Hydrological response to a seafloor spreading episode on the Juan de Fuca ridge


Seafloor hydrothermal systems are known to respond to seismic and magmatic activity along mid-ocean ridges, often resulting in locally positive changes in hydrothermal discharge rate, temperature and microbial activity, and shifts in composition occurring at the time of earthquake swarms and axial crustal dike injections1,2,3,4,5,6,7,8,9,10. Corresponding regional effects have also been observed11. Here we present observations of a hydrological response to seafloor spreading activity, which resulted in a negative formation-fluid pressure transient during and after an earthquake swarm in the sediment-sealed igneous crust of the Middle Valley rift of the northernmost Juan de Fuca ridge. The observations were made with a borehole seal and hydrologic observatory originally established in 1991 to study the steady-state pressure and temperature conditions in this hydrothermally active area12,13. The magnitude of the co-seismic response is consistent with the elastic strain that would be expected from the associated earthquakes, but the prolonged negative pressure transient after the swarm is surprising and suggests net co-seismic dilatation of the upper, permeable igneous crust. The rift valley was visited four weeks after the onset of the seismic activity, but no signature of increased hydrothermal activity was detected in the water column. It appears that water, not magma, filled the void left by this spreading episode.

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Figure 1: Locations of events of the September 2001 seismic swarm in Middle Valley, northern Juan de Fuca Ridge, determined from offshore SoSUS hydroacoustic array tertiary-phase data (error less than 4 km)22, along with earthquake locations determined using Western Canadian Seismic Network data (http://www.pgc.nrcan.gc.ca/seismo) and mechanisms (compressional quadrants shown solid) constrained by moment tensor solutions (J.Risteau, personal communication).
Figure 2: Formation pressure measured hourly in ODP Hole 857D before and after the September 2001 seismic swarm.
Figure 3: Simultaneous seismic and hydrologic records of the 2001 Middle Valley earthquake swarm.
Figure 4: Head resulting from an impulse propagating axisymmetrically in a uniform porous medium, calculated at a range of 30 km using a line-source approximation for a hydrologic slug test16, which is valid at large distances from the source well.


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We acknowledge the Ocean Drilling Program for ongoing support for long-term borehole monitoring experiments, T. Pettigrew, R. Meldrum, and R. Macdonald for engineering assistance, R. Thomson and S. Mihaley for assistance with the post-swarm water-column survey on board CHS Tully, M. Fowler for assistance with SoSUS data processing, and J. Risteau for earthquake moment tensor calculations. Financial support was provided by the US National Science Foundation and by the Geological Survey of Canada.

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Correspondence to Earl Davis.

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