The National Science Foundation (NSF) seismic survey vessel Marcus G. Langseth recently arrived in Seattle following a six-week-long expedition to study an area of the Pacific Ocean that has been the site of past "megathrust" earthquakes, said to be the largest earthquakes that occur on the planet.
The vessel carried an embarked team of scientists from New York City's Columbia University, the University of Texas, Woods Hole Oceanographic Institution, the University of Washington, Oregon State University, and the National Oceanic and Atmospheric Administration (NOAA) to investigate the Cascadia Subduction Zone, a fault zone that lies off the coasts of Oregon, Washington State, and British Columbia.
Columbia University geophysicist Suzanne Carbotte said the zone has been "eerily quiet" recently, with little seismicity detected within the Oregon to Washington portion. Scientists believe this lack of seismicity reflects the "locked" state of the megathrust fault at present, with stress quietly accumulating as the Juan de Fuca plate system continues to dive (subduct) beneath North America.
Carbotte said that all or part of that built-up stress will eventually be released in "the next great earthquake."
During the cruise, the researchers used sound to probe under the seafloor looking for the megathrust fault deep beneath several kilometres of sediments that cover the down-going Juan de Fuca plate.
Carbotte added that the NSF-funded survey was the first-ever seismic imaging study to span almost the entire Cascadia subduction zone. Modern advanced seismic imaging technology was used to detect and characterise fine-scale structures within the subduction zone to help address a range of scientific questions pertaining to earthquake and tsunami hazards within the Pacific Northwest region.
Marcus G. Langseth proved ideal for the expedition, being equipped with a high-quality sound source that uses compressed air. The vessel is also capable of towing an array of hydrophones up to 15 kilometres long to listen to the echoes returned from the seafloor and deep below.
The small scout vessel Rachel Carson sailed some five kilometres ahead of Marcus G. Langseth throughout the cruise. Those on the scout vessel were tasked with keeping the crew of the larger vessel apprised of any nearby marine mammals that could otherwise be adversely affected by the underwater noise generated by the sound source.
