letters to nature value of roughly 1 m is suggested if the area of rupture extended 2. Embley, R. W., Chadwick, W. W., Jonasson, I. R., Butterfield, D. A. & Baker, E. T. Initial results of the over the 3-km thickness of the crust that has been observed to rapid response to the 1993 CoAxial event: Relationships between hydrothermal and volcanic processes. Geophys. Res. Lett. 22, 143–146 (1995). 20 host microseismicity , and along the 30-km length of the axis 3. Von Damm, K. L. et al. Evolution of East Pacific Rise hydrothermal vent fluids following a volcanic defined by the concentration of seismicity seen in Figs 1 and 3. A eruption. Nature 395, 47–50 (1995). value of 0.5 m is suggested if rupture penetrated the full thickness of 4. Dziak, R. P., Fox, C. G. & Schreiner, A. E. The June-July 1993 seismoacoustic event at CoAxial Segment, Juan de Fuca Ridge: Evidence for a lateral dike injection. Geophys. Res. Lett. 22, 135–138 (1995). the crust. 5. Sohn, R. A., Fornari, D. J., Von Damm, K. L., Hildebrand, J. A. & Webb, S. C. Seismic and Although the general character of the predicted ‘slug-test’ tran- hydrothermal evidence of a cracking event on the East Pacific Rise crest near 98 50 0 N. Nature 396, sient (Fig. 4) is remarkably similar to that of the observed transient 159–161 (1998). (Fig. 2), the results must be considered with appropriate caution, 6. Delaney, J. R. et al. The quantum event of ocean crustal accretion: Impacts of diking at mid-ocean ridges. Science 281, 222–230 (1998). because there are large uncertainties in applying such a simple 7. Dziak, R. P. & Fox, C. G. The January 1998 earthquake swarm at Axial Volcano, Juan de Fuca Ridge: model. For example, (1) the model considers only lateral diffusion. Hydroacoustic evidence of seafloor volcanic activity. Geophys. Res. Lett. 26, 3429–3432 (1999). Ventilation through the sediment section or through faults and 8. Lupton, J. E., Baker, E. T. & Massoth, G. J. Helium, heat, and the generation of hydrothermal event permeable basement outcrops (as reflected by the year-long recov- plumes at mid-ocean ridges. Earth Planet. Sci. Lett. 171, 343–350 (1999). 9. Johnson, H. P. et al. Earthquake-induced changes in a hydrothermal system at the Endeavour ery to the normal formation state, established by the thermal and Segment, Juan de Fuca Ridge. Nature 407, 174–177 (2000). hydrologic structure of the valley13) will ‘dilute’ the magnitude of 10. Lilley, M. D., Butterfield, D. A., Lupton, J. E. & Olson, E. J. Magmatic events can produce rapid changes the initial transient, and vertical diffusion from any extension of the in hydrothermal vent chemistry. Nature 422, 878–881 (2003). 11. Davis, E. E., Wang, K., Thomson, R. E., Becker, K. & Cassidy, J. F. An episode of seafloor spreading and dilatant volume below the transmissive uppermost crust will add to associated plate deformation inferred from crustal fluid pressure transients. J. Geophys. Res. 106, and stretch the source signal. (2) Considering the dilatation to be 21953–21963 (2001). concentrated as a vertical line-source ‘implosion’ is unquestionably 12. Davis, E. E., Becker, K., Pettigrew, T., Carson, B. & Macdonald, R. CORK: A hydrologic seal and to oversimplify. Distributed dilatancy along the axis of the valley will downhole observatory for deep-ocean boreholes. Proc. ODP Init. Rep. 139, 43–53 (1992). 13. Davis, E. E. & Becker, K. Formation temperatures and pressures in a sedimented rift hydrothermal create diffusional directivity (enhancing propagation across strike), system: Ten months of CORK observations, Holes 857D and 858G. Proc. ODP Sci. Res. 139, 649–666 and reduce the effective range between the source and the obser- (1994). vation site along axis. (3) Perhaps the greatest shortcoming of the 14. Jonsson, S., Segall, P., Pedersen, R. & Bjornsson, G. Post-earthquake ground movements correlated to slug-test analogue is that it does not consider the initial distribution pore-pressure transients. Nature 424, 179–183 (2003). 15. Okada, Y. Internal deformation due to shear and tensile faults in a half space. Bull. Seismol. Soc. Am. of elastic strain associated with the spreading event, in which 82, 1018–1040 (1992). quadrants of contraction (and hence volumes of positive anomalous 16. Cooper, H. H. Jr, Bredehoeft, J. D. & Papadopulos, I. S. Response of a finite diameter well to an pressure) lie next to quadrants of dilatation. Diffusion between instantaneous charge of water. Wat. Resour. Res. 3, 263–269 (1967). 17. Becker, K., Morin, R. H. & Davis, E. E. Permeabilities in the Middle Valley hydrothermal system these quadrants will produce an ‘annihilation’ effect near nodal measured with packer and flowmeter experiments. Proc. ODP. Sci. Res. 139, 613–626 (1994). planes and complicate any pressure transient generated by net 18. Bessler, J. U., Smith, L. & Davis, E. E. Hydrologic and thermal conditions at a sediment/basalt dilatation. Despite these uncertainties, however, the simple interface: Implications for interpretation of field measurements at Middle Valley. Proc. ODP Sci. Res. concentrated-source model provides a useful first-order tool for 139, 667–678 (1994). 