Researchers lowered ocean-bottom seismometers from the researcher vessel Welcoma to measure seismic attenuation at the mid-ocean ridge. Photo by Dave O'Gorman/UC-Santa Barbara
May 24 (UPI) -- The small tectonic plate Juan de Fuca is born just a few hundred miles off the coast of the Pacific Northwest. But the plate's birthplace lies more than a mile beneath the ocean surface, limiting scientists ability to study it.
New seismic attenuation data -- the measure of lost seismic energy -- has offered fresh insights into the nature of the mid-ocean ridge system where Juan de Fuca is formed.
The mid-ocean ridge system is a narrow strip of volcanoes that circles the globe. Melted rock rising from the planet's mantle forms new crust along the ridge as it cools. The system produces 70 percent of Earth's tectonic plates.
According to the latest analysis -- detailed in the journal Science Advances -- the ridge's molten reservoirs extend much deeper than originally thought.
Scientists are interested in Juan de Fuca because the miniature plate is the single greatest geophysical threat to the United States. Researchers believe Juan de Fuca's subduction beneath North America's continental plate could yield megaquakes several orders of magnitude more powerful than the San Andreas Fault's largest tremors.
To study the origins of Juan de Fuca, scientists used a new method for measuring seismic attenuation. Seismic waves are generated by geologic phenomena like earthquakes and volcanoes. As the waves propagate outward from their source they dissipate. The energy loss happens as the waves spread out, but attenuation is also affected by what's called the quality factor -- the squishiness of the planet's composition.
"If I were to give you a well-made bell and you were to strike it once, it would ring for a long time," Zachary Eilon, a geophysicist at the University of California, Santa Barbara, said in a news release. "That's because very little of the energy is actually being lost with each oscillation as the bell rings. That's very low attenuation, very high quality. But if I give you a poorly made bell and you strike it once, the oscillations will die out very quickly. That's high attenuation, low quality."
In addition to measuring the amount of energy lost as seismic waves reach the mid-ocean ridge system. researchers looked at how different frequencies are slowed down by the region's composition.
"This new, more robust way of measuring attenuation is a breakthrough that can be applied in other systems around the world," Eilon said. "Attenuation is a very hard thing to measure, which is why a lot of people ignore it. But it gives us a huge amount of new information about the Earth's interior that we wouldn't have otherwise."
The data showed the magmatic plumbing of the mid-ocean ridge system extends more than 124 mile beneath the ocean floor. Researchers originally believed the geologic processes that yield fresh oceanic crust occur just a few miles beneath the seafloor.
"We've never had the ability to measure attenuation this way at a mid-ocean ridge before, and the magnitude of the signal tells us that it can't be explained by shallow structure," said Eilon.
Beyond Juan de Fuca and the mid-ocean ridges, researchers say their new analysis method will allow scientists to better understand the structure of the ocean floor.
"These new ocean bottom data, which are really coming out of technological advances in the instrumentation community, will give us new abilities to see through the ocean floor," Eilon said. "This is huge because 70 percent of the Earth's surface is covered by water and we've largely been blind to it -- until now."