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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.

Earthquakes near oceanic trenches are important for studying incoming plate bending and updip thrust zone seismogenesis, yet are poorly constrained using seismographs on land. We use an ocean bottom seismograph (OBS) deployment spanning both the incoming Pacific Plate and the forearc to study seismicity near the Mariana Trench. The yearlong deployment in 2012-2013 consisted of 20 broadband OBSs and 5 suspended hydrophones, with an additional 59 short period OBSs and hydrophones recording for 1 month. We locate 1,692 earthquakes using a nonlinear method with a 3D velocity model constructed from active source profiles and surface wave tomography results. Events occurring seaward of the trench occur to depths of~35 km below the seafloor, and focal mechanisms of the larger events indicate normal faulting corresponding to plate bending. Significant seismicity emerges about 70 km seaward from the trench, and the seismicity rate increases continuously towards the trench, indicating that the largest bending deformation occurs near the trench axis. These plate-bending earthquakes occur along faults that facilitate the hydration of the subducting plate, and the lateral and depth distribution of earthquakes is consistent with low-velocity regions imaged in previous studies. The forearc is marked by a heterogeneous distribution of low magnitude (<5 M w ) thrust zone seismicity, possibly due to the rough incoming plate topography and/or serpentinization of the forearc. A sequence of thrust earthquakes occurs at depths~10 km below seafloor and within 20 km of the trench axis, demonstrating that the megathrust is seismically active nearly to the trench. Plain Language Summary Studying earthquakes near oceanic trenches is important forunderstanding subduction zones but can be difficult using only distant land-based instruments. This study uses seismographs designed to work on the seafloor to study earthquakes near the central Mariana Trench. As the subducting plate bends, faults form due to the increased extensional stress. These faults can become pathways for water to penetrate into this subducting plate. Understanding the amount of water stored in the plate is essential for constraining the global water cycle, and the earthquake distribution allows us to determine the distribution of active faults. We found that the earthquakes occur shallower than 35-km depth and within 70 km seaward of the trench. We also study earthquakes occurring along the megathrust, which is the interface between the subducting plate and the overriding plate that is prone to seismic activity. We found that the earthquakes show a patchy distribution, indicating that the megathrust interface is not uniform. This could be related to topographic features on the subducting plate interacting with the plate above. We also observed earthquakes at depths shallower than 10 km below the seafloor, indicating that the megathrust can rupture close to the trench.

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