19. Davis, E. E. & Villinger, H. Tectonic and thermal structure of the Middle Valley sedimented rift, estimating net dilatation and the hydraulic transmission properties northern Juan de Fuca Ridge. Proc. ODP Init. Rep. 139, 9–41 (1992). of the formation, and for guiding a more complete analysis that will 20. Golden, C. E., Webb, S. C. & Sohn, R. A. Hydrothermal microearthquake swarms beneath active vents take distributed co-seismic deformation and three-dimensional at Middle Valley, northern Juan de Fuca Ridge. J. Geophys. Res. 108, doi:10.1029/2001JB000226 (2003). diffusion into account. 21. Wang, K. Applying fundamental principles and mathematical models to understand processes and estimate parameters. In Hydrogeology of the Oceanic Lithosphere (eds Davis, E. E. & Elderfield, H.) We note that the co- and post-seismic pressure transients observed (Cambridge Univ. Press, in the press). in ODP Hole 857D represent a significant fraction (totalling nearly 22. Fox, C. G., Dziak, R. P.,Marumoto, H. & Schreiner, A. E. Potential for monitoring low-level seismicity 20%) of the deep-seated buoyancy that drives hydrothermal dis- on the Juan de Fuca Ridge using military hydrophone arrays. Mar. Technol. Soc. J. 27, 22–30 (1994). charge at a nearby vent field. A pressure differential of þ80 kPa was Acknowledgements We acknowledge the Ocean Drilling Program for ongoing support for long- measured across the sediment section in ODP Hole 858G, which was term borehole monitoring experiments, T. Pettigrew, R. Meldrum, and R. Macdonald for drilled 1.6 km to the north into a sediment-buried permeable base- engineering assistance, R. Thomson and S. Mihaley for assistance with the post-swarm water- ment edifice beneath the vent field, sealed, and instrumented at the column survey on board CHS Tully, M. Fowler for assistance with SoSUS data processing, and same time as ODP Hole 857D12,13. Unfortunately, the seals at this J. Risteau for earthquake moment tensor calculations. Financial support was provided by the US National Science Foundation and by the Geological Survey of Canada. high-temperature site failed long before the earthquake swarm, but it is reasonable to conclude that basement there experienced a similar Competing interests statement The authors declare that they have no competing financial co- and post-seismic reduction in pressure. The lack of any signs of interests. augmented hydrothermal discharge searched for during a water- column investigation of the valley carried out shortly after the swarm Correspondence and requests for materials should be addressed to E.E.D. ([email protected]). ended is consistent with this conclusion, as well as with our primary conclusion that the permeable crust at this site suffered net dilatancy at the time of the seismic activity. A Methods .............................................................. An outline of the relationships among strain, total stress, and pore fluid pressure in a poroelastic medium is provided in refs 11 and 21. The elastic properties involved are either Cladogenesis and morphological established by laboratory data (at conditions corresponding to the formation temperature and pressure of 280 8C and 30 MPa, respectively), by observations of cored material, or by diversification in passerine birds observed formation-fluid pressure response to tidal loading. These include: fluid 29 21 compressibility, b f ¼ 1.3 £ 10 Pa ; solid constituent compressibility, 211 21 Robert E. Ricklefs b s ¼ 2.0 £ 10 Pa ; one-dimensional tidal loading efficiency, g ¼ 0.14; matrix frame 211 21 compressibility, b m ¼ 6.8 £ 10 Pa ; porosity, n ¼ 0.15; and Poisson’s ratio, n ¼ 0.25. 29 21 Department of Biology, University of Missouri-St Louis, 8001 Natural Bridge The resulting linear coefficient relating strain and fluid pressure is 0.29 £ 10 Pa . The relationship between permeability k and hydraulic diffusivity h involves a subset of these Road, St Louis, Missouri 63121-4499, USA properties plus viscosity m ¼ 1024 Pa s (appropriate for the formation temperature): ............................................................................................................................................................................. k ¼ myh, where y, the storage compressibility, is a function of n, b f, b s and b m as outlined Morphological diversity tends to increase within evolving in ref. 21. lineages over time1,2, but the relative roles of gradual evolution- 3 Received 15 January; accepted 14 June 2004; doi:10.1038/nature02755. ary change (anagenesis) and abrupt shifts associated with 4 1. Baker, E. T. et al. Hydrothermal event plumes from the CoAxial seafloor eruption site, Juan de Fuca speciation events (cladogenesis, or ‘punctuated equilibrium’) 5 Ridge. Geophys. Res. Lett. 22, 151–154 (1995). have not been resolved for most groups of organisms . However, 338 © 2004 Nature Publishing Group NATURE | VOL 430 | 15 JULY 2004 | www.nature.com/nature letters to nature these two modes of evolution can be distinguished by the fact that multiple regressions.