Incoming plate structure at the Japan Trench subduction zone revealed in densely spaced reflection seismic profiles

Abstract

The structure of the incoming plate is an important element that is often considered to be related to the occurrence of great earthquakes in subduction zones. In the Japan Trench, where the 2011 Tohoku earthquake occurred, we collected seismic profiles along survey lines separated by 2–8 km to examine the structural characteristics of the incoming Pacific plate in detail. The average thickness of the incoming sediments was < 500 m along most of the Japan Trench, and it was < 300 m at ~ 38° N, where the large shallow megathrust slip occurred during the 2011 Tohoku earthquake. We mapped bending-related normal faults, including their dip direction and amount of throw. The numbers of eastward (oceanward) and westward (trenchward) dipping normal faults were generally comparable in the Japan Trench. Eastward dipping normal faults were dominant in the northern and southern parts of the Japan Trench, whereas westward dipping normal faults were more numerous in the central part. Graben-fill sediments deposited at the landward edge of the graben were bounded by eastward dipping normal faults. Trench-fill sediments were also observed along the trench axis. The sediment fills locally increased the thickness of the input sediments where they were deposited. The along-axis variation in input sediment and sediment fill distribution, and the variations in normal fault dip direction between the central and other parts of the Japan Trench may correspond to different slip styles along the plate boundary.

Influence of outer-rise faults on shallow décollement heterogeneity and sediment flux at the Japan trench

“…We map seismic units on the incoming and overriding plate in the study area using the seismic stratigraphy of Nakamura et al (2013Nakamura et al ( , 2020Nakamura et al ( , 2023 (Figure 2a). This work defines four seismic units (SU1 though SU4) based on seismic characteristics and correlations to known seismic and lithostratigraphic units of DSDP Site 436 and International Ocean Drilling Program (IODP) Expedition 343 Core C0019 (Nakamura et al, 2013).…”

“…This unit is correlative to incoming plate sediments composed of diatomaceous muds with a basal pelagic clay layer (Nakamura et al, 2013). We include trench fill deposits, parallel and continuous reflections that overly the incoming plate sediments near the deformation front, as part of the SU2 map unit (Nakamura et al, 2023). Seismic unit SU3 contains high amplitude, sub-horizontal, semi-continuous reflections and is correlated to a chert layer stratigraphically below SU2.…”

“…Prior mapping demonstrates that the décollement is localized within SU1 or within or at the base of SU2 and is generally parallel to the top of the SU3 and SU4 units in the subducting plate (Nakamura et al, 2013(Nakamura et al, , 2020. Normal faults on the incoming plate and subducting plate beneath the prism are identified based on truncation of units SU2, SU3, and SU4 where they are offset at a high angle (>50°) by landward or seaward dipping faults (e.g., Boston et al, 2014;Nakamura et al, 2023).…”

“…The incoming Pacific plate has a relatively thin sediment cover (~50-600 m) (Boston et al, 2014;Nakamura et al, 2023) due to limited biologic productivity in the north Pacific gyre and the relatively small volume of trench fill derived from the nearby continent and trench slopes (Moore et al, 2015;Ikehara et al, 2017). The incoming plate stratigraphy recovered at Deep Sea Drilling Project (DSDP) Leg 56/57 Site 436 (Figure 1) consists of basaltic crust overlain by ~100-160 m of Cretaceous to Oligocene chert, ~20-50 m of Eocene to early Miocene smectite-rich clay, and ~200-350 m of Miocene to Quaternary biogenic mud (Kodaira et al, 2020;Moore et al, 2015;Nakamura et al, 2013;Shipboard Scientific Party, 1980).…”

Patterns of Causative Faults of Normal Earthquakes in the Fluid‐Rich Outer Rise of Northeastern Japan, Constrained With 3D Teleseismic Waveform Modeling

“…For the other four events (20110322, 20110603, 20110817, and 20170920), we find that all of them tend to take place on the trench-ward dipping faults (Figure 9). It is interesting that Nakamura et al (2023) used seismic profiles to infer the dipping directions of the outer-rise normal faults in the region and also found that similar results of more trench-ward dipping faults than that dip ocean-ward. Note that the centroid depths of these events gradually increase with distance from the trench, which could be explained by a deeper neutral stress axis (the transition from extension to compression) related to the bending and elastic thickness of the subducting plate.…”