Surface meltwater that reaches the base of an ice sheet creates a mechanism for the rapid response of ice flow to climate change. The process whereby such a pathway is created through thick, cold ice has not, however, been previously observed. We describe the rapid (<2 hours) drainage of a large supraglacial lake down 980 meters through to the bed of the Greenland Ice Sheet initiated by water-driven fracture propagation evolving into moulin flow. Drainage coincided with increased seismicity, transient acceleration, ice-sheet uplift, and horizontal displacement. Subsidence and deceleration occurred over the subsequent 24 hours. The short-lived dynamic response suggests that an efficient drainage system dispersed the meltwater subglacially. The integrated effect of multiple lake drainages could explain the observed net regional summer ice speedup.

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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...

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.[1] Mid-ocean ridge transform faults (RTFs) vary strongly along strike in their ability to generate large earthquakes. This general observation suggests that local variations in material properties along RTFs exert a primary control on earthquake rupture dynamics. We explore these relationships by examining the seismic structure of two RTFs that have distinctly different seismic coupling. We determine the seismic velocity structure at the Gofar and Quebrada faults on the East Pacific Rise (EPR) using P wave traveltime tomography with data from two active-source wide-angle refraction lines crossing the faults. We image low-velocity zones (LVZs) at both faults, where P wave velocities are reduced by as much as 0.5-1.0 km/s ($10-20%) within a several kilometer wide region. At the Gofar fault, the LVZ extends through the entire crust, into the seismogenic zone. We rule out widespread serpentinization as an explanation for the low velocities, owing to the lack of a corresponding signal in the locally measured gravity field. The reduced velocities can be explained if the plate boundary region is composed of fault material with enhanced fluid-filled porosity (1.5-8%). Local seismic observations indicate that the high-porosity region lies within a $10 km long portion of the fault that fails in large swarms of microearthquakes and acts as a barrier to the propagation of large (M $ 6.0) earthquakes. Tomographic images of fault structure combined with observed earthquake behavior suggest that EPR transform segments capable of generating large earthquakes have relatively intact gabbro within the seismogenic zone, whereas segments that slip aseismically or via earthquake swarms are composed of highly fractured, ≥2 km wide damage zones that extend throughout the crust.

[1] The Cocos plate experiences extensional faulting as it bends into the Middle American Trench (MAT) west of Nicaragua, which may lead to hydration of the subducting mantle. To estimate the along strike variations of volatile input from the Cocos plate into the subduction zone, we gathered marine seismic refraction data with the R/V Marcus Langseth along a 396 km long trench parallel transect offshore of Nicaragua and Costa Rica. Our inversion of crustal and mantle seismic phases shows two notable features in the deep structure of the Cocos plate: (1) Normal oceanic crust of 6 km thickness from the East Pacific Rise (EPR) lies offshore Nicaragua, but offshore central Costa Rica we find oceanic crust from the northern flank of the Cocos Nazca (CN) spreading center with more complex seismic velocity structure and a thickness of 10 km. We attribute the unusual seismic structure offshore Costa Rica to the midplate volcanism in the vicinity of the Galápagos hot spot. (2) A decrease in Cocos plate mantle seismic velocities from ∼7.9 km/s offshore Nicoya Peninsula to ∼6.9 km/s offshore central Nicaragua correlates well with the northward increase in the degree of crustal faulting outboard of the MAT. The negative seismic velocity anomaly reaches a depth of ∼12 km beneath the Moho offshore Nicaragua, which suggests that larger amounts of water are stored deep in the subducting mantle lithosphere than previously thought. If most of the mantle low velocity zone can be interpreted as serpentinization, the amount of water stored in the Cocos plate offshore central Nicaragua may be about 2.5 times larger than offshore Nicoya Peninsula. Hydration of oceanic lithosphere at deep sea trenches may be the most important mechanism for the transfer of aqueous fluids to volcanic arcs and the deeper mantle.

[1] Seismic reflection and refraction data from the SE Greenland margin provide a detailed view of a volcanic rifted margin from Archean continental crust to near-toaverage oceanic crust over a spatial scale of 400 km. The SIGMA III transect, located $600 km south of the Greenland-Iceland Ridge and the presumed track of the Iceland hot spot, shows that the continent-ocean transition is abrupt and only a small amount of crustal thinning occurred prior to final breakup. Initially, 18.3 km thick crust accreted to the margin and the productivity decreased through time until a steady state ridge system was established that produced 8-10 km thick crust. Changes in the morphology of the basaltic extrusives provide evidence for vertical motions of the ridge system, which was close to sea level for at least 1 m.y. of subaerial spreading despite a reduction in productivity from 17 to 13.5 km thick crust over this time interval. This could be explained if a small component of active upwelling associated with thermal buoyancy from a modest thermal anomaly provided dynamic support to the rift system. The thermal anomaly must be exhaustible, consistent with recent suggestions that plume material was emplaced into a preexisting lithospheric thin spot as a thin sheet. Exhaustion of the thin sheet led to rapid subsidence of the spreading system and a change from subaerial, to shallow marine, and finally to deep marine extrusion in $2 m.y. is shown by the morphological changes. In addition, comparison to the conjugate Hatton Bank shows a clear asymmetry in the early accretion history of North Atlantic oceanic crust. Nearly double the volume of material was emplaced on the Greenland margin compared to Hatton Bank and may indicate east directed ridge migration during initial opening.