Earthquakes that happen greater than 300 kilometers beneath the Earth are poorly understood. UC Davis geophysicist Magali Billen modeled stresses in a sinking tectonic plate at a subduction zone. On this video, yellow areas on the sinking plate present the place deep earthquakes are almost definitely to happen, as a result of the plate is each robust and deforming quickly. This work can clarify why earthquakes cluster at sure depths and lead to a greater understanding of the causes of deep earthquakes. Credit score: Magali Billen, UC Davis
A brand new understanding of our planet’s deepest earthquakes might assist unravel one of the most mysterious geophysical processes on Earth.
Deep earthquakes — these at the least 300 kilometers beneath the floor — don’t sometimes trigger harm, however they’re usually broadly felt. These earthquakes can present very important clues to understanding plate tectonics and the construction of the Earth’s inside. Due to the extraordinarily excessive temperature and pressures the place deep earthquakes happen, they doubtless stem from totally different bodily and chemical processes than earthquakes close to the floor. However it’s exhausting to collect details about deep earthquakes, so scientists don’t have a strong rationalization for what causes them.
“We will’t straight see what’s taking place the place deep earthquakes happen,” stated Magali Billen, professor of geophysics in the UC Davis Division of Earth and Planetary Sciences.
Billen builds numerical simulations of subduction zones, the place one plate sinks beneath one other, to higher perceive the forces controlling plate tectonics. Her latest work helps clarify the distribution of deep earthquakes, displaying that they most frequently strike in areas of “excessive pressure” the place a sinking tectonic plate bends and folds.
“These fashions present compelling proof that pressure price is a vital think about controlling the place deep earthquakes happen,” she stated.
The brand new understanding that deformation is a significant factor in deep earthquakes ought to assist scientists resolve which mechanisms set off deep earthquakes and may present new constraints on subduction zone construction and dynamics, Billen stated.
“As soon as we perceive deep earthquake physics higher, we can be ready to extract much more details about the dynamics of subduction, the key driver of plate tectonics,” she stated.
Her findings have been printed right now (Could 27, 2020) in the journal Science Advances.
Deep earthquakes happen in subduction zones — the place one of the tectonic plates floating on the floor of the Earth dives beneath one other and is “subducted” into the mantle. Inside the sinking slabs of crust, earthquakes cluster at some depths and are sparse in others. For instance, many slabs exhibit giant gaps in seismic exercise beneath 410 kilometers in depth.
The gaps in seismicity line up with areas of the slab which might be deforming extra slowly in the numerical fashions, Billen stated.
“Deformation just isn’t the similar in all places in the plate,” Billen stated. “That’s actually what’s new right here.”
Billen’s analysis was not initially supposed to examine deep earthquakes. Somewhat, she was attempting to perceive the sluggish back-and forth movement of deep ocean trenches, the place plates bend downward in subduction zones.
“I made a decision out of curiosity to plot the deformation in the plate, and after I checked out the plot, the very first thing that popped in my thoughts was ‘wow, this appears to be like like the distribution of deep earthquakes,’” she stated. “It was a complete shock.”
Billen’s mannequin incorporates the newest information about phenomena akin to the density of minerals, totally different layers in the sinking plate, and experimental observations of how rocks behave at excessive temperatures and pressures.
“That is the first mannequin that basically brings collectively the bodily equations that describe the sinking of the plates and key bodily properties of the rocks,” Billen stated.
The outcomes can’t distinguish between attainable causes for deep earthquakes. Nevertheless, they do present new methods to discover what causes them, Billen stated.
“Considering the added constraint of strain-rate ought to assist to resolve which mechanisms are energetic in the subducting lithosphere, with the chance that a number of mechanisms could also be required,” she stated.
The undertaking was supported by a fellowship from the Alexander von Humboldt Basis and an award from the Nationwide Science Basis. The Computational Infrastructure for Geodynamics helps the CitcomS software program used for the numerical simulations.
Reference: “Deep slab seismicity restricted by price of deformation in the transition zone” by Magali I. Billen, 27 Could 2020, Science Advances.