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.

S U M M A R YIn 1997, a seismic experiment using an airgun array and ocean bottom seismographs (OBSs) was performed in the forearc region of the northeastern Japan (NEJ) arc. The objectives of this experiment were to clarify seismic structures of the forearc region off Sanriku, Japan using airgun-OBS data and to understand the nature of the Japan trench seismogenic zone. Fundamental features of the structure are as follows:(i) The subduction angle becomes steeper from east to west, from 3 to 8 to 11 • , with the top of the plate located at depths of 11 km (40 km west from the trench), 21 km (120 km west from the trench) and 28 km (140 km west from the trench), respectively. (ii) The NEJ Moho beneath the forearc is defined at a depth of approximately 20 km. (iii) P-wave velocity of the island arc upper mantle is approximately 8 km s −1 . (iv) The P-wave velocity of the Cretaceous sedimentary layer exhibits significant lateral variability (4.0-5.5 km s −1 ) in the E-W direction.(v) The upper crust of NEJ forearc also has the significant lateral heterogeneity (5.0-6.3 km s −1 ) in the E-W direction, but not in the N-S direction. (vi) The middle crust defined on land extends in some locations 40 km east of the coastline. (vii) Lateral variability in the P-wave velocity of the lower crust (6.7-7.0 km s −1 ) is small compared with that of the upper crust.According to these results, it is suggested that the rupture areas of large earthquakes (>M7) in this region are limited within the contact zone between the NEJ arc crust and the subducting oceanic crust. Although a thin layer with low velocity is probably present within this contact zone, the serpentinization of uppermost mantle wedge of the NEJ arc by reaction between the mantle materials and the water from the thin layer beneath forearc region is limited. This might indicate that a significant volume of water and sediments subduct with the slab to depths of over 20 km.

In models of 1-segment lumbar fusion surgery, the positions of pedicle screws were closely linked with corresponding disc stresses and facet contact forces. However, even in cases of facet violation by pedicle screws, removal of the pedicle screw after fusion completion can reduce facet contract forces and disc stresses under both extension and torsional moments.

Inflammatory cytokines (IL-6, TNF-α, PGE2, and NO) seem to play a crucial role in hypertrophy and ossification of LF. Degenerated, herniated intervertebral disks, and facet arthrosis may influence LF through inflammatory cytokines and cause hypertrophy and ossification of LF.