Kiyokazu Oohashi

Pelagic smectite as an important factor in tsunamigenic slip along the Japan Trench

The very large slip on the shallow portion of the subduction interface during the 2011 Tohoku-oki earthquake (M w 9.0) caused a huge tsunami along the northeast coast of Honshu, Japan. In order to elucidate the mechanics of such tsunamigenic slip, the Integrated Ocean Drilling Program Expedition 343 (Japan Trench Fast Drilling Project, JFAST), was carried out one year after the earthquake and succeeded in recovering rocks constituting the active plate boundary fault. Our mineralogical analyses using X-ray diffraction reveal that the shallow portion of the fault zone that caused the earthquake is significantly enriched in smectite compared to the surrounding sediments, which may be intimately linked to the tsunamigenic shallow faulting. For comparison, we also analyzed mineralogical features of incoming sediments just prior to subduction, recovered on the outer rise of the Japan Trench (Site 436, Deep Sea Drilling Project Leg 56), and found a characteristic smectite-rich horizon in the uppermost ∼5 m of the pelagic clay layer. This horizon should be mechanically weak and will become the future plate boundary fault, as observed in the JFAST cores. The smectite-rich deposits are broadly distributed in the northwestern Pacific Ocean, and may therefore potentially enhance conditions for large shallow slip during earthquakes that occur over a broad area of the Japan Trench plate boundary, which would result in large tsunamis for this region.

Dynamic weakening of smectite‐bearing faults at intermediate velocities: Implications for subduction zone earthquakes

The hydrous clay mineral smectite, which is pervasive in sediments on subducting oceanic plates, is thought to weaken and stabilize subduction thrust faults. However, these frictional properties of smectite alone cannot explain the large coseismic slip in the vicinity of a trench. Here we performed friction experiments to demonstrate the rate dependence of friction at slip rates from 30 µm/s to 1.3 m/s for water-saturated smectite-quartz mixtures with various smectite contents, so as to shed light on the frictional response of smectite-bearing faults at intermediate to high slip rates. At slip rates of 30 to 150 µm/s, the friction coefficients decreased gradually from 0.5–0.6 to 0.1 with an increase in smectite content from 20 to 50 wt %. In contrast, at slip rates higher than 1.3 mm/s, friction exhibited marked slip weakening, resulting in low friction coefficients of 0.1–0.05, even for low smectite contents (roughly <30 wt %). Drastic slip weakening occurred at smectite contents of 10–30 wt % at slip rates of ~10 mm/s, which is 1 to 2 orders of magnitude lower than the slip rate at which slip weakening was observed in previous experiments on various rock types. The intermediate velocity weakening could be attributed to a rise in pore pressure caused by both shear-enhanced compaction and microscopic thermal pressurization of pore fluids. This process could weaken the fault even below seismic slip rates, leading to an acceleration of fault motion and potentially facilitating large coseismic slip and a stress drop in the vicinity of a trench.

Carbon-forming reactions under a reducing atmosphere during seismic fault slip

Graphite is a well-known solid lubricant and can be as important as clay minerals in reducing the frictional strength of faults. Some natural fault zones contain carbonaceous material (CM) even where host rocks do not contain it, and seismic fault motion can promote the graphitization of low-grade CM. Thus, the origin of CM in fault zones is an important issue in fault mechanics. Previous high-velocity friction experiments have revealed various chemical reactions in fault zones during seismic fault motion, but most experiments have been conducted in an atmosphere under oxic conditions. Here we report experimental results on Carrara marble (free of CM), conducted under N2 or H2 atmospheres at a slip rate of 1.3 m/s and normal stresses of 2.0–3.1 MPa. A small amount of blackish material formed in generated gouge only under reducing conditions with the H2 atmosphere, and Raman spectroscopic analysis revealed the presence of CM (amorphous carbon) in the material. The CM is attributable to (1) the generation and pyrolitic dissociation of CH4, and/or (2) a reduction reaction of emitted CO2 due to calcite decomposition. We confirmed the formation of CH4 using gas chromatography. The CM produced in experiments resembles CM in the Nojima fault (Japan) gouge in terms of Raman spectra. The granitic host rock of this fault is free of CM, and calcite is precipitated close to the CM; therefore, the CM probably formed through processes similar to those simulated in our experiments. Future research should investigate the amount and origin of CM in natural fault zones.

Stress State in the Kumano Basin and in Slope Sediment Determined From Anelastic Strain Recovery: Results From IODP Expedition 338 to the Nankai Trough

Three-dimensional, in situ stresses in the Kumano Basin and slope sediment (IODP Sites C0002 and C0022) in the Nankai Trough, southwest Japan, have been determined using the anelastic strain recovery (ASR) of core samples. Two samples taken from Hole C0002J, located in the bottom of the Kumano Basin, indicate that the maximum principal stress, σ1, is vertical. The intermediate principal stress, σ2, is oriented ENE–WSW, parallel to the trench axis. These stress orientations are similar to those obtained using ASR and borehole breakout methods in previous expeditions. In contrast, a sample from the lower section of the slope sediment (Hole C0022B), located beneath the megasplay fault, is characterized by σ1 plunging moderately to the ESE and σ3 oriented near-horizontally, trending NNE–SSW. The direction of maximum horizontal stress obtained from ASR (WNW–ESE) is similar to that inferred from borehole breakouts in an adjacent hole (NW–SE). Trench-normal compression and a near-vertical σ2 are also inferred from focal mechanisms of very-low-frequency earthquakes within the Nankai accretionary prism, and from borehole breakouts in the hanging wall of the megasplay fault. These observations suggest that the horizontal compressional regime extends to a shallower level than previously thought, likely due to the shallow portion of the megasplay fault accumulating tectonic stress in response to plate convergence.

A structural traverse across the Shimanto belt in western Shikoku, Japan

The Cretaceous and Tertiary Shimanto accretionary complex is largely characterized by imbricated thrust slices of trench-fill and ocean-floor sediments, and is thought as an ancient analog of the Nankai accretionary prism. Recent studies on a thermal structure and fault rock analysis for the Shimanto accretionary complex in the central and eastern Shikoku revealed that it has suffered earthquake faulting along the out-of-sequence thrusts associated with tectonic uplift. However, special distributions of thermal and tectonic structures are remaining unclear since those in the western part of Shikoku are poorly understood. In the presentation, we demonstrate the distributions and details of deformed rocks (e.g. melange and brittle faults), geological structure, and vitrinite reflectance across the Shimanto belt in western Shikoku.