Small quake shakes up hydrothermal vents

The geological tremblings caused by a small earthquake off the coast of Washington State in June 1999 died out rather quickly and caused no damage onshore, but ripples are still spreading through the scientific community.

The June 8, 1999, undersea earthquake affected hydrothermal vents along a segment of the Juan de Fuca Ridge west of the Washington coast. University of Washington

Data collected by sensors located in the undersea neighborhood where the earthquake struck are now changing ideas about how fluids circulate within rocks beneath the seafloor. Research reported in the Sept. 14 Nature by several groups of investigators suggests that the flow of water to hydrothermal vents might be much more complex than geologists had thought.

On June 8, 1999, a magnitude 4.5 earthquake occurred 3.5 miles beneath the seafloor about 180 miles off the coast of Washington’s Olympic Peninsula. Fortuitously, the temblor’s epicenter was just west of the Juan de Fuca Ridge. Several groups of scientists had deployed sensors along the ridge to study hydrothermal vents.

About 4 days after the earthquake, the volume and temperature of water spilling from a cluster of hydrothermal vents located about 5 miles from the temblor’s epicenter began to rise. On June 19, instruments recorded similar changes at two nearby vent sites, says H. Paul Johnson, a geophysicist at the University of Washington in Seattle and lead author of the report.

“It’s pure luck that we were out there,” Johnson says. “It’s a bigger stroke of luck that other scientists’ sensors were also at nearby vents.” He candidly admits that he might have written off the changes as bad data if they’d been recorded at only one vent.

In late June, the temperature at the three sites began to oscillate through a range of 10ºC. Each site had its own fluctuation cycle, which ranged between 8 and 12 days. The cycles likely continued beyond the 80 days recorded, though Johnson notes that no one can be sure because all the instruments at the vents were collected in early September. The researchers only discovered the anomalies when they downloaded their data in the laboratory.

At first, Johnson says, he and his colleagues had interpreted the temperature-fluctuation data as “equipment suddenly gone nuts.” Then, after a chance conversation with a colleague who knew of the undersea earthquake, Johnson realized that the observations were a geological windfall. For the past 20 years, geologists had seen few changes in flow rates and temperatures at hydrothermal vents and therefore assumed these characteristics are stable.

Johnson and his coauthors propose several explanations for the fluctuations in flow rate and temperature recently recorded. The earthquake could have caused cracks in the seafloor to extend further into deeper, hotter rocks—in effect, turning up the thermostat. Or, Johnson says, the shock of the quake could have shaken loose mats of bacteria-deposited sulfur, which periodically would plug the vents.

“These hydrothermal vents are remote places covered by thousands of meters of water,” says Edmond A. Mathez, research geologist at the American Museum of Natural History in New York City. “This [finding] illustrates just how little we know about midocean ridges.”

Johnson will return to the Juan de Fuca Ridge later this month to deploy 20 improved sensors at the previously studied sites and at other vents along the ridge. “At this point, all we know is that there’s been a change in fluid circulation,” Johnson says.

“Now, we’d like to understand what caused it.”

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