Kohtaro Ujiie

New insights into deformation and fluid flow processes in the Nankai Trough accretionary prism: Results of Ocean Drilling Program Leg 190

Moore, G. F., Taira, A., Klaus, A., Becker, L., Boeckel, B., Cragg, B. A., Dean, A., Fergusson, C. L., Henry, P., Hirano, S., Hisamitsu, T. et al. (2001). New insights into deformation and fluid flow processes in the Nankai Trough accretionary prism: Results of Ocean Drilling Program Leg 190. Geochemistry, Geophysics, Geosystems, 2, Article No: 2001GC000166.

Low Coseismic Shear Stress on the Tohoku-Oki Megathrust Determined from Laboratory Experiments

Deep Drilling for Earthquake Clues The 2011 Mw 9.0 Tohoku-Oki earthquake and tsunami were remarkable in many regards, including the rupturing of shallow trench sediments with huge associated slip (see the Perspective by Wang and Kinoshita). The Japan Trench Fast Drilling Project rapid response drilling expedition sought to sample and monitor the fault zone directly through a series of boreholes. Chester et al. (p. 1208) describe the structure and composition of the thin fault zone, which is predominately comprised of weak clay-rich sediments. Using these same fault-zone materials, Ujiie et al. (p. 1211) performed high-velocity frictional experiments to determine the physical controls on the large slip that occurred during the earthquake. Finally, Fulton et al. (p. 1214) measured in situ temperature anomalies across the fault zone for 9 months, establishing a baseline for frictional resistance and stress during and following the earthquake. The Tohoku-Oki earthquake occurred along a thin, clay-rich fault zone in the basal strata of the subducting plate. Large coseismic slip was thought to be unlikely to occur on the shallow portions of plate-boundary thrusts, but the 11 March 2011 Tohoku-Oki earthquake [moment magnitude (Mw) = 9.0] produced huge displacements of ~50 meters near the Japan Trench with a resultant devastating tsunami. To investigate the mechanisms of the very large fault movements, we conducted high-velocity (1.3 meters per second) friction experiments on samples retrieved from the plate-boundary thrust associated with the earthquake. The results show a small stress drop with very low peak and steady-state shear stress. The very low shear stress can be attributed to the abundance of weak clay (smectite) and thermal pressurization effects, which can facilitate fault slip. This behavior provides an explanation for the huge shallow slip that occurred during the earthquake.

Structure and composition of the plate-boundary slip zone for the 2011 Tohoku-Oki earthquake.

Deep Drilling for Earthquake Clues The 2011 Mw 9.0 Tohoku-Oki earthquake and tsunami were remarkable in many regards, including the rupturing of shallow trench sediments with huge associated slip (see the Perspective by Wang and Kinoshita). The Japan Trench Fast Drilling Project rapid response drilling expedition sought to sample and monitor the fault zone directly through a series of boreholes. Chester et al. (p. 1208) describe the structure and composition of the thin fault zone, which is predominately comprised of weak clay-rich sediments. Using these same fault-zone materials, Ujiie et al. (p. 1211) performed high-velocity frictional experiments to determine the physical controls on the large slip that occurred during the earthquake. Finally, Fulton et al. (p. 1214) measured in situ temperature anomalies across the fault zone for 9 months, establishing a baseline for frictional resistance and stress during and following the earthquake. The Tohoku-Oki earthquake occurred along a thin, clay-rich fault zone in the basal strata of the subducting plate. The mechanics of great subduction earthquakes are influenced by the frictional properties, structure, and composition of the plate-boundary fault. We present observations of the structure and composition of the shallow source fault of the 2011 Tohoku-Oki earthquake and tsunami from boreholes drilled by the Integrated Ocean Drilling Program Expedition 343 and 343T. Logging-while-drilling and core-sample observations show a single major plate-boundary fault accommodated the large slip of the Tohoku-Oki earthquake rupture, as well as nearly all the cumulative interplate motion at the drill site. The localization of deformation onto a limited thickness (less than 5 meters) of pelagic clay is the defining characteristic of the shallow earthquake fault, suggesting that the pelagic clay may be a regionally important control on tsunamigenic earthquakes.

Seismic slip propagation to the updip end of plate boundary subduction interface faults: Vitrinite reflectance geothermometry on Integrated Ocean Drilling Program NanTro SEIZE cores

Seismic faulting along subduction-type plate boundaries plays a fundamental role in tsunami genesis. During the Integrated Ocean Drilling Program (IODP) Nankai Trough Seismogenic Zone Experiment (NanTro SEIZE) Stage 1, the updip ends of plate boundary subduction faults were drilled and cored in the Nankai Trough (offshore Japan), where repeated large earthquakes and tsunamis have occurred, including the A.D. 1944 Tonankai (Mw = 8.1) earthquake. Samples were obtained from the frontal thrust, which connects the deep plate boundary to the seafloor at the toe of the accretionary wedge, and from a megasplay fault that branches from the plate boundary decollement. The toe of the accretionary wedge has classically been considered aseismic, but vitrinite reflectance geothermometry reveals that the two examined fault zones underwent localized temperatures of more than 380 °C. This suggests that frictional heating occurred along these two fault zones, and implies that coseismic slip must have propagated at least one time to the updip end of the megasplay fault and to the toe of the accretionary wedge.

