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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. 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)

   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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3. 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)

   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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4. Earth’s Upper Crust Seismically Excited by Infrasound from the 2022 Hunga Tonga–Hunga Ha’apai Eruption, Tonga

   https://doi.org/10.1785/0220220252  

   Anthony, Robert E. ; Ringler, Adam T. ; Tanimoto, Toshiro ; Matoza, Robin S. ; De Angelis, Silvio ; Wilson, David C. ( December 2022 , Seismological Research Letters)

   Abstract Records of pressure variations on seismographs were historically considered unwanted noise; however, increased deployments of collocated seismic and acoustic instrumentation have driven recent efforts to use this effect induced by both wind and anthropogenic explosions to invert for near-surface Earth structure. These studies have been limited to shallow structure because the pressure signals have relatively short wavelengths (<∼300 m). However, the 2022 eruption of Hunga Tonga–Hunga Ha’apai (also called “Hunga”) volcano in Tonga generated rare, globally observed, high-amplitude infrasound signals with acoustic wavelengths of tens of kilometers. In this study, we examine the acoustic-to-seismic coupling generated by the Hunga eruption across 82 Global Seismographic Network (GSN) stations and show that ground motion amplitudes are related to upper (0 to ∼5 km) crust material properties. We find high (>0.8) correlations between pressure and vertical component ground motion at 83% of the stations, but only 30% of stations show this on the radial component, likely due to complex tilt effects. We use average elastic properties in the upper 5.2 km from the CRUST1.0 model to estimate vertical seismic/acoustic coupling coefficients (SV/A) across the GSN network and compare these to recorded observations. We exclude many island stations from these comparisons because the 1° resolution of the CRUST1.0 model places a water layer below these stations. Our simple modeling can predict observed SV/A within a factor of 2 for 94% of the 51 non-island GSN stations with high correlations between pressure and ground motion. These results indicate that analysis of acoustic-to-seismic coupling from the eruption could be used to place additional constraints on crustal structure models at stations with collocated seismic and pressure sensors. Ultimately, this could improve tomographic imaging models, which rely on methods that are sensitive to local structure. 

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5. High Bulk and Shear Attenuation Due to Partial Melt in the Tonga‐Lau Back‐arc Mantle

   https://doi.org/10.1029/2019JB017527  

   Wei, S. Shawn ; Wiens, Douglas A. ( January 2020 , Journal of Geophysical Research: Solid Earth)