NEWPORT, Ore. - During the past 12 years, researchers at Oregon State University's Hatfield Marine Science Center have recorded more than 30,000 earthquakes in the Pacific Ocean off the Northwest coast - few of which have ever shown up on land-based seismic equipment.
The earthquakes, most having a magnitude of 2.0 to 4.0, originate where the Juan de Fuca plate, which is creeping inland, is gradually is being subducted beneath the North American plate. Scientists from OSU and the National Oceanic and Atmospheric Administration utilize a network of underwater military hydrophones to listen to the sounds of seafloor earthquakes and other phenomena from their laboratories at the Newport center.
Robert Dziak, who has dual appointments with OSU and NOAA's Pacific Marine Environmental Laboratory, said the use of the U.S. Navy's hydrophones is providing critical data to scientists.
"It is the only real-time hydrophone system in the world - at least for civilians," Dziak said. "It allows us to listen to the earthquakes as they occur and when something unusual happens, we can send out a group of scientists to study the events as they unfold."
The hydrophone system - called the Sound Surveillance System, or SOSUS - was used during the decades of the Cold War to monitor submarine activity in the northern Pacific Ocean. As the Cold War ebbed, these and other unique military assets were offered to civilian researchers performing environmental studies, Dziak said.
This past November, the Navy completed a series of repairs on the hydrophone arrays used to monitor the Juan de Fuca Ridge earthquakes. At the same time, the OSU/NOAA team installed a new data acquisition center at the naval air station on Whidbey Island near Seattle, where the hydrophone data is encrypted and sent to OSU's Hatfield Marine Science Center for decoding and analysis.
The sharpened new system is up and working - and revealing that the earthquakes are ongoing.
The number of earthquakes offshore initially stunned researchers because they weren't being detected on land - even by the most sensitive seismometers. The scientists also discovered that these quakes occurred daily, but every so often there would be a "swarm" of as many as a thousand quakes in a three-week period.
"In the last 10 years, I've seen seven of these swarms," Dziak said. "The plate doesn't move in a continuous manner and some parts move faster than others. When it gets caught up and meets resistance, these swarms occur and when they do, lava breaks through onto the seafloor.
"Usually, the plate moves at about the rate a fingernail might grow - say three centimeters a year," he added. "But when these swarms take place, the movement may be more like a meter in a two-week period."
Since the scientists began using the hydrophones to monitor quakes - and observe seafloor spreading - the research has paid unexpected dividends. While one swarm was taking place, a research vessel dispatched to the region began taking surveys of the water column and seafloor and discovered a large colony of micro-organisms that had been lying dormant within the shallow ocean crust and were "awakened" by the heat of the volcanic spreading.
"The heat from the lava was like activating yeast," Dziak said. "The bacteria could have been down there literally for centuries. It gives us another little clue to how life may have formed on Earth, and may be lying dormant on other planets."
The hydrophone research also has recorded vocalizations of marine mammals, and a research team led by David Mellinger, Kate Stafford and Sharon Nieukirk has been able to detect regional differences in the sounds made by blue whales and fin whales. Like humans, whales from different parts of the globe apparently have their own "accents."
One of the keys to the success of hydrophone use is the existence of a two-dimension underwater layer of certain water pressure, salinity and temperature that creates a "sound channel." Located about 1,000 feet below the surface, this layer focuses sound energy horizontally like a wire.
"During a test, we exploded a stick of dynamite below the surface and it didn't register on land at all," Dziak said. "Yet hydrophones recorded it as a magnitude 1 event 6,000 miles away. That's why the hydrophone array is so important to our research."
Now the OSU/NOAA researchers are working on a portable hydrophone system developed by Haru Matsumoto that they can deploy in hotspots around the globe. During the first tests, they attached the instruments - which are housed in a titanium casing that looks like a diving air tank - to upright moorings anchored on the ocean floor. The recording equipment was powered by 50 C-cell batteries.
"The limitation was that is was not in real time," Dziak said. "The portables weren't powerful enough to transmit data, so it has to be recorded. We'd have to haul them up every few months to see what happened and, if there was an earthquake swarm, we wouldn't know it until a year later."
A new prototype has been tested that is based on a buoyancy system. When the machine records an earthquake event, it ascends to the surface and transmits a satellite signal to alert the researchers to unusual seismic activity.
"The portable hydrophones will give us the ability to study and compare different areas around the world," Dziak said. "The East Pacific Rise off South America is the fastest seafloor spreading area in the world, but it is a lot quieter in terms of earthquakes. The crust is very thin and moves so fast that we don't get a lot of noise. The mid-Atlantic region, in comparison, has infrequent quakes, but they are bigger. Next year, we hope to set up a system near Antarctica.
"Most of the models on seafloor spreading and plate tectonics are based on magnetic anomalies that have been recorded every million years or so," he added. "Now, for the first time, we are able to determine exactly how these tectonic plates are moving."
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