Stagnant forearc mantle wedge inferred from mapping of shear-wave anisotropy using S-net seafloor seismometers

Abstract

Shear-wave anisotropy in Earth’s mantle helps constrain the lattice-preferred orientation of anisotropic minerals due to viscous flow. Previous studies at the Japan Trench subduction zone using land-based seismic networks identified strong anisotropy in the mantle wedge, reflecting viscous flow induced by the subducting slab. Here we map anisotropy in the previously uninvestigated offshore region by analyzing shear waves from interplate earthquakes that are recorded by a new seafloor network (the S-net). The newly detected anisotropy is not in the mantle wedge but only in the overlying crust (∼0.1 s time delay and trench-parallel fast direction). The distinct lack of anisotropy indicates that the forearc mantle wedge offshore is decoupled from the slab and does not participate in the viscous flow, in sharp contrast with the rest of the mantle wedge. A stagnant forearc mantle wedge provides a stable and cold tectonic environment that is important for the petrological evolution and earthquake processes of subduction zones.

Mantle Flow Patterns Beneath the Junction of Multiple Subduction Systems Between the Pacific and Tethys Domains, SE Asia: Constraints From SKS‐Wave Splitting Measurements

“…Note that TNTI is located in the forearc, in which the structures above a down‐going slab primarily include the mantle wedge “nose” and the overriding crust (Figure 7a). The former has been growingly recognized to be isotropic due to its stagnant status (Uchida et al., 2020), whereas the latter's contribution to the anisotropy is also insignificant because the global average splitting time from the crust has been suggested to be on the order of 0.1 s (P. G. Silver, 1996). To make a conspicuous contribution to the observed anisotropy, another model is thus required.…”

Mantle Wedge Seismic Anisotropy and Shear Wave Splitting: Effects of Oblique Subduction

“…The overall pattern of fast direction and delay time in the arc region is remarkably similar to the ones observed in the two‐layer models (Text S2 in Supporting Information S1) with 30–60° a axis azimuthal offset, and thus two‐layer models with horizontal anisotropy may provide a reasonable approximation to the subarc mantle. For regions with sufficient observations, potentially with a range of initial polarizations, such as in NE Japan (e.g., Uchida et al., 2020), the characteristics of the two layers may be constrained based on the SWS parameter distribution; a similar approach has been applied for resolving mantle anisotropy in other tectonic settings (for example, beneath the central Appalachians by Aragon et al. (2017)).…”

Improving the Constraint on theMw7.1 2016 Off‐Fukushima Shallow Normal‐Faulting Earthquake With the High Azimuthal Coverage Tsunami Data From the S‐Net Wide and Dense Network: Implication for the Stress Regime in the Tohoku Overriding Plate

“…In response to the Tohoku earthquake, a new wide offshore deep‐ocean observation network, Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench (S‐net), has been constructed off eastern Japan (Aoi et al., 2020; Kanazawa et al., 2016; Mochizuki et al., 2016; Uehira et al., 2016, Figure 1a). Recent studies have started to utilize S‐net ocean‐bottom seismometers to investigate the seismotectonics and geodynamics in the Tohoku subduction zone (Dhakal et al., 2021; Hua et al., 2020; Matsubara et al., 2019; Nishikawa et al., 2019; Sawazaki & Nakamura, 2020; Takagi et al., 2019, 2020; Tanaka et al., 2019; Uchida et al., 2020; Yu & Zhao, 2020). The S‐net also incorporates ocean‐bottom pressure gauges (OBPGs), which are expected to be utilized for tsunami forecasts (e.g., Aoi et al., 2019; Inoue et al., 2019; Mulia & Satake, 2021; Tanioka, 2020; Tsushima & Yamamoto, 2020; Wang & Satake, 2021; Yamamoto, Aoi, et al., 2016; Yamamoto, Hirata, et al., 2016).…”