Stress State in the Largest Displacement Area of the 2011 Tohoku-Oki Earthquake

Stressed Out Large seismic events such as the 2011 magnitude 9.0 Tohoku-Oki earthquake can have profound effects not just on the severity of ground motion and tsunami generation, but also on the overall state of the crust in the surrounding regions. Lin et al. (p. 687) analyzed the stress 1 year after the Tohoku-Oki earthquake and compared it with the estimated stress state before the earthquake. In situ resistivity images were analyzed from three boreholes drilled into the crust across the plate interface where the earthquake occurred. Stress values indicate a nearly complete drop in stress following the earthquake such that the type of faulting above the plate boundary has changed substantially. These findings are consistent with observations that the sea floor moved nearly 50 meters during the earthquake. Borehole stress measurements indicate a nearly total stress drop in the region of largest slip. The 2011 moment magnitude 9.0 Tohoku-Oki earthquake produced a maximum coseismic slip of more than 50 meters near the Japan trench, which could result in a completely reduced stress state in the region. We tested this hypothesis by determining the in situ stress state of the frontal prism from boreholes drilled by the Integrated Ocean Drilling Program approximately 1 year after the earthquake and by inferring the pre-earthquake stress state. On the basis of the horizontal stress orientations and magnitudes estimated from borehole breakouts and the increase in coseismic displacement during propagation of the rupture to the trench axis, in situ horizontal stress decreased during the earthquake. The stress change suggests an active slip of the frontal plate interface, which is consistent with coseismic fault weakening and a nearly total stress drop.

Rapid pulses of uplift, subsidence, and subduction erosion offshore Central America: Implications for building the rock record of convergent margins

Integrated Ocean Drilling Program (IODP) Expedition 334 to southern Costa Rica, Central America, documented unprecedented subduction erosion in an area of active seismic slip. Widespread subduction erosion of the upper plate initiated when the Cocos Ridge, an overthickened aseismic ridge, arrived at the Middle America Trench. Subduction erosion was coeval with the rapid formation of deposition centers on the forearc of the upper plate. The completely recovered shelf sequence constrains a short (<2 m.y.) interval of extreme subsidence (∼1200 m) with a rapid pulse occurring during the first ∼0.3 m.y. This event removed an estimated 1.2 × 106 km3 of forearc material at a rate of ∼1690 km3/m.y./km of trench during a time of rapid (∼1035 m/m.y.) shelf sediment accumulation. At this erosive margin, a sediment-starved trench persisted, in spite of abundant sediment supply, because subduction erosion led to the creation of forearc basins. Similar rapid pulses of subduction erosion may punctuate the evolution of many margins, contributing disproportionately to crustal recycling at subduction zones with implications for the evolution of continental crust and mountain belts, and recycling of continental material into the mantle.

Evolution and kinematics of an ancient décollement zone, mélange in the Shimanto accretionary complex of Okinawa Island, Ryukyu Arc

Detailed structural analysis of the melange in the Shimanto accretionary complex of Okinawa Island in the Ryukyu Arc elucidates the spatial distribution and temporal progression of decollement-related deformation. Early deformation took place in the footwall of the decollement and is characterized by subhorizontal layer-parallel extension and subsequent heterogeneous shear of partially lithified sediments, resulting in alternation of asymmetric (e.g. S–C fabrics) and layer-parallel extensional (boudined layers) fabrics. Mud intrusions suggest that high fluid pressure was locally generated in the footwall of the decollement. Late deformation is marked by the partitioning of deformation in the melange: the lower structural level of the melange was pervasively sheared along microfaults whereas the upper structural level was flattened with development of planar pressure solution cleavage. Because shear is concentrated in the lower structural level, late deformation is considered to have occurred in the hanging wall of the decollement. Kinematic indicators suggest that shear directions during decollement-related deformation reflect the relative plate motion during early Tertiary. After the melange was incorporated into the accretionary prism, crenulation cleavage and cylindrical upright folds were formed under subhorizontal shortening.

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.

Progressive illitization in fault gouge caused by seismic slip propagation along a megasplay fault in the Nankai Trough

The question of whether coseismic ruptures along megasplay faults in accretionary prisms (i.e., large landward-dipping thrust faults branching from the plate boundary) reach the seafloor is critical for assessing the risk of tsunami disaster. However, samples from active megasplay faults have not previously been available. Here we present geochemical and mineralogical data of megasplay fault samples obtained from the shallow (

Off‐scraping accretionary process under the subduction of young oceanic crust: The Shimanto Belt of Okinawa Island, Ryukyu Arc

The Eocene Kayo Formation exposed at the southwestern end of the Shimanto Belt in the Ryukyu Arc records the progressive development of off-scraping accretion accompanied by increase in lithification and burial as well as synaccretionary thermal imprint. The formation consists almost wholly of coherent trench turbidites supplied by axial currents. Detailed structural analysis has revealed that two successive deformation stages are recognized in the Formation. The early deformation (stage 1) occurred by horizontal shortening related to the initial stage of off-scraping accretion. In this stage, sands were water-rich and nearly unlithified, and the competence contrast between sandstone and shale was low. The late deformation (stage 2) is marked by the formation of seaward verging imbricate fold-and-thrust system within the accretionary wedge. Compared with stage 1, sands in stage 2 were more dewatered and lithified, and the competence contrast between sandstone and shale increased. In accordance with this deformation history, structures caused by high pore fluid pressure indicate increased cohesion of sands during off-scraping accretion. Studies of the vitrinite reflectance and the illite crystallinity clearly suggest that the Formation became buried and suffered thermal imprint; pressure solution cleavage developed as an axial planar to the stage 2 folds. Thermal imprint on the Formation is characterized by high paleotemperatures at approximately 250°C–300°C in a region of the prehnite-pumpellyite facies metamorphism. This thermal event took place during and slightly after the stage 2 probably associated with subduction of a young and hot oceanic plate.