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Intermediate-depth earthquakes (focal depths 70–300 km) are enigmatic with respect to their nucleation and rupture mechanism and the properties controlling their spatial distribution. Several recent studies have shown a link between intermediate-depth earthquakes and the thermal-petrological path of subducting slabs in relation to the stability field of hydrous minerals. Here we investigate whether the structural characteristics of incoming plates can be correlated with the intermediate-depth seismicity rate. We quantify the structural characteristics of 17 incoming plates by estimating the maximum fault throw of bending-related faults. Maximum fault throw exhibits a statistically significant correlation with the seismicity rate. We suggest that the correlation between fault throw and intermediate-depth seismicity rate indicates the role of hydration of the incoming plate, with larger faults reflecting increased damage, greater fluid circulation, and thus more extensive slab hydration.

The firn layer covers 98% of Antarctica's ice sheets, protecting underlying glacial ice from the external environment. Accurate measurement of firn properties is essential for assessing cryosphere mass balance and climate change impacts. Characterizing firn structure through core sampling is expensive and logistically challenging. Seismic surveys, which translate seismic velocities into firn densities, offer an efficient alternative. This study employs Distributed Acoustic Sensing technology to transform an existing fiber‐optic cable near the South Pole into a multichannel, low‐maintenance, continuously interrogated seismic array. The data resolve 16 seismic wave propagation modes at frequencies up to 100 Hz that constrain P and S wave velocities as functions of depth. Using co‐located geophones for ambient noise interferometry, we resolve very weak radial anisotropy. Leveraging nearby SPICEcore firn density data, we find prior empirical density‐velocity relationships underestimate firn air content by over 15%. We present a new empirical relationship for the South Pole region. Plain Language Summary: Firn, the layer of compacted snow merging into glacial ice covering Antarctica, acts as an insulating blanket that mitigates environmental perturbations to the polar ice sheet. Understanding the density and seismic characteristics of the firn layer helps scientists better infer its properties and variation, including factors relevant to glacial stability and sea level change. Firn density is the major uncertainty source for measuring ice sheet mass changes via satellite and airborne sensing. Traditional methods of assessing firn density involve drilling or snow pit analyses and are expensive and time‐consuming. We utilize the rapidly developing technology of Distributed Acoustic Sensing to transform a data communication cable near the South Pole into a dense array of seismic sensors, allowing us to noninvasively estimate firn properties by studying seismic waves propagating in the firn to assess its physical properties. Our findings suggest that previous parameterizations overestimate firn density by over 5% and underestimate its air content by over 15% and highlight the value of seismology for advancing glaciological and polar region's climate research. Key Points: Distributed Acoustic Sensing repurposes an 8 km fiber‐optic cable at the South Pole into a dense seismic arrayGathered data resolve 16 dispersion modes at frequencies up to 100 Hz that constrain P‐ and S‐wave velocities in the firn layerPrevious density‐velocity empirical relations overestimate the dry firn density at South Pole [ABSTRACT FROM AUTHOR]

We develop a 3‐D isotropic shear velocity model for the Alaska subduction zone using data from seafloor and land‐based seismographs to investigate along‐strike variations in structure. By applying ambient noise and teleseismic Helmholtz tomography, we derive Rayleigh wave group and phase velocity dispersion maps, then invert them for shear velocity structure using a Bayesian Monte Carlo algorithm. For land‐based stations, we perform a joint inversion of receiver functions and dispersion curves. The forearc crust is relatively thick (35–42 km) and has reduced lower crustal velocities beneath the Kodiak and Semidi segments, which may promote higher seismic coupling. Bristol Bay Basin crust is relatively thin and has a high‐velocity lower layer, suggesting a dense mafic lower crust emplaced by the rifting processes. The incoming plate shows low uppermost mantle velocities, indicating serpentinization. This hydration is more pronounced in the Shumagin segment, with greater velocity reduction extending to 18 ± 3 km depth, compared to the Semidi segment, showing smaller reductions extending to 14 ± 3 km depth. Our estimates of percent serpentinization from VS reduction and VP/VS are larger than those determined using VP reduction in prior studies, likely due to water in cracks affecting VS more than VP. Revised estimates of serpentinization show that more water subducts than previous studies, and that twice as much mantle water is subducted in the Shumagin segment compared to the Semidi segment. Together with estimates from other subduction zones, the results indicate a wide variation in subducted mantle water between different subduction segments. Plain Language Summary: This study uses seismic data from the 2018–2019 Alaska Amphibious Community Seismic Experiment and other land stations to image the 3‐D seismic velocity structure of the Alaska subduction zone. The analysis combines constraints from both Rayleigh waves and converted body waves. The results provide insight into the distinct lateral variations observed for many properties of the subduction zone. Thick, low‐velocity forearc crust is found beneath the Kodiak and Semidi segments, which may be related to the higher seismic coupling in these regions. The Bristol Bay Basin has a thin crust with a high velocity lower layer, suggesting a dense mafic lower crust emplaced by the extensional processes that formed the basin. Low velocities in the incoming plate near the trench in the Shumagin segment indicate pronounced mantle hydration, extending to about 18 km below the Moho. Together with estimates from other subduction zones, the results indicate a wide variation in subducted mantle water between different subduction segments. Key Points: Crustal thickness of the inner forearc (35–42 km) generally exceeds that of the volcanic arc, but becomes variable in the Shumagin segmentThe Shumagin segment has more incoming plate mantle hydration than the Semidi segment, aligning with abundant plate bending normal faultsHydration extends to depths of 18 km below the Moho, indicating more water subducts than most previous estimates [ABSTRACT FROM AUTHOR]