Geological setting

The 2011 Tohoku earthquake occurred on the megathrust fault surface west of the Japan Trench where the Pacific plate of Cretaceous age subducts below Honshu Island. The subduction zone is characterized by a relatively rapid convergence rate of ~85 mm/y (e.g., Apel et al., 2006; DeMets et al., 2010; Argus et al., 2011), a high rate of seismic activity, and a deep trench (Kanamori et al., 2006; Hashimoto et al., 2009; Simons et al., 2011). The historical record for the region includes 13 Mw 7 and 5 Mw 8 earthquakes over the last 400 years. The convergent margin of the Japan Trench displays the features generally associated with subduction erosion (von Huene et al., 1994, 2004; Tsuru et al., 2000), specifically, evidence of subsidence in the Neogene with associated extensional faulting in the middle slope region, horst and graben structures in the upper portion of the subducting plate, and a relatively small frontal prism (5–15 km wide) containing landward-dipping reflectors and a backstop bounding the frontal prism on the landward side.

The structure and lithology of the forearc region of northern Japan were investigated during Deep Sea Drilling Project (DSDP) Legs 56 and 57 (Arthur and Adelseck, 1980), and later observatories were established in the forearc during Ocean Drilling Program (ODP) Leg 186 (Sacks, Suyehiro, Acton, et al., 2000). ODP Leg 186 Sites 1150 and 1151 are located above the region of large fault slip during the Tohoku earthquake, ~100 km north of the epicenter (e.g., Lin et al., 2011). DSDP Leg 56 and 57 Sites 434–441 are located approximately 50–100 km further north (Fig. F1). In this region, the Japan Trench system consists of a deep-sea terrace, inner trench slope, midslope terrace, trench lower slope, the trench, and outer trench slope (Arthur and Adelseck 1980; Tsuru et al., 2002). A forearc basin formed at the deep-sea terrace contains sequences of Neogene sediments as thick as 5 km overlying a Cretaceous unconformity, which correlates with a regional unconformity and relates to geologic features on land. The overlying sediments extend trenchward through the midslope terrace to the backstop boundary, and the frontal prism forms the trench lower slope (e.g., Tsuru et al., 2000). Seismic profiling indicates the structure in the northern Japan Trench is similar through the region to the south that ruptured during the Tohoku event (Tsuru et al., 2002).

The frontal prism is characterized by lower seismic velocity than the region landward of the backstop, and the prism displays disrupted-to-chaotic reflection patterns that likely indicate strong deformation (e.g., Tsuru et al., 2000). Coring of the toe region of the frontal prism at Site 434 revealed the prism is composed of a highly disrupted, very uniform hemipelagic deposit (Shipboard Scientific Party, 1980a). The major constituents are terrigenous silty clay, biogenic silica, and vitric ash. Biostratigraphic observations indicate structural complexity in the prism with repetition of assemblages that could record slumping, sliding, and faulting. Significant induration of the sediments occurs at depths below ~100 m, and the mudstones recovered are highly fractured with slickensided faces. The highly fractured and disrupted structure contributed to the difficulties of coring and poor core recovery at Site 434.

Seismic studies/Site survey

The offshore Tohoku region is well characterized from decades of data collection, including some high-resolution surveys of bathymetric and seismic reflection data taken after the 2011 Tohoku earthquake (Kodaira et al., 2012; Fujiwara et al., 2011). A differential bathymetry analysis across the trench axis eastward of the hypocentral area, using data collected along the same track before and after the earthquake, shows large topographical changes on the landward side of the trench. From analysis of differential bathymetry, Fujiwara et al. (2011) demonstrate approximately 50 m of horizontal movement toward the southeast and 10 m of upward movement during the earthquake. In addition, a large submarine landslide scarp on the lower trench wall and associated slump deposits in the trench are indicated by the distinct negative and positive seafloor elevation changes at the trench. A critical result is that the coseismic displacement of the 2011 Tohoku earthquake extended all the way to the landslide scarp, if not to the trench axis proper (Kodaira et al., 2012). The pervasive and similar magnitude of displacement along the profile shows that the frontal prism was displaced as a coherent unit along a fairly uniform dipping detachment surface. Analysis of other data sets also points to coseismic slip on the order of 50 m that extends to the trench (e.g., Ito et al., 2011; Ide et al., 2011; Simons et al., 2011; Fujii et al., 2011).

Based on the above observations and the geometric continuity with the deeper main detachment surface, the plate interface is considered to be the most likely fault surface of large displacement during the earthquake (e.g., Ito et al., 2011; Kodaira et al., 2012).

Japan Agency for Marine-Earth Science and Technology (JAMSTEC) carried out high-resolution seismic surveys in the area of the large seafloor displacement. Multichannel seismic data perpendicular to the trench axis along Line HD33B (Fig. F2) show a strong reflector at ~800 meters below seafloor (mbsf) that is thought to be the sediment/basalt interface of the subducting Pacific plate. A weak reflector about 50 m above this strong reflector is interpreted as the megathrust fault zone and is the main target of JFAST drilling.

The selection of the drilling site was constrained by a maximum penetration depth of 1000 mbsf for riserless drilling and a maximum water depth of 7000 m. In general, the plate interface shallows toward the trench but the water deepens. Generally, the plate interface at a water depth of ~7000 m is shallower near 38°N and deeper further north. On the basis of existing seismic data, the primary site (JFAST-3) was identified as a location that minimizes both water depth and fault depth below the seafloor (Fig. F3).