The Mw=9 2011 Tohoku-oki earthquake ruptured to the Japan Trench, with largest coseismic slip (c. 50 m) unexpectedly occurring on the shallow part of the décollement. The JFAST Project, Integrated Ocean Drilling Program (IODP) Expedition 343/343T, successfully located and sampled the shallow part of the subduction thrust shear zone (Chester et al. 2013a,b). Temperature data from a downhole observatory confirm that a thin and weak clay rich layer, identified in logging-while-drilling data and core-sample observations, is the plate boundary fault that accommodated the large slip of the earthquake rupture, as well as most of the kilometres interplate motion at the drill site (Chester et al. 2013b; Fulton et al. 2013; Lin et al. 2013; Ujiie et al. 2013). The décollement separates folded and faulted frontal prism sediments in the overriding plate from incoming flat-lying sediments along the top of the subducting plate (Chester et al., 2013b). Observed stratigraphic discontinuities at the boundary and inside the recovered fault material (Chester et al. 2013a) suggest that it contains multiple slip surfaces, many of them probably not recovered. Core analysis shows that the décollement is localized upon a strongly deformed 5≤m thick layer of smectite-rich clay, likely derived from the Paleogene to middle Miocene Pacific Plate pelagic sediments. A pervasive scaly fabric, defined by polished lustrous surfaces, commonly striated, enclosing lenses of less fissile material (phacoids), which are self-similar at scales ranging from a few micrometers to centimeters, is distributed throughout the clay. The spacing of the surfaces increase from millimeter scale near the top of the recovered core to centimetre scale, toward the lower tectonic contact, reflecting a decrease in the magnitude of shear strain. In the upper highly sheared section, one extremely narrow discontinuity, crosscuts this fabric, truncating without deflection the foliations that are not parallel across the contact. While the scaly fabric is indicative of distributed shear across the recovered interval (~1 m) the sharp discontinuity, resulted from localized deformation and similar to those observed at coseismic slip rates in friction experiments, could record seismic slip although not necessarily that of the Tohoku-Oki earthquake
Internal structure of the shallow plate boundary slip zone for the 2011 Tohoku-Oki Earthquake sampled during the Japan Trench Fast Drilling Project (JFAST) / Remitti, Francesca; Kirkpatrick, J.; Ujiie, K.; Mishima, T.; Chester, F.; Rowe, C.; Regalla, C.; Moore, C.; Toy, V.; Kameda, J.; Bose, S.; Wolfson Schwehr, M.. - In: RENDICONTI ONLINE DELLA SOCIETÀ GEOLOGICA ITALIANA. - ISSN 2035-8008. - STAMPA. - 31 Supplemento N°1:(2014). (Intervento presentato al convegno 87° Congresso della Società Geologica Italiana e 90° Congresso della Società Italiana di Mineralogia e Petrologia tenutosi a Milano nel 10-12 Settembre 2014).