Seismic Anisotropy and Mantle Flow in the Sumatra Subduction Zone Constrained by Shear Wave Splitting and Receiver Function Analyses

Abstract

To systematically investigate seismic azimuthal anisotropy in the Sumatra subduction zone and probe mantle dynamics associated with the subduction of the Australian Plate beneath the Sunda Plate, a total of 169 pairs of teleseismic XKS (including PKS, SKKS, SKS) and 115 pairs of local S splitting parameters are obtained using broadband seismic data recorded at ~70 stations. Additionally, crustal anisotropy in the overriding Sunda Plate is measured by analyzing the moveout of P‐to‐S conversions from the Moho using a sinusoidal function. Comparison between the three sets of anisotropy measurements obtained using shear waves with different depths of origin suggests that (1) the crust of the Sunda Plate is anisotropic with mostly trench‐parallel fast orientations and a mean splitting time of 0.28 ± 0.05 s; (2) the mantle wedge is azimuthally anisotropic with dominantly trench‐parallel fast orientations and splitting times ranging from 0.22 to 0.81 s, which generally increase with the focal depth; and (3) subslab anisotropy is mostly trench‐normal beneath the fore‐arc region with an averaged splitting time of 1.48 ± 0.06 s, and becomes trench‐parallel beneath the arc and back‐arc areas with a mean splitting time of 0.33 ± 0.04 s. The resulting lateral and vertical distributions of anisotropy obtained using splitting of three types of shear waves advocate the presence of an entrained subslab flow that is deflected by the mantle transition zone. The flow enters the mantle wedge through a slab window and flows horizontally parallel to the trench.

Slab Dehydration and Mantle Upwelling in the Vicinity of the Sumatra Subduction Zone: Evidence from Receiver Function Imaging of Mantle Transition Zone Discontinuities

“…Advective upwelling of hot material through the slab window can lead to reduced velocities in the upper mantle, and at the same time depress the d410 due to the higher temperature relative to the ambient mantle in the upper-most MTZ. The slab window in the study area, whose presence is also suggested by a recent seismic anisotropy study based on shear wave splitting analysis (Kong et al, 2020), is probably associated with the IFZ and linked with the feeder system of the Toba supervolcano (Koulakov et al, 2016;Liu et al, 2018).…”

The Geometry of the Subducted Slab Beneath Sumatra Revealed by Regional and Teleseismic Traveltime Tomography

“…(2016) inferred a tear in the slab beneath the Toba caldera to interpret the abovementioned low‐velocity anomalies and high VP/VS ratios. A recent study of seismic anisotropy also indicated the presence of a slab tear beneath Toba caldera (Kong et al., 2020). Their shear wave splitting measurement reflect both lateral and vertical distributions of anisotropy beneath the Toba caldera and surrounding areas (Kong et al., 2020).…”

“…A recent study of seismic anisotropy also indicated the presence of a slab tear beneath Toba caldera (Kong et al., 2020). Their shear wave splitting measurement reflect both lateral and vertical distributions of anisotropy beneath the Toba caldera and surrounding areas (Kong et al., 2020). These anisotropy patterns are attributed to mantle flow that enters the mantle wedge through the slab window resulting from the slab tear.…”

“…By combining the bending of the subducted slab in the upper mantle revealed by a previous regional tomography model (including Sumatra and Java) (Luehr et al, 2013) with their tomographic results, Koulakov et al (2016) inferred a tear in the slab beneath the Toba caldera to interpret the abovementioned low-velocity anomalies and high VP/VS ratios. A recent study of seismic anisotropy also indicated the presence of a slab tear beneath Toba caldera (Kong et al, 2020). Their shear wave splitting measurement reflect both lateral and vertical distributions of anisotropy beneath the Toba caldera and surrounding areas (Kong et al, 2020).…”

Seismic Anisotropy and Mantle Deformation Beneath the Central Sunda Plate

“…Several seismic anisotropy investigations have been conducted in the central Sunda Plate and surrounding areas to explore the upper mantle seismic anisotropy and associated mantle deformation (Figure S1; Collings et al., 2013; Di Leo et al., 2012; Hammond et al., 2010; Kong, Gao, Liu, Zhang, et al., 2020; Wang & He, 2020; Xue et al., 2013; Y. Yu et al., 2018). While some SWS studies using local S and XKS (including SKS, SKKS, and PKS) waves suggested that the mantle wedge of the Sumatra subduction zone is generally isotropic (Hammond et al., 2010), trench‐orthogonal azimuthal anisotropy has been observed and attributed to a corner flow system in the mantle wedge using data recorded by two temporary seismic networks (Collings et al., 2013).…”