rav. Science 255. 165 11 992). 5. D.'s. Fisher, M. P. A. ~j s h e r , '~. A. Huse, Phys. Rev. 5 4 3 , 130 (1991). 6. G. Blatter, M. V. Feiqel'man, V. B. Geshkenbein, A. I. Larkin, V. M. ~inokur, Rev. Mod. M. Lelovic, P. Kr~shnaraj, N. G. Eror, U. Balachandran, ibid. 242, 246 (1 995). 11. Q. Li, H. J. Wiesmann, M. Suenaga, L. Motow~dlo, P. Haldar, Appl. Phys. Lett. 66, 637 (1 995). 12. P. Majewski, Adv. Mater. 6, 593 (1 994). 13. The problem of thermally activated flux motion is less severe In the HTSC YBa, Cu, O, (YBCO) and hence it offers better intrinsic behav~or at high temperatures and magnetic fields. The processing strategies developqd for BSCCO fail to yield viable YBCO wires as a result of poor intergranular current flow. Recent work suggests, however, that good alignment between grains can be achieved in thick films deposited on nickel tapes by Ion beam depos~tion [X. D. Wu et a/., Appl. Phys. Lett. 67, 2397 (1 99511. The commercial viabil~ty of th~s strategy remains to be demonstrated. 14. D. R. Nelson and V. M. Vinokur, Phys. Rev. Lett. 68, 2398 (1 9 9 2 ) ; , Phys. Rev. 5 48,13060 (1 993). 15. T. Hwa, P. Le Doussal, D. R. Nelson, V. M. Vinokur, Phys. Rev. Lett. 71, 3545 (1 993). 16. L. C~vale et a/., ibid. 67, 648 (1991); M. Konczykowski et a/., Phys. Rev. 5 44, 7167 (1991); R. C. 995). 25. A recent report of carbon nanotube-BSCCO composites (24) showed some evidence of J, improvement; however, the J, value of both the reference and nanorod-conta~ning sample in this report were lower than the good-quality BSCCO samples reported previously and in the present study. It is thus difficult to conclude that there is an improvement in behav~or upon adding nanotubes. In addition, th~s report and our own stud~es show that few nanotubes survive the synthesis process, leaving in doubt their ab~lity to create well-defined columnar defects in the HTSCs. 982).32. Nanorod-HTSC composites have also been successfully prepared with TI Ba, Ca, Cu, O, and T12Ba2Ca,Cu,010 materials. Preliminary measurements show that there are significant enhancements in J, for these composites (P. Yang and C. M. Lieber, unpublished results). . 34. The actual density of columnar defects that can pin flux lines may be larger than that corresponding to the dens~ty of MgO nanorods; that is, lattice strains associated w~th nanorod-BSCCO interfaces can lead to dislocations and other correlated defects that exhibit columnarlike pinning behavior. 35. The density of nanorods oriented close to the c axis was about 1 x 101° cm-'; a s~m~lar dens~ty was determined for nanorods oriented In the ab plane. Although this density is sign~ficantly lower than that obtained by heavy-ion and proton irradiation, we have not tr~ed to maximize the dens~ty of MgO nanorods and also believe that the density of correlated defects is probably significantly higher than that of nanorods (34). 36. C. P. Bean, Rev. Mod. Phys. 36, 31 (1964). 37. An inverse dependence of J, on defect size was also reported previously for Y,BaCuO, inclus~ons of 1 to 10 p m in diameter in YBC...

The boundaries of three major plates (Africa, India, and Antarctica) meet in a triple junction in the Indian Ocean near 25°S, 70°E. Using observed bathymetry and magnetic anomalies, we locate the junction to within 5 km and show that it is a ridge‐ridge‐ridge type. Relative plate motion is N60°E at 50 mm/yr (full rate) across the Central Indian Ridge, N47°E at 60 mm/yr across the Southeast Indian Ridge, and N3°W at 15 mm/yr across the Southwest Indian Ridge; the observed velocity triangle is closed. Poles of instantaneous relative plate motion are determined for all plate pairs. The data in the South Atlantic and Indian oceans are consistent with a rigid African plate without significant internal deformation. Two of the ridges at the triple junction are normal midocean spreading centers with well‐defined median valleys. The Southwest Indian Ridge, however, has a peculiar morphology near the triple junction, that of an elongate triangular deep, with the triple junction at its apex. The floor of the deep represents crust formed at the Southwest Indian Ridge, and the morphology is a consequence of the evolution of the triple junction and is similar to that at the Galapagos Triple Junction. Though one cannot determine with precision the stability conditions at the triple junction, the development of the junction over the last 10 m.y. can be mapped, and the topographic expressions of the triple junction traces may be detected on the three plates.

No abstract

Abstract. Hole 504B provides the only opportunity to directly correlate seismic velocity structure to the lithology and physical properties of upper ocean crust, providing a baseline for comparison with seismic measurements elsewhere. We determine P and S velocities from vertical seismic profiles (

Recent drilling on the crest of the Blake Ridge during Ocean Drilling Program Leg 164 has provided an opportunity to compare estimates of attenuation from seismic data with direct samples of hydrate and gas in this region with the objective of using attenuation to remotely quantify hydrate and gas. Hydrate formation at the sediment grain contacts rather than in the pore spaces may significantly decrease the seismic attenuation. Because attenuation may be estimated from single-channel data, it would be more useful in hydrate detection than velocity, which requires more expensive multichannel data. In this analysis both single-channel seismic data and vertical seismic profile (VSP) data were inverted using a spectral modeling technique. For the single-channel data, this was performed using a simulated annealing algorithm, and for VSP data, the model updates were implemented manually. The results from both independent data sets are consistent with each other in each of the hydrate-and gas-stability zones. Values of the quality factor Q for hydrate-bearing sediments fall within the range expected of nonhydratebearing, fine-grained marine sediments, ranging from ~90 to 600, suggesting that small amounts of hydrate do not significantly affect Q. All values of Q less than ~90 were associated with gassy sediments; some were as low as Q = 6. Q within the hydratestability field changes systematically, reaching a minimum directly below the ridge crest. As expected, Q in the gassy sediments appears to correlate inversely with reflection strength.