Sediment provenance and controls on slip propagation: Lessons learned from the 2011 Tohoku and other great earthquakes of the subducting northwest Pacific plate

Abstract

The ~50 m slip of the Tohoku earthquake occurred along a very fine grained red-brown smectitic clay horizon subducting in the Japan Trench. This clay, cored in the plate boundary fault at Integrated Ocean Drilling Program Expedition 345, Site C0019, correlates with similar pelagic clay recovered seaward of the trench at Deep Sea Drilling Project Sites 436 and 1149. Comparable clays occur throughout the northwest Pacific Basin. Backtracking of ocean drilling Sites 436, C0019, and 1149 indicates that they formed during the Early Cretaceous at the Kula-Pacific Ridge. These sites traveled northwestward through the equatorial zone, accumulating siliceous and calcareous oozes until ca. 100-85 Ma. Sites 436, C0019, and 1149 then entered the realm of pelagic clay deposition where they remained until ca. 15 Ma. From ca. 15 Ma to the present, Sites 436, C0019, and 1149 accumulated clays and silty clays with variable amounts of siliceous microfossils and volcanic ash, representing the transition from deep-sea conditions to a continental margin sedimentary environment. The predicted backtracked vertical sequence of sediments fits well with the cores at Sites 436, 1149, and C0019, after accounting for structural complications in the latter.Pelagic clay occurs in numerous boreholes penetrating the relatively smooth ocean floor of the Pacific plate north and northeast of the Tohoku earthquake. Here the widespread pelagic clay apparently fosters tsunami and tsunamigenic earthquakes. Seamounts rising above the normal oceanic crust accumulated sequences of calcareous sediments as their crests remained above the calcite compensation depth for most of their history. A seafloor including pelagic clay and carbonate-covered seamounts occurs south and southeast of the southern extent of the Tohoku earthquake rupture zone. This area has no historic tsunami or tsunamigenic earthquakes along the Japan and Izu-Bonin Trenches with the possible exception of the poorly located Enpo earthquake of A.D. 1677. We believe that the seamounts incoming on the oceanic plate to the south and southeast of the Tohoku rupture zone interfere with long-distance propagation of slip in the pelagic clay, limiting earthquake magnitude, shallow slip, and tsunami generation.

Lithologic control of frictional strength variations in subduction zone sediment inputs

“…At Site 436, four major units were identified: diatomaceous silty clay and claystone (unit I, further subdivided into IA and IB based on higher induration in IB), yellowish-brown diatomaceous claystone (unit II), pelagic clay (unit III), and chert and porcellanite (unit IV) (Shipboard Scientific Party, 1980). Thus, the three lowermost units at Site 436 correlate with the underthrust section at Site C0019 (Chester et al, 2013b;Kirkpatrick et al, 2015;Moore et al, 2015). At Site C0019, there are two clay-rich layers: the pelagic clay that correlates with unit III at Site 436, and the scaly clay that defines the plate boundary fault zone.…”

Structure and lithology of the Japan Trench subduction plate boundary fault

“…The pelagic clay in the Pacific Plate at the JFAST site (core 21R) is Cretaceous (see section 3.2). Although the ages of these two units are different, they may be conformable units within the same sequence so either could correlate with the clay in core 17R, as noted by Moore et al [2015]. The radiolarian microfossil assemblage in the short mudstone interval shows that it is the same age as the footwall units (Late Miocene).…”

“…Based on the grain size distribution, volcaniclastic content and microfossil assemblages, the tan-brown mudstone in cores 18R-20R at the JFAST site correlates to Unit 2 at the nearest incoming plate reference site, DSDP Site 436 (for a detailed discussion of the stratigraphic correlation between Site C0019 and DSDP Site 436, see Nakamura et al [2013] and Moore et al [2015]). The Cretaceous pelagic clay and chert in cores 20R-21R at the JFAST site correlate with Cretaceous chert (Unit 3) at the DSDP reference site.…”

“…The lack of microfossils in the smectite-rich clay in core 17R means the age of the clay is currently poorly constrained. It could be stratigraphically equivalent to either the pelagic clay near the base of the cored interval (core 20R) or the Neogene pelagic clay at Site 436 [Moore et al, 2015]. In either case, there must be an age gap at the base of the frontal prism, and a significant age reversal at the base of core 17R.…”

Multiple major faults at the Japan Trench: Chemostratigraphy of the plate boundary at IODP Exp. 343: JFAST

“…Previous studies of the core used a nearby site in the Western Pacific (DSDP Site 436) as a reference site for the incoming plate stratigraphy Moore et al, 2015). The correlation between the two ocean drilling sites has been based largely on radiolarian biostratigraphic ages and lithologic characteristics of sediments (Chester et al, 2012).…”