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Spatial Variations of Incoming Sediments at the Northeastern https://doi.org/10.1130/G46757.1 Manuscript received 14 July 2019 Revised manuscript received 4 February 2020 Manuscript accepted 10 February 2020 © 2020 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 30 March 2020 Spatial variations of incoming sediments at the northeastern Japan arc and their implications for megathrust earthquakes Gou Fujie1, Shuichi Kodaira1, Yasuyuki Nakamura1, Jason P. Morgan2, Anke Dannowski3, Martin Thorwart4, Ingo Grevemeyer3 and Seiichi Miura1 1 Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokohama 236-0001, Japan 2 Department of Ocean Science and Engineering, Southern University of Science and Technology (SUSTech), Shenzhen 518055, China 3 GEOMAR–Helmholtz-Centre for Ocean Research Kiel, Kiel 24148, Germany 4 Institute of Geosciences, Christian-Albrechts University Kiel, Kiel 24118, Germany ABSTRACT subduction) show that the origin of smectite The nature of incoming sediments is a key controlling factor for the occurrence of megath- at the plate boundary fault is from pelagic red- rust earthquakes in subduction zones. In the 2011 Mw 9 Tohoku earthquake (offshore Japan), brown clay within the incoming sediments (Ka- smectite-rich clay minerals transported by the subducting oceanic plate played a critical role in meda et al., 2015; Moore et al., 2015). Thus, the development of giant interplate coseismic slip near the trench. Recently, we conducted inten- the composition of incoming sediments is also sive controlled-source seismic surveys at the northwestern part of the Pacific plate to investigate a key factor shaping the occurrence of mega- the nature of the incoming oceanic plate. Our seismic reflection data reveal that the thickness thrust earthquakes. of the sediment layer between the seafloor and the acoustic basement is a few hundred meters In the past, due to relatively poor seismic in most areas, but there are a few areas where the sediments appear to be extremely thin. Our coverage, spatial variations in incoming sedi- wide-angle seismic data suggest that the acoustic basement in these thin-sediment areas is not ments have not been well constrained. Recently, the top of the oceanic crust, but instead a magmatic intrusion within the sediments associated we conducted intensive multichannel seismic with recent volcanic activity. This means that the lower part of the sediments, including the (MCS) reflection surveys and wide-angle seis- smectite-rich pelagic red-brown clay layer, has been heavily disturbed and thermally metamor- mic reflection and refraction surveys on the phosed in these places. The giant coseismic slip of the 2011 Tohoku earthquake stopped in the northwestern part of the Pacific plate with the vicinity of a thin-sediment area that is just beginning to subduct. Based on these observations, goal of revealing the nature of the subduction we propose that post-spreading volcanic activity on the oceanic plate prior to subduction is inputs to the northeastern Japan arc (Fig. 1A). a factor that can shape the size and distribution of interplate earthquakes after subduction In this study, we present an improved picture through its disturbance and thermal metamorphism of the local sediment layer. of the spatial variations in incoming sediments and discuss its implications for subduction zone INTRODUCTION trench (>50 m) occurred within a thin smectite- earthquakes. The occurrence and magnitude of thrust rich clay layer at the plate boundary (Chester earthquakes in subduction zones is closely linked et al., 2013). Because smectite is an extremely DATA ACQUISTION to interplate seismic coupling. This coupling, in weak mineral whose presence can dramati- Since 2009, we have conducted extensive turn, is generally thought to be related to the sur- cally change both the static and dynamic fric- controlled-source seismic surveys, along lines face topography and surface materials that form tion along a fault, the presence of an ultraweak as much as several hundred kilometers long, that the incoming oceanic plate. Large geometrical smectite-rich clay layer is now thought to be a mainly focus on the impact of plate bending–re- irregularities like seamounts tend to hinder long- prerequisite for giant coseismic slip (Ujiie et al., lated faulting prior to subduction (Fujie et al., range coseismic rupture propagation (Wang and 2013) like that observed at Tohoku. Incoming 2013, 2018; Kodaira et al., 2014) (Fig. 1A, thick Bilek, 2014). In contrast, thick sediments can sediments of the northwestern Pacific plate are black lines). MCS data were collected by towing smooth out low seafloor relief and result in a ho- generally divided into three parts: the lower- a 6-km-long, 444-channel hydrophone streamer mogenized interplate coupling (e.g., Ruff, 1989). most sediments are a chert unit, overlain by thin cable