Seismicity of the Incoming Plate and Forearc Near the Mariana Trench Recorded by Ocean Bottom Seismographs

Abstract

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.

Upper Mantle Hydration Indicated by Decreased Shear Velocity Near the Southern Mariana Trench From Rayleigh Wave Tomography

“…For example, previous studies suggested the neutral plane with a global average depth of 30-40 km marks the bottom boundary of the mantle hydration, assuming that pore water cannot penetrate the compressional stress regime (Chapple & Forsyth, 1979). Based on near-field OBS observations, the extensional earthquakes are estimated at the depth of ∼20 km below the Moho in the central Mariana, which is much deeper than previous results (Emry et al, 2014) and consistent with the estimated hydration extent derived from the SV-wave velocities (Eimer et al, 2020).…”

Constraints on Bend‐Faulting and Mantle Hydration at the Marianas Trench From Seismic Anisotropy

“…The refraction data sample only a few km beneath the Moho, and cannot provide information on how deep bend-related faulting and hydration extend. However, other observations, including Rayleigh wave dispersion and anisotropy (Cai et al, 2018) and earthquake hypocenters (Eimer et al, 2020;Emry et al, 2014), show that extensional faulting extends to greater depths than sampled in this study.…”

Geology, environment, and life in the deepest part of the world’s oceans

“… 37 , 38 A well-known example of the hydration of the incoming lithosphere comes from the Middle America Trench, where active bending-related faulting across the entire ocean trench slope penetrates at least 20 km into the plate, promoting the deep percolation of water and hydration of the crust and upper mantle. 22 , 39 , 40 The hydration of upper mantle rock has also been documented below the Moho of the subducting slab at the northern central Chile, 41 central Chile, 23 southernmost Mariana, 42 and Mariana 43 trenches, as shown by a notable reduction in compressional mantle velocities, with a penetration depth of 1–2 km, 22 and even 5–7 km, into the mantle. 41 , 44 , 45

Figure 3 Bending-related normal faults and fracture zones in the southern Mariana Trench (A) Topographic map showing abundant bending-related normal faults occurring on the incoming plate of the southern Mariana Trench.

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“…59 The work by Du et al 59 provides direct evidence that the structural deformation of the incoming plate could substantially influence the hydration of upper basaltic crust, which fosters chemical exchange between the upper ocean crust and seawater in a new way. 43 These newly discovered fluid discharge fields are exhibited by small-scale mud volcanoes and pockmarks in association with varied altered mafic rocks that crop out on the trench slope and are mainly composed of iddingsite. 59 Chemically and physically, this phenomenon differs from the serpentine mud volcano formation associated with the shallower Mariana forearc region.…”