Seismic Evidence for Water Transportation in the Forearc off Northern Japan

Abstract

The water cycle plays an essential role in arc volcanism, earthquake generation, mantle rheology, and thermal structure of subduction zones. Previous seismic studies have revealed strong structural heterogeneities in the megathrust zone in Northeast Japan and Hokkaido. However, water transportation in the forearc region remains poorly understood due to the lack of long-term seismic observatories at the seafloor. Using high-quality data recently recorded by the permanent ocean-bottom-seismometer network (S-net) in the Pacific Ocean off Northern Japan, we study the fine three-dimensional P wave velocity (V p ) structure of the crust and upper mantle beneath the Kuril and Tohoku forearc region. Our results reveal high velocities in the mantle-wedge corner, implying a low degree of serpentinization there. We suggest that the forearc mantle in the study region is cold and anhydrous. The slab interface under the forearc area may have a low permeability, which controls the fluid flux to the mantle wedge and the overriding plate. A low-V p layer atop the subducting Pacific plate is interpreted as the subducted oceanic crust. Dehydration of the subducted oceanic crust occurs at depths of 80-120 km, providing a large volume of water to the overriding mantle wedge to produce arc volcanoes, and part of the water may migrate upward to the shallow area. Large megathrust earthquakes (M w ≥ 6.0) mainly occurred around low-V p patches in the megathrust zone. Destruction of the low-permeability slab interface would result in fluid flow upward, which may trigger large megathrust earthquakes and seismicity in the mantle wedge under the forearc.

Role of warm subduction in the seismological properties of the forearc mantle: An example from southwest Japan

“…The absence of the serpentinite layer in the models supports a number of geophysical observations in northeast Japan, such as the small delay times of shear-wave splitting (~0.3 s) (12,14), the paucity of deep nonvolcanic tremors, and the cold and dry nature of the forearc mantle (Fig. 1) (20,35). The study by (36) reported the occurrence of shallow low-frequency nonvolcanic tremors at 10-to 25-km depths on the plate interface, implying no subarc serpentinite layer.…”

Seismic Anisotropy and Intraslab Hydrated Faults Beneath the NE Japan Forearc

“…Recently, a permanent seismic network consisting of 150 ocean bottom seismometer (OBS) stations, the Seafloor Observation Network for Earthquakes and Tsunamis (S‐net), has been installed around the Japan Trench and the Kuril trench (Kanazawa, 2013; Figure 1a), which provides an unprecedented opportunity to improve our understanding of subduction dynamics in the East Japan forearc region (e.g., Hua et al., 2020; Nishikawa et al., 2019; Uchida et al., 2020; Yu & Zhao, 2020). P ‐wave arrival times of local earthquakes recorded at 612 seismic stations on and around the Japan Islands, including 462 onshore network stations and 150 offshore S‐net stations (Figure 1a), were carefully selected for conducting Vp anisotropic tomography.…”

Improving the Constraint on theMw7.1 2016 Off‐Fukushima Shallow Normal‐Faulting Earthquake With the High Azimuthal Coverage Tsunami Data From the S‐Net Wide and Dense Network: Implication for the Stress Regime in the Tohoku Overriding Plate

“…In response to the Tohoku earthquake, a new wide offshore deep‐ocean observation network, Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench (S‐net), has been constructed off eastern Japan (Aoi et al., 2020; Kanazawa et al., 2016; Mochizuki et al., 2016; Uehira et al., 2016, Figure 1a). Recent studies have started to utilize S‐net ocean‐bottom seismometers to investigate the seismotectonics and geodynamics in the Tohoku subduction zone (Dhakal et al., 2021; Hua et al., 2020; Matsubara et al., 2019; Nishikawa et al., 2019; Sawazaki & Nakamura, 2020; Takagi et al., 2019, 2020; Tanaka et al., 2019; Uchida et al., 2020; Yu & Zhao, 2020). The S‐net also incorporates ocean‐bottom pressure gauges (OBPGs), which are expected to be utilized for tsunami forecasts (e.g., Aoi et al., 2019; Inoue et al., 2019; Mulia & Satake, 2021; Tanioka, 2020; Tsushima & Yamamoto, 2020; Wang & Satake, 2021; Yamamoto, Aoi, et al., 2016; Yamamoto, Hirata, et al., 2016).…”