Nav: Home

The birth and death of a tectonic plate

May 24, 2017

Several hundred miles off the Pacific Northwest coast, a small tectonic plate called the Juan de Fuca is slowly sliding under the North American continent. This subduction has created a collision zone with the potential to generate huge earthquakes and accompanying tsunamis, which happen when faulted rock abruptly shoves the ocean out of its way.

In fact, this region represents the single greatest geophysical hazard to the continental United States; quakes centered here could register as hundreds of times more damaging than even a big temblor on the San Andreas Fault. Not surprisingly, scientists are interested in understanding as much as they can about the Juan de Fuca Plate.

This microplate is "born" just 300 miles off the coast, at a long range of underwater volcanoes that produce new crust from melt generated deep below. Part of the global mid-ocean ridge system that encircles the planet, these regions generate 70 percent of the Earth's tectonic plates. However, because the chains of volcanoes lie more than a mile beneath the sea surface, scientists know surprisingly little about them.

UC Santa Barbara geophysicist Zachary Eilon and his co-author Geoff Abers at Cornell University have conducted new research -- using a novel measurement technique -- that has revealed a strong signal of seismic attenuation or energy loss at the mid-ocean ridge where the Juan de Fuca Plate is created. The researchers' attenuation data imply that molten rock here is found even deeper within the Earth than scientists had previously thought. This in turn helps scientists understand the processes by which Earth's tectonic plates are built, as well as the deep plumbing of volcanic systems. The results of the work appear in the journal Science Advances.

"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, an assistant professor in UCSB's Department of Earth Science. "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."

According to Eilon's calculations, the narrow strip underneath the mid-ocean ridge, where hot rock wells up to generate the Juan de Fuca Plate, has very high attenuation. In fact, its levels are as high as scientists have seen anywhere on the planet. His findings also suggest that the plate is cooling faster than expected, which affects the friction at the collision zone and the resulting size of any potential megaquake.

Seismic waves begin at an earthquake and radiate away from it. As they disperse, they lose energy. Some of that loss is simply due to spreading out, but another parameter also affects energy loss. Called the quality factor, it essentially describes how squishy the Earth is, Eilon said. He used the analogy of a bell to explain how the quality factor works.

"If I were to give you a well-made bell and you were to strike it once, it would ring for a long time," he explained. "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."

Eilon looked at the way different frequencies of seismic waves attenuated at different rates. "We looked not only at how much energy is lost but also at the different amounts by which various frequencies are delayed," he explained. "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," Eilon added. "But it gives us a huge amount of new information about the Earth's interior that we wouldn't have otherwise."

Next year, Eilon will be part of an international effort to instrument large unexplored swaths of the Pacific with ocean bottom seismometers. Once that data has been collected, he will apply the techniques he developed on the Juan de Fuca in the hope of learning more about what lies beneath the seafloor in the old oceans, where mysterious undulations in the Earth's gravity field have been measured.

"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.

"The Pacific Northwest project was an incredibly ambitious community experiment," he said. "Just imagine the sort of things we'll find out once we start to put these instruments in other places."
-end-


University of California - Santa Barbara

Related Tectonic Plates Articles:

The birth and death of a tectonic plate
Geophysicist Zachary Eilon developed a new technique to investigate the underwater volcanoes that produce Earth's tectonic plates
From where will the next big earthquake hit the city of Istanbul?
Scientists reckon with an earthquake with a magnitude of 7 or greater in this region in the coming years.
Deep-seated tectonic genesis of large earthquakes in North China
In the 1960s-1970s, North China has undergone a series of strong earthquakes.
Tectonic shift?
A recent study by researchers at the University of Delaware, the University of Oxford and the University of Minnesota, Twin Cities, provides a new data set that scientists can use to better understand plate tectonics -- the movement of the earth's outer layer.
SLU geologists discover how a tectonic plate sank
Saint Louis University researchers report new information about conditions that can cause the Earth's tectonic plates to sink into the Earth.
Oceans may be large, overlooked source of hydrogen gas
Serpentinized rocks formed near fast-spreading tectonic plates under Earth's seafloor could be a large and previously overlooked source of free hydrogen gas, a Duke University study finds.
Young bowhead whales may cease growing lengthwise to grow head and baleen plates
Young bowhead whales may cease growing lengthwise and undergo severe bone loss to help grow their enormous head and baleen plates, according to a study published June 22, 2016 in the open-access journal PLOS ONE by John George from North Slope Borough Department of Wildlife Management, Alaska, and colleagues.
UNC-Chapel Hill scientists find likely cause for recent southeast US earthquakes
The southeastern United States should, by all means, be relatively quiet in terms of seismic activity.
Study: Ancient tectonic activity was trigger for ice ages
Continental shifting may have acted as a natural mechanism for extreme carbon sequestration.
Hi-tech opens up Earth's secrets
A JCU scientist has developed a hi-tech animation of millions of years of tectonic plate movements that could lead to new mineral discoveries and help predict volcanic eruptions.

Related Tectonic Plates Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Anthropomorphic
Do animals grieve? Do they have language or consciousness? For a long time, scientists resisted the urge to look for human qualities in animals. This hour, TED speakers explore how that is changing. Guests include biological anthropologist Barbara King, dolphin researcher Denise Herzing, primatologist Frans de Waal, and ecologist Carl Safina.
Now Playing: Science for the People

#SB2 2019 Science Birthday Minisode: Mary Golda Ross
Our second annual Science Birthday is here, and this year we celebrate the wonderful Mary Golda Ross, born 9 August 1908. She died in 2008 at age 99, but left a lasting mark on the science of rocketry and space exploration as an early woman in engineering, and one of the first Native Americans in engineering. Join Rachelle and Bethany for this very special birthday minisode celebrating Mary and her achievements. Thanks to our Patreons who make this show possible! Read more about Mary G. Ross: Interview with Mary Ross on Lash Publications International, by Laurel Sheppard Meet Mary Golda...