Analysis of Hydraulic Seismicity at the TAG Hydrothermal Mound

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ABSTRACTOCE-0647221Intellectual Merit. This project will address fundamental issues regarding the mechanics of fluid flow within a massive sulfide deposit by analyzing seismicity data from a unique ocean bottom experiment conducted at the TAG hydrothermal field on the Mid-Atlantic Ridge (26degreesN) for 9 months from July, 2003 to March, 2004. A network of five ocean-bottom seismometers (OBS) was deployed around the perimeter of the active TAG mound, which has a diameter of ~200 m. These seismometers detected ~40,000 short, impulsive events that appear to be flow-induced events from hydraulic processes within the mineral deposit, and we will conduct seismological analyses of these events to assess key problems such as the geometry of the fracture network that provides pathways for hydrothermal circulation, and the volume of fluid reservoirs that decompress during hydrofracturing. The scientific payoff from our proposed analysis will be enhanced by the fact that we also acquired exit-fluid temperature time-series data from 21 sites on the active TAG mound during the microearthquake survey. The flow-induced events can thus be placed in the context of the perturbation patterns observed in the temperature records, which will allow us to study the relationship between faulting/fracturing and fluid flow at the scale of an individual massive sulfide deposit something that has never before been possible. Broader Impacts. Mineral deposition in fault-controlled deep-sea hydrothermal systems is focused in discrete zones over long periods of time, which leads to the generation of massive ore deposits. It can be very hard to know the size of these deposits because of their remote position on the deep seafloor, and because most of the deposit is underground, but this research will allow us to determine the location of the cracks that provide flow pathways in the shallow subsurface. Knowing the geometry of fluid circulation will provide excellent constraints on the subsurface extent of the mineral deposit. These deep-sea vent fields also support exotic chemosynthetic biological communities that derive metabolic energy from chemical reactions with the metal-rich hydrothermal fluid. Knowing the geometry of fluid circulation will also help us understand the physical characteristics of the subsurface biosphere inhabited by the microbes at the base of these ecosystems.The research will also contribute to the graduate education and doctoral thesis research of a student in the MIT/WHOI Joint Program in Oceanography.