High‐resolution seismic imaging in the Japan Trench axis area off Miyagi, northeastern Japan

Abstract

Newly obtained high‐resolution seismic data reveal the detailed structure of the Japan Trench axis off Miyagi, Japan, in the region of large shallow slip during the 11 March 2011 M9 Tohoku earthquake. Correlation of seismic images with previous drilling results identifies a possible basal chert‐rich layer and hemipelagic/pelagic mudstones overlying igneous Pacific crust. Mapping of acoustic basement depicts the subduction of horst‐and‐graben topography. The basement and basal chert are subducted beneath a seismically chaotic frontal prism, but the majority of overlying hemipelagic mudstones is offscraped and imbricated at the trench axis as a result of plate boundary compression. A possible décollement is imaged as a seaward dipping reflection at landward part of the trench graben and was likely generated by loading and failure of underthrust sediments. Collectively, these analyses provide a structural framework for understanding sedimentary inputs and the localization of shallow plate boundary slip at the Japan Trench.

The damage is done: Low fault friction recorded in the damage zone of the shallow Japan Trench décollement

“…The frontal prism above the décollement contains imbricated, off-scraped sedimentary rocks that are faulted and folded [Chester et al, 2013a;Nakamura et al, 2013]. Imbricate faults in the prism are part of the plate-boundary system, Figure 8.…”

“…Hanging wall damage decay exponents are less than the footwall. This may be because the frontal prism has undergone more displacement (as suggested by tectonic reconstructions [Chester et al, 2013b;Nakamura et al, 2013]). If so, the hanging wall damage decay exponents for core-scale (Figure 6b) and microcopic ( Figure 6c) structure observations are roughly within the observed range of values for a displacement slightly greater than 3.2 km.…”

Outer-rise normal fault development and influence on near-trench décollement propagation along the Japan Trench, off Tohoku

“…However, the large grid spacing between regional lines will mask small-scale trends. Because the Japan Trench accretes a large portion of sediment at the toe (Figure 7; Kodaira et al 2012;Nakamura et al 2013), sediment variations of 600 m may still affect prism morphology because they are a large percentage of the total sediment thickness. However, due to the fluctuations in sediment thickness and lack of regional trends, prism morphology and growth in our survey area owing to the incoming sediments is difficult to constrain.…”

“…Seismic sections of a near-trench subducting graben (Figure 7; Kodaira et al 2012;Nakamura et al 2013) reveal a low-angle, seaward-dipping reflector descending into the graben from the adjacent horst block. Imbricate structures sole into the same plane, which is thus interpreted to be the décollement.…”

“…Differential bathymetry from before and after the earthquake also revealed the largest vertical displacements near the trench axis , and seismic reflection data imaged frontal thrust development after the event . Seismic data collected in 2011 suggested that the décollement steps down into a local graben at the trench axis Nakamura et al 2013) rather than remaining at a constant depth above the overriding plate and 'bulldozing' the sediment from the top of the incoming horst blocks (e.g., Hilde 1983). Subduction of active bending-related normal faults may have larger implications for deformation of the near-trench upper plate and may be a small contributor to tsunamigenesis, with large seafloor displacements but small areal extent.…”