and using the large tuned airgun array of Fault zone materials control the mechani- pelagic red-brown sediments, with the top unit R/V Kairei of the Japan Agency for Marine- cal behavior of a plate boundary fault. For ex- a thick hemipelagic sediment layer (Shipboard Earth Science and Technology (JAMSTEC; Yo- ample, results from the Integrated Ocean Drill- Scientific Party, 1980; Moore et al., 2015). kohama, Japan) (total volume of 7800 in3). The ing Program (IODP) Expedition 343 after the Mineralogical analyses of drilling cores from tow depths of the airgun array and the streamer 2011 Mw 9 Tohoku earthquake (offshore Japan) both IODP Expedition 343 (post-subduction) cable were 10 and 12 m, respectively (see the showed that the giant coseismic slip near the and Deep Sea Drilling Project Site 436 (pre- GSA Data Repository1 for methodology). 1GSA Data Repository item 2020180, additional MCS reflection profiles and representative OBS record sections, as well as methodology, is available online at http://www.geosociety.org/datarepository/2020/, or on request from [email protected]. The data used in this study can be accessed through JAMSTEC (http:// www.jamstec.go.jp/obsmcs_db/e/index.html). CITATION: Fujie, G., et al., 2020, Spatial variations of incoming sediments at the northeastern Japan arc and their implications for megathrust earthquakes: Geology, v. 48, p. 614–619, https://doi.org/10.1130/G46757.1 614 www.gsapubs.org | Volume 48 | Number 6 | GEOLOGY | Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/6/614/5051324/614.pdf by guest on 30 September 2021 142°E 144°E 146°E 148°E 150°E 142°E 144°E 146°E 148°E 150°E 142°E 144°E 146°E 148°E 150°E Figure 1. (A) Seismic 44°N survey lines in the north- A B C western Pacific margin (offshore Japan). (B) Sediment thickness in 42°N two-way traveltime along multichannel seismic (MCS) lines. Red circles show petit-spot volca- 40°N nism cluster sites, where many small and young volcanoes are observed 38°N (Hirano et al., 2006). Petit-spot sites A and C correspond to apparent thin sediment cover. Most 36°N other thin-sediment areas correspond to seamounts and small seafloor bulges. 34°N (C) Coseismic slip dis- tribution of the 2011 02468 0200 400600 800 Bathymetry (km) Sediment thickness (ms.) Tohoku earthquake (black contours) (Iinuma et al., 2012). Contour interval is 10 m, and shaded area represents large-slip (>30 m) area. Distribution of chert unit along MCS lines is plotted in red. After the 2011 Tohoku earthquake, we are commonly reflective, and there common- extract one-dimensional (1-D) Vp-depth pro- also conducted another type of seismic survey ly exist patchy reflective zones between them files from the 2-D Vp model every 10 km and that focused on the traces of giant coseismic (Shipboard Scientific Party, 1980) Fig. 2C( ). categorize them as belonging to three possible slip near the trench (Kodaira et al., 2012; Na- However, in the thin-sediment areas, we do not segments: (1) the bend fault segment, where a kamura et al., 2013). We used a small-offset observe the characteristic appearance of the horst-and-graben structure caused by bend fault- MCS system (1.2-km-long, 192-channel hy- chert unit (Fig. 2D). The absence of the chert ing is observed; (2) the thin-sediment segment, drophone streamer cable, total airgun array vol- unit implies that the lower part of the sediments, where sediments appear to be extremely thin ume of 380 in3) and collected data along >100 including the pelagic red-brown clay, is missing (area C of Fig. 1B); and (3) the thick-sediment densely aligned short survey lines across the because the chert unit is the lowermost part of segment (Figs. 2E and 2F). Japan Trench (Fig. 1A, thin black lines). Tow the sediments and the the clay layer is locat- In general, the oceanic crust in the north- depths of the airguns and streamer cable were ed immediately above the chert (Moore et al., western Pacific plate consists of upper crust relatively shallow (5 m and 6 m, respectively) 2015). We carefully investigated all MCS pro- (oceanic layer 2, with large Vp gradient) and to better focus on the sedimentary structure. files and mapped areas where we could clearly lower crust (oceanic layer 3, with almost Despite significant differences in the survey recognize the characteristic appearance of the constant Vp). All 1-D Vp profiles are basi- configuration, spatial variations in sediment chert unit. We confirm a good correlation be- cally consistent with this general structure, thickness are well constrained by both surveys tween sediment thickness and the distribution but there are intriguing differences among after applying standard post-stack time migra- of the chert unit, which suggests that the pe- segments. tion (Figs. 2 and 3). lagic clay is missing in the thin-sediment areas The oceanic crust in the thick-sediment seg- (Fig. 1C).
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