The Juan de Fuca is a small tectonic plate that sits several hundred miles off the Pacific Northwest coast. But it is potentially a perfect storm it slowly slides under the North American continent with the potential to generate massive earthquakes with accompanying tsunamis.
Because the Juan de Fuca plate represents the single greatest geophysical hazard to the continental United States, scientists are keen to learn more about it. It is understood that quakes from this area could be hundreds of times more damaging than a big quake from the San Andreas Fault.
The Juan de Fuca plate started its life 300 miles off the coast, along a lengthy range of underwater volcanoes that are producing new crusts deep below the sea. This area is part of the global mid-ocean ridge system that encircles the planet — it is these regions where 70 percent of the Earth’s tectonic plates are generated.
Scientists have struggled to understand these chains of volcanoes as they lie more than a mile beneath the sea surface. However, using an unusual measurement technique, geophysicist Zachary Eilon and his co-author of the paper Geoff Abers have found seismic attenuation, or energy loss, at the mid-ocean ridge where the Juan de Fuca Plate was created.
The geophysicist’s data indicates that molten rock here is found deeper within the Earth than scientists had previously thought. Knowing this will help scientists understand how the Earth’s tectonic plates are built, as well as the deep volcanic systems. Eilon said in a statement:
“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.
“Whatever is down there causing all this seismic energy to be lost extends really deep, at least 200 kilometers beneath the surface. That’s unexpected, because we think of the processes that give rise to this — particularly the effect of melting beneath the surface — as being shallow, confined to 60 km or less.”
In their paper, the researchers have calculated that in the narrow strip underneath the mid-ocean ridge, there is very high attenuation, where hot rock wells up to generate the Juan de Fuca Plate. Going further, they state that the levels are as high as scientists have seen anywhere on the planet.
It was also found that the plate is cooling much faster than expected. This affects the friction at the collision zone and the size of any potential megaquake (seismic waves begin at an earthquake and radiate away from it).
While traveling through the water, they lose energy. Some of the loss is due to the wave spreading out; however, there is another factor that also affects energy loss. It’s called the quality factor. To explain this factor, Eilon uses the analogy of a bell, saying:
“If I were to give you a well-made bell and you were to strike it once, it would ring for a long time.
“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.”
The researchers observed the way different frequencies of seismic waves attenuated at different rates. Eilon explained:
“We looked not only at how much energy is lost, but also at the different amounts by which various frequencies are delayed.
“This new, more robust way of measuring attenuation is a breakthrough that can be applied in other systems around the world. 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.”
Next year, there will be an international effort from researchers, including Eilon, to instrumentize large unexplored areas of the Pacific with ocean bottom seismometers. After all the data has been collected, Eilon, using the same techniques he used on the Juan de Fuca, hopes to learn more about what it truly lying beneath the seafloor. Eilon explained that:
“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.
“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.
“The Pacific Northwest project was an incredibly ambitious community experiment. Just imagine the sort of things we’ll find out once we start to put these instruments in other places.”
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