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The seismologist Deborah Kilb was wading through the records of earthquakes of California of the last four decades when she noticed something strange – a series of deep earthquakes that had occurred under the Sierra Nevada at a depth where the crust of the earth would generally be too hot and high pressure for seismic activity.
“In northern California, the data (from the earthquake) descended about 10 kilometers (6 miles). In Southern California, they will deepen a little more than 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 in which existing Quests usually occur in these regions.
But the earthquakes it found taking place near the central region of the mountain range were up to twice deeper – and seem to be in progress.
“The fact that we saw seismicity below 20 kilometers (12.4 miles) – like 20 kilometers at 40 kilometers (25 miles) – is very strange,” said Kilb. “This is not something that you would generally see in crustal earthquakes.”
Kilb reported the data from Vera Schulte-Pelkum, a researcher at the Cooperative Institute for Research in Environmental Sciences and Associate Research Professor of Geological Sciences at the University of Colorado Boulder. Schulte-Pelkum was already studying the particular rocky imprint of Sierra Nevada, which had shown deep rocky deformations in the same area.
Using new data, researchers have imagined Sierra Nevada thanks to a technique known as the receiver 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 taking off, a process scientifically known as the lithospheric foundation. Kilb and Schulte-Pelkum pointed out the results in December in the journal Geophysical research letters.
The hypothesis has aligned with previous speculations according to which the area had undergone a lithospheric foundation, which occurs when the external layer of the earth sinks into the lower layer of the coat. Now, the study authors believe that the process is underway and is currently progressing north of the mountain chain, according to the study.
“We compared the notes and realized that my signs of strange rocky fabric (the arrangement of the rocks) and its strange deep earthquakes were in the same area,” said Schulte-Pelkum. “So we decided to look at it more closely and found this whole story.”
What is happening under Sierra Nevada could offer rare information about how continents have formed, said Schulte-Pelkum. The observation could also help scientists identify more areas where this process occurs and better understand earthquakes and the functioning of our planet, she added.
The upper layer of the earth, the lithosphere, is made up of the rigid crust and the upper part of the coat, which is in a denser, but more fluid state. This layer also contains the oceanic crust of the earth – a thinner and denser layer under the oceans – and the continental crust which is above this layer. But the way in which these underlaying manages to exist in this ideal state, with the continents above, is a mystery, said Schulte-Pelkum.
“The continents just passed above the current sea level, fortunately for us, because … they are made of less dense minerals on average,” said Schulte-Pelkum. “To make it sit above (first of all), you have to get rid of some of the dense things.”
The lithospheric foundation is the process of the densest materials drawn downwards, while the less dense material emerges at the top, resulting in the creation of land. “He pours part of these more dense things in this solid and sticky coat layer below and somehow to detach it so that he stops getting things less dense above,” she explained.
In the imagery of the earth under the Sierra Nevada, the researchers found a separate layer in the coat at around 40 to 70 kilometers (25 to 43 miles) deep. This layer had specific imprints that have gradually changed due to the North, showed the data.
If we had to have a block of clay which had clay spots of different colors everywhere and tightened the clay in their hands, the spots would begin to turn into scratches – this is similar to the way in which rocky deformations appear, said Schulte -Pelkum.
In the south of Sierra, dense rocks had the strongest inherent stripes and turned out to have already sheailed away from the crust, while in the central region, this process seems to be in progress. In the north of Sierra, there is currently no signs of deformation. This separate layer inside the mantle would also explain the deep earthquakes that the kilb found, because the crust in the central region is exceptionally thick to be lowered and is also colder than the material of the hot coat generally found at these depths.
“Rock takes a long time to warm up or cool off. So if you move things, you know, pulling it down or pushing it, it takes a while to adjust its temperature,” said Schulte-Pelkum.
The evidence of this process was difficult to find. It is not visible above the ground, and it is an extremely slow process. Scientists theorize that the South Sierra finished the lithospheric foundation process about 4 to 3 million years ago, according to the study.
It seems that these natural events occasionally occur in the world, said Schulte-Pelkum. “Geologically speaking, this is a fairly rapid process with long periods of stability between the two … This (Lithosphere Foundation) probably started to occur a long time ago when we started to build continents, and (the continents) became larger over time. It is therefore just a kind of punctuated and localized thing,” she added.
Sierra Nevada has been a subject of debate for decades in the geology community due to an anomaly Found in the coat located under the large valley.
While some scientists think that the lithospheric foundation has caused this characteristic, other scientists think that it can be caused by a subduction, that is to say when an oceanic plaque flows under a less dense plate, such as the continental crust, and changes the landscape, said Mitchell McMillan, research geologist and postdoctoral scholarship shelf, which was not involved in the study.
“There are really two competing hypotheses to explain all this data, and you do not really get it very often in geology. … So, this document will add to all this discussion in a really neat manner,” he said.
A more in -depth study in this field could also help scientists better understand how the land is evolving on long time scales. If the lithospheric foundation continues under the mountain range, we can speculate that the earth will continue to stretch vertically, changing the appearance of the landscape now, said McMillan. But that could take several hundred to a few million years old, he added.
In general, large mountain belts, or wherever there is BatholithThis is where you expect to find these events, said McMillan. The Andes, a long mountain range in South America, is an example of another place where scientists speculate once on the lithosphere foundation and could still occur today, he added.
“I think this study in particular (underlines) the importance of linking these different data sets,” said McMillan.
By better understanding this process, scientists can know more about the functions of the planet and what is happening below its surface, including the presence of earthquakes that have been linked to this process, said McMillan. Furthermore, the planet Venus, which has no plates tectonics like the earth, has signs of these founding lithospheric events, and by understanding the process on earth, we can start applying it to Venus, he said.
“It’s really fascinating to think about how you could be … By hiking in Sierra or in the buttresses, or even everywhere else on a continent. And, you know, there are things that are very deep under you that we do not know,” said Schulte-Pelkum.
“We owe in a way our existence on earth to these processes which occur. If the earth had not made continents, we would be very different creatures. … We have evolved because the planet has evolved as it did. So, in a way, the whole system is being part, I think, she added.