Hydrothermal circulation at the axis of mid- ocean ridges affects the chemistry of the lithosphere and overlying ocean, supports chemosynthetic biological communities and is responsible for significant heat transfer from the lithosphere to the ocean(1-3). It is commonly thought that flow in these systems is oriented across the ridge axis, with recharge occurring along off- axis faults(4-6), but the structure and scale of hydrothermal systems are usually inferred from thermal and geochemical models constrained by the geophysical setting(7-9), rather than direct observations. The presence of microearthquakes may shed light on hydrothermal pathways by revealing zones of thermal cracking where cold sea water extracts heat from hot crustal rocks, as well as regions where magmatic and tectonic stresses create fractures that increase porosity and permeability. Here we show that hypocentres beneath a well- studied hydrothermal vent field on the East Pacific Rise cluster in a vertical pipe- like zone near a small axial discontinuity, and in a band that lies directly above the axial magma chamber. The location of the shallow pipe- like cluster relative to the distribution and temperature of hydrothermal vents along this section of the ridge suggests that hydrothermal recharge may be concentrated there as a consequence of the permeability generated by tectonic fracturing. Furthermore, we interpret the band of seismicity above the magma chamber as a zone of hydrothermal cracking, which suggests that hydrothermal circulation may be strongly aligned along the ridge axis. We conclude that models that suggest that hydrothermal cells are oriented across- axis, with diffuse off- axis recharge zones, may not apply to the fast- spreading East Pacific Rise.
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