Deep earthquakes reveal rare geological process under the nevada

Sismologist Deborah Kilb was analyzing California earthquake records from the last four decades when she noticed something strange: a series of deep earthquakes that had occurred under the snowed saw at a depth where the earth’s crust would usually be very hot and high pressure for seismic activity.

“In Northern California, usually the data (from earthquakes) reach up to about 10 kilometers. In California do Sul, they go a little deeper, up to 18 kilometers,” says Kilb, a researcher at the San Diego University Oceanography Institute, referring to the depths where earthquakes usually occur in these regions.

But the earthquakes she found occurring near the central region of the mountain range were up to twice on deep – and seem to be ongoing.

“The fact that we see some seismicity below 20 kilometers – like 20 to 40 kilometers – is very strange,” adds Kilb. “It’s not something you would normally see in crustal earthquakes.”

Kilb signaled the data to Vera Schulte-Pelkum, a researcher scientist at the Cooperative Institute for Research in Environmental Sciences and Associate Professor of Geological Sciences at the University of Colorado Boulder. Schulte-Pelkum was already studying the peculiar rock structure of Serra Nevada, which had shown deep rock deformations in the same area.

Using the new data, the researchers made images of Serra Nevada through a technique known as receiving function analysis, which uses seismic waves to map the internal structure of the earth. Scientists have found that in the central region of the mountain range, the earth’s crust is currently detaching, a process scientifically known as lithospheric sinking. Kilb and Schulte-Pelkum reported the findings in December in the magazine Geophysical Research Letters.

The hypothesis was aligned with previous speculation that the area had undergone lithospheric sinking, which happens when the outer layer of the earth sinks into the lower layer of the cloak. Now, the study’s authors believe the process is underway and currently progressing north of the mountain range, according to the study.

“We compared our notes and realized that my strange signs of rocky structure and its strange deep earthquakes were in the same area,” explains Schulte-Pelkum. “So we decided to look more closely and found this whole story.”

What is happening under the Nevada Serra could offer a rare insight on how continents formed, according to Schulte-Pelkum. The discovery could also help scientists identify more areas where this process is happening, as well as providing a better understanding of earthquakes and how our planet operates, he adds.

How the continents formed

The upper layer of the earth, the lithosphere, is composed of the rigid crust and the top of the cloak, which is in a denser but more fluid state. This layer also contains the earth’s oceanic crust – a thinner and denser layer below the oceans – and the continental crust above this layer.

But as these sub-kings can exist in this ideal state, with continents at the top, it is something mysterious, according to Schulte-Pelkum. “The continents ended up being above the current level of the sea, luckily our luck, because they are made of less dense minerals on average,” he says. “To make you get louder (first), you have to get rid of the dense material.”

Lytospheric sinking is the process of denser materials being pulled to the bottom, while the less dense material emerges at the top, resulting in land creation. “It is pouring out part of this denser material in this layer of solid and viscous cloak underneath and basically disconnecting it so that it stops pulling the less dense material above,” she explained.

In the earth images below the Serra Nevada, the researchers found a distinct layer inside the cloak about 40 to 70 kilometers deep. This layer had specific impressions that gradually changed toward the north, showed the data.

If someone had a clay block with spots of different colors and squeezed it between their hands, the spots would begin to turn into stripes-this is similar to how rock deformations appear, according to Schulte-Pelkum.

In the South Sierra, the dense rocks had the strongest inherent stripes and showed that they have already separated from the crust, while in the central region this process seems to be underway. In Sierra Norte, there are currently no signs of deformation. This distinct layer inside the cloak would also explain the deep earthquakes that Kilb found, as the crust in the central region is exceptionally thick because it has been pulled down and is also colder than the hot cloak material typically found in these depths.

“The rock takes a long time to warm or cool. So if you move something, you know, pulling down or pushing up, takes time to adjust your temperature,” says Schulte-Pelkum.

Evidence of this process has been difficult to find. It is not visible from the surface and is an extremely slow process. Scientists theorize that southern Sierra has completed the process of sinking lithospheric for about 4 million to 3 million years, according to the study.

It seems that these natural events happen occasionally around the world, explains Schulte-Pelkum. “Geologically speaking, this is a very fast process with long periods of stability between them. This (sinking lithospheric) probably started to happen long ago when we started building continents, and (the continents) got bigger over time. So it’s just something punctual and localized,” he adds.

Ongoing debate

Sierra Nevada has been the subject of debate for decades in the geological community due anomaly found in the cloak located under the large valley.

While some scientists believe that lithospheric sinking has caused this feature, other scientists think it can be caused by the subduction, which is when an ocean plate sinks under a less dense plate, such as the continental crust, and changes the landscape, according to Mitchell McMillan, a researcher and postdoctoral geologist at Georgia Tech, who was not involved in the study.

“There are really two competing hypotheses to explain all this data, and you do not find it very often in geology. So this article will contribute to this whole discussion in a very interesting way,” he says.

Additional studies in this area could also help scientists better understand how Earth evolves in long time scales. If lithospheric sinking remains under the mountain range, it can be speculated that the earth will continue to extend vertically, changing the way the landscape now looks like, according to McMillan. But that could take hundreds of thousands to a few million years, the expert adds.

In general, large mountain ranges, or anywhere where there is a Batolito, is where these events are expected, says McMillan. The Andes, a long mountain range in South America, is an example of another place where scientists speculate that lithospheric sinking occurred once and may still be happening today.

“I think this particular study (highlights) the importance of connecting these different data sets,” says McMillan.

By better understanding this process, scientists can learn more about the functions of the planet and what happens under its surface, including the occurrence of earthquakes that were linked to this process, according to McMillan.

Separately, the planet Venus, which has no signs such as Earth, has evidence of these lithospheric sinking events, and by understanding the process on Earth, we can begin to apply it to Venus.

“It’s really fascinating to think that you could be taking a walk in Sierra or the hills, or even elsewhere on a continent. And, you know, there are things happening very deeply under you that we don’t realize,” says Schulte-Pelkum.

“In a way, we owe our existence on Earth to these processes. If the earth had not formed continents, we would be very different creatures. We evolved because the planet has evolved the way it has evolved. So, understanding the whole system you are part of it has value – besides reducing monetary damage and human impacts during, for example, an earthquake,” he adds.

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This content was originally published in deep earthquakes reveal rare geological process under the snack on CNN Brazil.

Source: CNN Brasil