Megasplay faults, very long thrust faults that rise from the subduction plate boundary megathrust and intersect the sea floor at the landward edge of the accretionary prism, are thought to play a role in tsunami genesis. We imaged a megasplay thrust system along the Nankai Trough in three dimensions, which allowed us to map the splay fault geometry and its lateral continuity. The megasplay is continuous from the main plate interface fault upwards to the sea floor, where it cuts older thrust slices of the frontal accretionary prism. The thrust geometry and evidence of large-scale slumping of surficial sediments show that the fault is active and that the activity has evolved toward the landward direction with time, contrary to the usual seaward progression of accretionary thrusts. The megasplay fault has progressively steepened, substantially increasing the potential for vertical uplift of the sea floor with slip. We conclude that slip on the megasplay fault most likely contributed to generating devastating historic tsunamis, such as the 1944 moment magnitude 8.1 Tonankai event, and it is this geometry that makes this margin and others like it particularly prone to tsunami genesis.

We describe a group of alloys that exhibit "super" properties, such as ultralow elastic modulus, ultrahigh strength, super elasticity, and super plasticity, at room temperature and that show Elinvar and Invar behavior. These "super" properties are attributable to a dislocation-free plastic deformation mechanism. In cold-worked alloys, this mechanism forms elastic strain fields of hierarchical structure that range in size from the nanometer scale to several tens of micrometers. The resultant elastic strain energy leads to a number of enhanced material properties.Mechanical properties, such as strength, of metallic materials are strongly affected by metallurgical processes such as heat treatment and plastic working, which bring modifications in the microstructure. On the other hand, these processes have no substantial effect on physical properties such as elastic modulus and thermal expansion. The reason for this is that the changes that can be affected by plastic working and heat treatment do not extend to interatomic bonds or electronic states.We present a group of alloys that exhibit multiple "super" properties and drastic changes in physical properties after plastic working at room temperature. These alloys simultaneously offer super elasticity, super strength, super coldworkability, and Invar and Elinvar properties. The alloys consist of Group IVa and Va elements and oxygen and share the following three electronic magic numbers: (i) a compositional average valence electron number [electron/atom (e/a) ratio] of about 4.24; (ii) a bond order (Bo value) of about 2.87 based on the DV-X␣ cluster method, which represents the bonding strength (1-3); and (iii) a "d" electron-orbital energy level (Md value) of about 2.45 eV, representing electronegativity. The properties emerge only when all three of these magic numbers are satisfied simultaneously. Various alloy composition combinations meet these criteria, such as Ti-12Ta-9Nb-3V-6Zr-O and Ti23Nb-0.7Ta-2Zr-O [mole percent (mol %)], wherein each alloy has a simple body-centered cubic (bcc) crystal structure. In order to exhibit these properties, each alloy system requires substantial cold working and the presence of a certain amount of oxygen, restricted to an oxygen concentration of 0.7 to 3.0 mol %.Typical properties of the alloys are shown in Fig. 1 for samples before and after cold swaging with 90% reduction in area (4). Tensile stress-strain curves shown in Fig. 1A indicate that cold working substantially decreases the elastic modulus and increases the yield strength and confirm nonlinearity in the elastic range, with the gradient of each curve decreasing continuously to about 1/3 its original value near the elastic limit. As a result of this decrease in elastic modulus and nonlinearity, elastic deformability after cold working reaches 2.5%, which is at least double the value before cold working. Generally, large elastic deformations that occur in so-called "super-elastic alloys" are known to be reversible martensitic transformations resulting from deformation, d...

