Abstract
The 2011 Mw9.0 Tohoku-oki earthquake ruptured to the trench with maximum coseismic slip located on the shallow portion of the plate boundary fault. To investigate the conditions and physical processes that promoted slip to the trench, Integrated Ocean Drilling Program Expedition 343/343T sailed 1 year after the earthquake and drilled into the plate boundary ∼7 km landward of the trench, in the region of maximum slip. Core analyses show that the plate boundary décollement is localized onto an interval of smectite-rich, pelagic clay. Subsidiary structures are present in both the upper and lower plates, which define a fault zone ∼5–15m thick. Fault rocks recovered from within the clay-rich interval contain a pervasive scaly fabric defined by anastomosing, polished, and lineated surfaces with two predominant orientations. The scaly fabric is crosscut in several places by discrete contacts across which the scaly fabric is truncated and rotated, or different rocks are juxtaposed. These contacts are inferred to be faults. The plate boundary décollement therefore contains structures resulting from both distributed and localized deformation. We infer that the formation of both of these types of structures is controlled by the frictional properties of the clay: the distributed scaly fabric formed at low strain rates associated with velocity-strengthening frictional behavior, and the localized faults formed at high strain rates characterized by velocity-weakening behavior. The presence of multiple discrete faults resulting from seismic slip within the décollement suggests that rupture to the trench may be characteristic of this margin.
Highlights
The devastating 2011 Mw9 Tohoku-oki earthquake ruptured a 200 x 380 km area of the Japan Trench plate boundary where the Pacific Plate subducts beneath Japan
We infer that the formation of both of these types of structures is controlled by the frictional properties of the clay: the distributed scaly fabric formed at low strain rates associated with velocity-strengthening frictional behavior, and the localized faults formed at high strain rates characterized by velocity-weakening behavior
Core recovered from in and around the décollement at the Japan Trench Fast Earthquake Drilling Project (JFAST) study site shows that the frontal prism near the trench is composed of moderately to steeply dipping, off-scraped Miocene and younger hemipelagic mudstones
Summary
The devastating 2011 Mw9 Tohoku-oki earthquake ruptured a 200 x 380 km area of the Japan Trench plate boundary where the Pacific Plate subducts beneath Japan. M. Schleicher et al, Response of natural smectite to seismogenic heating and implications for the 2011 Tohoku earthquake in the Japan Trench, submitted to Geology, 2014], and rock friction experiments on samples of the core show that the clay is frictionally weak, velocity-weakening at seismic slip rates [Ujiie et al, 2013], and is weaker than the surrounding mudstones [Ikari et al, 2015]. Schleicher et al, Response of natural smectite to seismogenic heating and implications for the 2011 Tohoku earthquake in the Japan Trench, submitted to Geology, 2014], and rock friction experiments on samples of the core show that the clay is frictionally weak, velocity-weakening at seismic slip rates [Ujiie et al, 2013], and is weaker than the surrounding mudstones [Ikari et al, 2015] This combination of properties suggests that localization of slip onto the thin plate boundary fault is preferred during both seismic and aseismic deformation during the seismic cycle [Chester et al, 2013b]. These observations, along with details of the deformation fabrics within the décollement fault rocks, are used to infer the relative strain rates of formation and to develop an interpretation of the mechanical behavior of the shallow part of the décollement
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