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1. Seismic constraints on the hydrous state of the Tonga slab

   Wang, Fan; Wei, S. S. (January 2019, 2019 AGU Fall Meeting)
2. Stress Drops of Intermediate‐Depth and Deep Earthquakes in the Tonga Slab

   https://doi.org/10.1029/2022JB025109  

   Tian, Dongdong; Wei, S. Shawn; Wang, Wei; Wang, Fan (October 2022, Journal of Geophysical Research: Solid Earth)

   Abstract Multiple physical mechanisms have been proposed to explain the cause of intermediate‐depth and deep earthquakes, but they are still under debate. Source parameters such as stress drop, have the potential to provide insight into their physical mechanisms. We develop a modified spectral decomposition method to analyze 1‐year seismic data from temporary land‐based and ocean bottom seismographs in a complex subduction zone. By applying this method to investigate 1,083 intermediate‐depth and deep earthquakes in the Tonga slab, we successfully resolve the source spectra and stress drops of 743M_W2.6–6.0 earthquakes. Although the absolute stress drops are subject to the choices of source model parameters, the relative stress drops are more reliably resolved. The median stress drop of Tonga earthquakes does not change with respect to magnitude but decreases with depth by 2–3 times in two separate depth ranges of 70–250 and 400–600 km, corresponding to intermediate‐depth and deep earthquakes, respectively. The median stress drops show spatial variations, with two high‐stress‐drop (five times higher than the ambient value) regions, coinciding with strong local deformation where the Tonga slab bends or tears. In the Tonga double seismic zone at 120–300 km depths, the median stress drop appears smaller in the lower plane than in the upper plane, suggesting a slower rupture velocity or a higher fluid content in the lower‐plane region. Our results suggest that intermediate‐depth and deep earthquakes in the Tonga slab generally follow the earthquake self‐similar model and favor the fluid‐related embrittlement hypothesis for both groups of earthquakes. 

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3. Toroidal flow around the Tonga slab moved the Samoan plume during the Pliocene

   https://doi.org/10.1130/G51588.1  

   Konrad, Kevin; Jackson, Matthew; Steinberger, Bernhard; Koppers, Anthony; Balbas, Andrea; Finlayson, Valerie; Konter, Jasper; Price, Allison (January 2024, Geology)

   Abstract Age-progressive seamount tracks generated by lithospheric motion over a stationary mantle plume have long been used to reconstruct absolute plate motion (APM) models. However, the basis of these models requires the plumes to move significantly slower than the overriding lithosphere. When a plume interacts with a convergent or divergent plate boundary, it is often deflected within the strong local mantle flow fields associated with such regimes. Here, we examined the age progression and geometry of the Samoa hotspot track, focusing on lava flow samples dredged from the deep flanks of seamounts in order to best reconstruct when a given seamount was overlying the mantle plume (i.e., during the shield-building stage). The Samoan seamounts display an apparent local plate velocity of 7.8 cm/yr from 0 to 9 Ma, 11.1 cm/yr from 9 to 14 Ma, and 5.6 cm/yr from 14 to 24 Ma. Current fixed and mobile hotspot Pacific APM models cannot reproduce the geometry of the Samoa seamount track if a long-term fixed hotspot location, currently beneath the active Vailulu’u Seamount, is assumed. Rather, reconstruction of the eruptive locations of the Samoan seamounts using APM models indicates that the surface expression of the plume migrated ~2° northward in the Pliocene. Large-scale mantle flow beneath the Pacific Ocean Basin cannot explain this plume migration. Instead, the best explanation is that toroidal flow fields—generated by westward migration of the Tonga Trench and associated slab rollback—have deflected the conduit northward over the past 2–3 m.y. These observations provide novel constraints on the ways in which plume-trench interactions can alter hotspot track geometries. 

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4. Seismic Evidence for Plume‐Slab Interaction by High‐Resolution Imaging of the 410‐km Discontinuity Under Tonga

   https://doi.org/10.1029/2019GL084164  

   Li, Luchen; Chen, Yi‐Wei; Zheng, Yingcai; Hu, Hao; Wu, Jonny (December 2019, Geophysical Research Letters)

   Abstract The Tonga‐Samoa system provides a unique tectonic context to study how a cold subducting slab interacts with a hot rising mantle plume. Here we present a 3‐D high‐resolution image of the 410‐km mantle discontinuity (the410) using seismic signals excited by deep‐focus earthquakes. The410is found to be ~30 km shallower inside the Tonga slab relative to the ambient mantle and ~20 km deeper further to the northwest under Fiji Islands. The downward deflection of the410under Fiji supports the hypothesis of a plume migration around the northern edge of the Tonga slab from Samoan hot spot to under Fiji due to fast trench rollback. The 50‐km topography difference in the410between the plume and the slab corresponds to a temperature difference of ~500 ± 100 K. The Samoan plume is inferred to be 200 ± 50 K hotter than the ambient mantle and supports a thermal origin for the plume. 

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5. Slab horizontal subduction and slab tearing beneath East Asia

   https://doi.org/doi.org:10.1029/2018GL081703  

   Ma. P., Liu (May 2019, Geophysical research letters)