New multichannel seismic reflection data collected over the Nankai Trough image the accretionary complex in two areas: the International Program of Ocean Drilling leg 87 transect area (western area) and the region of upcoming Ocean Drilling Program leg 131 (eastern area). The incoming Shikoku Basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites. The basin section is overlain by a trench wedge that is 12-16 km wide and 350-750 m thick at the thrust front. Accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge. The zone in the western area is 4.5 km wide and is characterized by "kink" folds; the zone in the eastern area is only 2.5 km wide and does not exhibit such folds. The frontal thmsts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section. The overthmsting sediments form fault-bend folds over these ramps. Thrust spacing at the toe of the slope is 1.5-2.5 km. The second thrust cuts up from an inferred dtcollement within the Shikoku Basin sedimentary section. In the eastern area, a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6.3 km seaward of the thrust front and underlies the entire protothmst zone. This reflector continues with reversed polarity under the accretionary complex and is at the level of the basal dtcollement. The underlying basin pelagic section is apparently thmst undisturbed beneath the accretionary prism. The reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface. This decrease in velocity and density may indicate that the dtcollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism. The reflections from the first and second thmsts are also reversed polarity, possibly indicating that they also are pathways of fluid expulsion. The critical wedge taper of the westem area is greater than that of the eastern area, an observation that is consistent with the existence of an overpressured dtcollement in the eastern area. MOOREET AL.: STRU• OFTHEN••OU• 8755 Am., 66, 117-133, 1986.

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A newly discovered group of alloys, called Gum Metals, approaches ideal strength in bulk form, exhibits significant plastic deformation prior to failure, and shows no indications of conventional-dislocation activity. Two conditions must be met for a material to exhibit this "ideal" behavior: (1) the stress required to trigger conventional-dislocation plasticity in the material must exceed its ideal strength, and (2) the material must be intrinsically ductile when stressed to ideal strength. Gum Metals satisfy both criteria, explaining their remarkable mechanical properties.

We documented regional and local variations in basement relief, sediment thickness, and sediment type in the Shikoku Basin, northern Philippine Sea Plate, which is subducting at the Nankai Trough. Seismic reflection data, tied with ocean drilling program drill cores, reveal that variations in the incoming sediment sequences are correlated with basement topography. We mapped the three-dimensional seismic facies distribution and measured representative seismic sequences and units. Trench-parallel seismic profiles show three regional provinces in the Shikoku Basin that are distinguished by the magnitude of basement relief and sediment thickness: Western (<200-400 m basement relief, >600 m sediment thickness), Central (>1500 m relief,~2000 m sediments), and Eastern (<600 m relief,~1200 m sediments) provinces. The total thickness of sediment in basement lows is as much as six times greater than that over basement highs. Turbidite sedimentation in the Shikoku Basin reflects basement control on deposition, leading to the local presence or absence of turbidite units deposited during the middle Oligocene to the middle Miocene. During the first phase of sedimentation, most basement lows were filled with turbidites, resulting in smooth seafloor morphology that does not reflect basement relief. A second phase of turbidite deposition in the Eastern Province was accompanied by significant amounts of hemipelagic sediments interbedded with turbidite layers compared to the other provinces because of its close proximity to the Izu-Bonin Island Arc. Both regional and local variations in basement topography and sediment thickness/type have caused lateral heterogeneities on the underthrusting plate that will, in turn, influence lateral fluid flow along the Nankai accretionary prism.

Three-dimensional seismic refl ection data reveal the presence of a low seismic velocity zone (LVZ) with weak refl ectivity character along the Nankai accretionary prism. This LVZ is intercalated between an upper, offscraped layer and a lower, underthrusting layer in the outer accretionary wedge. Wide-angle ocean bottom seismograph data also support the presence of the LVZ, which is estimated to be a maximum of ~2 km thick, ~15 km wide, and ~120 km long. The LVZ could be an underthrust package underplated in response to the lateral growth of the Nankai accretionary prism. Underplating of the underthrusting layer beneath the overlying offscraped layer would maintain a critical taper of the accretionary prism so that the offscraped layer can continue to grow seaward. The LVZ could have elevated fl uid pressure, leading to rigidity reduction of the entire outer accretionary wedge. The rigidity-lowered outer wedge, containing the LVZ, may be more easily uplifted and thus eventually foster tsunami generation during a Nankai megathrust earthquake. If the fl uid-rich LVZ supplies a signifi cant amount of the fl uid to the megasplay fault zone at depth, it may affect stick-slip behavior of the fault.