Seismometer array will monitor natural hazards.
One of the most under-appreciated earthquake hazard zones in the United States is getting extra attention.
Just off the Pacific Northwest coast, the Juan de Fuca tectonic plate dives beneath North America, shuddering and occasionally releasing the accumulated stress as a major earthquake. A magnitude-9 quake struck here in 1700, triggering a tsunami that hit Japan, and the chances of another quake of that size are 1 in 500 each year. Yet the Cascadia zone remains relatively sparsely studied.
Now geophysicists have taken advantage of grant money gleaned from last year's US economic stimulus bill to rig up the waters off the Cascadia coast with an unprecedented assembly of ocean-bottom seismometers. The US$10-million project is on a fast track to be fully set up by early next year. But the fact that the United States is only now putting such research resources into a long-known trouble zone shows how far even developed nations lag behind on earthquake studies.
"People haven't appreciated the level of hazard there for both an earthquake and a tsunami," says project member John Orcutt, a geophysicist at the Scripps Institution of Oceanography in La Jolla, California. "This study will over the long term provide a fundamental understanding of the Earth's structure and the hazard."
Cascadia already has a number of seismometers and other equipment on land to measure earthquake risk, including the Plate Boundary Observatory of geodetic equipment that stretches from California to Alaska. But the ocean-bottom component has been lacking; only with that, geophysicists say, can sea-floor movements along the edge of the Juan de Fuca plate be monitored effectively.
"This will be the most comprehensive study of a subduction zone conducted anywhere to date," says Richard Carlson, project manager for the Cascadia initiative at the National Science Foundation in Arlington, Virginia.
The initiative will beef up the land-based instruments already in place by adding seismometers and having geodetic data feed back in real time instead of after a delay. More significantly, it will also deploy 60 ocean-bottom seismometers off the coasts of Oregon and Washington, at depths of between about 300 metres and 1 kilometre.
The stations are being built by three venerable oceanographic institutions: the Lamont-Doherty Earth Observatory in Palisades, New York, the Woods Hole Oceanographic Institution in Woods Hole, Massachusetts, and Scripps. Each of the new ocean-bottom stations costs about $70,000 and includes a seismometer, pressure gauge, data recorder and special clock to match data to shore-based equipment.
Some project scientists are questioning whether the correct type of seismometer is being purchased. Because the economic stimulus money had to be spent within a certain period of time, the decision on which seismometer to use had to be made within about three months last summer. Orcutt, for one, says that he would have preferred a '240' seismometer with a higher bandwidth than the '120' instruments the National Science Foundation has bought from Nanometrics Inc. in Kanata, Ontario. The '240' seismometers can capture more data and would be more useful for future deployments, Orcutt says.
Carlson, for his part, says that the agency is comfortable with the type of seismometer it has chosen.
Meanwhile, project leaders are starting work on how to deploy the complex package of delicate instruments. The equipment must be tough enough to withstand being dropped off the side of a ship, then exposed to sea-floor conditions for at least a year, and retrieved by popping a remote-control release and floating to the surface.
Sometimes deployments go flawlessly, as when researchers dropped and recovered nearly 70 devices off British Columbia last summer. But equipment can go awry. For instance, three stations malfunctioned after a deployment off Newport, Oregon, several years ago. The problem, which involved the cable connection, wasn't discovered until after they had been in the water for a year. In other cases, the glass balls that buoy the instrument package to the surface have imploded, destroying the stations.
For the Cascadia deployment, project leaders are also building protective cages to shield the instrument from heavy currents and from being snagged by fishing trawlers.
But researchers can't always predict what challenges await them in the field. Anne Tréhu, a geophysicist at Oregon State University in Corvallis who led the earlier deployment, says that the team hadn't expected to receive several local visitors on the sea floor.
"We found octopus were a problem, too," she says. They nestled against the instruments, messing up the measurements of current flows.
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Dalton, R. Cascadia quake zone gets wired up. Nature (2010). https://doi.org/10.1038/news.2010.38