HR: 0800h
AN: T31B-0500    [Abstracts]
TI: Modeling Hydrothermal Response to Earthquakes at Mid-Ocean Ridges
AU: * Ramondenc, P
EM: pierre.ramondenc@ce.gatech.edu
AF: Georgia Institute of Technology, School of Civil and Environmental Engineering, Atlanta, GA 30332-0355 United States
AU: Germanovich, L N
EM: leonid.germanovich@ce.gatech.edu
AF: Georgia Institute of Technology, School of Civil and Environmental Engineering, Atlanta, GA 30332-0355 United States
AU: Lowell, R P
EM: bob.lowell@eas.gatech.edu
AF: Georgia Institute of Technology, School of Earth and Atmospheric Sciences, Atlanta, GA 30332-0340 United States
AB: Data from the 1995 EPR swarm [ Sohn et al., 1998] shows that hydrothermal discharge temperature suddenly increases within a few days subsequent to seismic activity, followed by a relatively rapid (~ months) decay to a level close to the pre-earthquake values, and finally a long-term (~ years) secular rise. Somewhat similar behavior has been detected after the 1999 JdFR swarm [ Johnson et al., 2000]. Existing models [ Wilcock, 2004] show that the spike in vent temperature some days following an earthquake can result from: (1) a step-temperature impulse at the base of the system, (2) increasing the fluid pressure at depth, or (3) increasing permeability in the upflow zone. While models (1) and (2) explain the temperature spike and the following drop, they do not explain the long-term rise. Conversely, approach (3) only applies to the latter case. Moreover, for EPR conditions, model (1) requires a fluid residence time in the upflow zone of less than an hour; and model (2) substantially underestimates the upflow zone permeability (< 0.01 darcy). Another approach is to recognize that heat conduction and fluid mixing in the shallow subsurface results in a thin thermal boundary layer near the seafloor. In this case, the temperature response to the earthquake need not propagate through the entire discharge zone, but need only affect the movement of the boundary layer. In our model, we also point out that the earthquake swarm associated with the event at EPR is located near the tips of the magma lens, suggesting that the earthquakes may be associated with dike emplacement as well as with faulting. The earthquakes on the JdFR could also be related to a diking event. Consequently, we incorporated the dike-enhanced heating and permeability that could be related to the earthquake swarm beneath the ridge into our model. Our results show that an increase in permeability at depth following an earthquake can sufficiently perturb the boundary layer to give the observed temperature increases at both EPR and JdFR sites. In this case, the fluid residence time in the discharge zone is ~ years. To address the decay of the temperature spike and the long-term temperature rise, we assume that coexistence of high-temperature discrete vents and nearby cool diffuse flow are indicative of a quasi-impermeable barrier caused by anhydrite precipitation. Then, the change in the boundary layer height at the top of the discharge zone following the dike-increased permeability induces lateral temperature and pressure gradients that intensify the shallow circulation of the colder fluid. Subsequently, the upflow is rapidly cooled via mixing. Yet, due to the overall permeability increase near the dike emplacement, a long-term secular temperature rise occurs. Johnson, H. P., M. Hutnak, R. P. Dziak, C. G. Fox, I. Urcuyo, J. P. Cowen, J. Nabelek, and C. Fisher (2000), Earthquake-induced changes in a hydrothermal system on the Juan de Fuca mid-ocean ridge, Nature, 407, 174-177. Sohn, R. A., D. J. Fornari, K. L. Von Damm, J. A. Hildebrand, and S. C. Webb (1998), Seismic and hydrothermal evidence for a cracking event on the East Pacific Rise crest at 9°50' N, Nature, 396, 159-161. Wilcock, W. S. D. (2004), Physical response of mid-ocean ridge hydrothermal systems to local earthquakes, Geochem. Geophys. Geosyst., 5, Q11009, doi:10.1029/2004GC000701.
DE: 8135 Hydrothermal systems (0450, 1034, 3017, 3616, 4832, 8424)
SC: Tectonophysics [T]
MN: Fall Meeting 2005