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Seismologist Deborah Kilb was wading through California earthquake records from the past four decades when she noticed something odd — a series of deep earthquakes that had occurred under the Sierra Nevada at a depth where Earth’s crust would typically be too hot and high pressure for seismic activity.
“In Northern California usually the (earthquake) data goes down to about 10 kilometers (6 miles). In Southern California, they’ll go down a little bit deeper into 18 kilometers (11 miles),” said Kilb, a researcher at the Scripps Institution of Oceanography at the University of California, San Diego, referring to the depths at which earthquakes usually occur in those regions.
But the earthquakes she found taking place near the central region of the mountain range were up to twice as deep — and appear to be ongoing.
“The fact that we see some seismicity that’s below 20 kilometers (12.4 miles) — like 20 kilometers to 40 kilometers (25 miles) — is very odd,” Kilb said. “It’s not something you would typically see in crustal earthquakes.”
Kilb flagged the data to Vera Schulte-Pelkum, a research scientist at the Cooperative Institute for Research in Environmental Sciences and an associate research professor of geological sciences at the University of Colorado Boulder. Schulte-Pelkum was already studying the Sierra Nevada’s peculiar rock footprint, which had shown deep rock deformations within the same area.
Using the newfound data, the researchers imaged the Sierra Nevada through a technique known as receiver function analysis, which uses seismic waves to map Earth’s internal structure. The scientists found that in the central region of the mountain range, Earth’s crust is currently peeling away, a process scientifically known as lithospheric foundering. Kilb and Schulte-Pelkum reported the findings in December in the journal Geophysical Research Letters.
The hypothesis lined up with previous speculation that the area had undergone lithospheric foundering, which happens when Earth’s outermost layer sinks into the lower layer of the mantle. Now, the study authors believe that the process is ongoing and is currently progressing to the north of the mountain range, according to the study.
“We compared notes and realized that my strange rock fabric (the arrangement of rocks) signals and her strange deep earthquakes were in the same area,” Schulte-Pelkum said. “So then we decided to look at it more closely, and found this whole story.”
What’s happening under the Sierra Nevada could offer rare insight into how the continents formed, Schulte-Pelkum said. The finding could also help scientists identify more areas where this process is happening as well as provide a better understanding of earthquakes and how our planet operates, she added.
Earth’s uppermost layer, the lithosphere, is made up of the rigid crust and the top part of the mantle, which is in a denser, but more fluid state. This layer also contains Earth’s oceanic crust — a thinner and denser layer below the oceans — and the continental crust that sits above this layer. But how these sublayers manage to exist in this ideal state, with the continents on top, is something of a mystery, Schulte-Pelkum said.
“The continents just happened to be sticking up above the current sea level, luckily for us, because … they’re made of less dense minerals on average,” Schulte-Pelkum said. “To make it sit higher (in the first place), you have to get rid of some of the dense stuff.”
Lithospheric foundering is the process of the denser materials being pulled to the bottom, while the less dense material emerges at the top, resulting in land creation. “It’s dumping some of this denser stuff into this gooey, solid mantle layer underneath and sort of basically detaching it so it stops pulling on the less dense stuff above,” she explained.
Within the imaging of Earth below the Sierra Nevada, the researchers found a distinct layer within the mantle about 40 to 70 kilometers (25 to 43 miles) deep. This layer had specific imprints that gradually changed due north, the data showed.
If one were to have a block of clay that had spots of different colored clay throughout, and squeezed the clay between their hands, the spots would start to turn into stripes — this is similar to how the rock deformations appear, Schulte-Pelkum said.
In the southern Sierra, the dense rocks had the strongest inherent stripes and were shown to have already sheared away from the crust, whereas in the central region this process appears to be ongoing. In the northern Sierra, there are currently no signs of deformation. This distinct layer within the mantle would also explain the deep earthquakes Kilb found, as the crust in the central region is unusually thick from being pulled down and is also colder than the hot mantle material typically found at those depths.
“Rock takes a really long time to warm up or cool down. So if you move some stuff, you know, by pulling it down or pushing it up, it takes a while for it to adjust its temperature,” Schulte-Pelkum said.
Evidence for this process has been hard to come by. It is not visible from above ground, and it’s an extremely slow process. Scientists theorize that the south Sierra finished the process of lithospheric foundering about 4 million to 3 million years ago, according to the study.
It appears that these natural events happen occasionally around the world, Schulte-Pelkum said. “Geologically speaking, this is a pretty quick process with long periods of stability in between. … This (lithosphere foundering) probably started happening a long time ago when we started building continents, and (the continents) have gotten bigger over time. So it’s just sort of this punctuated, localized thing,” she added.
The Sierra Nevada has been a topic of debate for decades in the geology community due to an anomaly found within the mantle located underneath the Great Valley.
While some scientists believe lithospheric foundering caused this feature, other scientists think it may be caused by subduction, which is when an oceanic plate sinks beneath a less dense plate, such as continental crust, and changes the landscape, said Mitchell McMillan, a research geologist and postdoctoral fellow at Georgia Tech, who was not involved with the study.
“There are really two competing hypotheses to explain all these data, and you don’t really get that very often in geology. … So this paper is going to add to that whole discussion in a really neat way,” he said.
Further study within this area could also help scientists better understand how the Earth evolves on long timescales. If the lithospheric foundering continues underneath the mountain range, one can speculate that the land will continue to stretch vertically, changing the way the landscape looks now, McMillan said. But that could take anywhere from several hundred thousand to a few million years, he added.
In general, large mountain belts, or anywhere there is a batholith, is where you expect to find these events, McMillan said. The Andes, a long mountain range in South America, is an example of another place where scientists speculate lithosphere foundering once occurred and could still be happening today, he added.
“I think this study in particular (highlights) the importance of tying together these different datasets,” McMillan said.
By better understanding this process, scientists can learn more about the functions of the planet and what happens beneath its surface, including the occurrence of earthquakes that have been linked to this process, McMillan said. Separately, the planet Venus, which does not have plate tectonics like Earth, has evidence of these lithospheric foundering events, and by understanding the process on Earth, we can start to apply it to Venus, he said.
“It’s really fascinating to think about how you could be … hiking in the Sierra or in the foothills, or even anywhere else on a continent. And, you know, there’s stuff going on really deep underneath you that we’re not aware of,” Schulte-Pelkum said.
“We sort of owe our existence on land to these processes happening. If the Earth hadn’t made continents, then we’d be very different creatures. … We evolved because the planet evolved the way it did. So just sort of understanding the whole system that you’re part of, I think, has value — beyond just less monetary damage and less human impact during, say, an earthquake,” she added.