HR: 13:55h
AN: S13F-02    [Abstracts]
TI: Microearthquakes Beneath the Endeavour Hydrothermal Vent Fields: Insights Into Reaction Zone Processes
AU: * Wilcock, W S
EM: wilcock@u.washington.edu
AF: University of Washington, School of Oceanography Box 357940, Seattle, WA 98195, United States
AU: Hooft, E E
EM: emilie@uoregon.edu
AF: University of Oregon, Department of Geological Sciences, Eugene, OR 97403, United States
AU: McGill, P R
EM: mcgill@mbari.edu
AF: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039, United States
AU: Toomey, D R
EM: drt@uoregon.edu
AF: University of Oregon, Department of Geological Sciences, Eugene, OR 97403, United States
AU: Barclay, A H
EM: barclay@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10963, United States
AU: Stakes, D S
EM: dstakes@cuesta.edu
AF: Cuesta College, Division of Physical Sciences, San Luis Obispo, CA 93403, United States
AU: Ramirez, T M
EM: Tony_Ramirez@AOAGeophysics.com
AF: AOA Geophysics, 7532 Sandholdt Road #6, Moss Landing, CA 95039, United States
AB: From 2003-2006, a novel seismic network comprising seven short-period corehole seismometers and a broadband Guralp CMG-1T OBS was deployed using remotely operated vehicles in a subseafloor configuration on the Endeavour segment of the Juan de Fuca mid-ocean ridge. The seismic monitoring array was one part of a multi-disciplinary prototype NEPTUNE experiment designed to investigate the linkages between seismic deformation, hydrothermal fluxes, and microbial productivity along oceanic plate boundaries. The seismic network recorded high-quality data that illustrate the advantages of using an ROV to deploy seismometers in well- coupled configurations that are also away from the effects of ocean currents. A preliminary analysis of the first year of Keck seismic data was undertaken during a research apprenticeship class taught in the fall of 2004 at the University of Washington's Friday Harbor Laboratories. Eight post- baccalaureate students obtained a preliminary catalog of nearly 13,000 earthquakes on the Endeavour segment. Two of the apprentices conducted a second-pass analysis to refine the locations of ~3000 earthquakes that are within or near the network. Further analysis of these proximal earthquakes has focused on the application of cross-correlation and relative relocation techniques, the determination of focal mechanisms using P-wave first motions and P- to S-wave amplitudes ratios, and improved estimates of earthquake magnitudes. The results show that the entire Endeavour segment was seismically active during 2003-2004. Within the network, the earthquakes are located in tight clusters centered at ~2 km depth in the inferred location of the hydrothermal reaction zone immediately above a crustal magma chamber imaged by seismic reflection studies. The number of earthquakes below each hydrothermal vent field correlates with the heat flux measured by other researchers and the vertical thickness of this reaction zone, inferred from the distribution of seismicity, is ~0.5-1 km. Preliminary focal mechanisms show a transition from ridge-perpendicular extension beneath the rise axis to ridge-perpendicular compression on either side over horizontal distances of <1 km. This is most simply explained if the stress field above the magma chamber is locally influenced by either the injection of magma or over-pressured hydrothermal fluids. This observation, in combination with a reaction zone thickness that is at least an order of magnitude larger than that inferred from thermal boundary layer theory, suggests that the heat transfer between the magmatic and hydrothermal systems is not simply the result of cold water attacking hot magma. Instead, it appears as though the magmatic system also attacks the reaction zone from below.
DE: 3017 Hydrothermal systems (0450, 1034, 3616, 4832, 8135, 8424)
DE: 3025 Marine seismics (0935, 7294)
DE: 3035 Midocean ridge processes
DE: 7245 Mid-ocean ridges
DE: 7280 Volcano seismology (8419)
SC: Seismology [S]
MN: 2007 Fall Meeting