle transition zone (MTZ) and a normal depth difference between the 520 and 660 discontinuities (d660‐d520) under central Alaska, indicating the presence of the subducted Pacific slab in the upper MTZ. A near‐normal MTZ and a significantly smaller d660‐d520 are resolved under southeastern Alaska, suggesting potential mantle upwelling in the lower MTZ. Beneath the Alaska Peninsula, the thinned MTZ implies that the Pacific slab may not have reached the MTZ in this region, which is also consistent with recent tomography models. Overall, the results demonstrate a bent or segmented Pacific slab with varying depths under central Alaska and the Alaska Peninsula.more » « less
-
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 (hereafter 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 United States than in the central-eastern United States. The 410-km and 660-km discontinuities are about 40–50 km deeper beneath the western United States than the central-eastern United States 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, modelling of layering in the upper mantle must be based on 3-D reference structures and accurate calculations of reverberation traveltimes.more » « less
-
Approximately two-thirds of Earth’s outermost shell is composed of oceanic plates that form at spreading ridges and recycle back to Earth’s interior in subduction zones. A series of physical and chemical changes occur in the subducting lithospheric slab as the temperature and pressure increase with depth. In particular, olivine, the most abundant mineral in the upper mantle, progressively transforms to its high-pressure polymorphs near the mantle transition zone, which is bounded by the 410 km and 660 km discontinuities. However, whether olivine still exists in the core of slabs once they penetrate the 660 km discontinuity remains debated. Based on SKS and SKKS shear-wave differential splitting times, we report new evidence that reveals the presence of metastable olivine in the uppermost lower mantle within the ancient Farallon plate beneath the eastern United States. We estimate that the low-density olivine layer in the subducted Farallon slab may compensate the high density of the rest of the slab associated with the low temperature, leading to neutral buoyancy and preventing further sinking of the slab into the deeper part of the lower mantle.more » « less