Scientists have identified the deepest earthquake ever. It occurred 751 kilometers deep, below the surface of the Earth. Specifically, within the lower mantle. The point is that for seismologists an earthquake at that point appeared and still appears as impossible to occur. This is because under extreme pressure rocks are more likely to bend and deform rather than crack with a sudden release of energy.
Yet it seems to have gone differently. Minerals, explains Pamela Burnley, a geomaterials expert at the University of Nevada, don’t always behave as expected. “Just because they should change doesn’t mean they will,” Burnley told Live Science. What the earthquake could reveal, then, is that the boundaries within the Earth are more blurred than is often believed, explains ScienceAlert.
The earthquake was recorded in June in the magazine Geophysical Research Letters: it was actually a small aftershock following a magnitude 7.9 earthquake that shook the Bonin Islands off mainland Japan in 2015. Researchers led by University of Arizona seismologist Eric Kiser detected the earthquake using the series of Hi-net seismic stations distributed in the country. This is the most powerful network in use in the world for identifying earthquakes: the earthquake was not at all perceptible on the surface and was of limited magnitude, so such sophisticated tools were needed to identify it.
In reality, the depth estimate awaits further confirmation but the discovery seems reliable. And very interesting: the vast majority of earthquakes are in fact superficial. It originates within the earth’s crust and upper mantle within the first hundred kilometers below the surface. The crust, in particular, measures an average of 20 kilometers: there the rocks are cold and fragile. When they go under stress they can’t keep themselves whole that much and they break releasing energy like a spring. Deeper, however, the rocks are warmer and in fact subjected to higher pressures, which is why they tend less to break – if anything, to change state and deform. But at this depth, earthquakes can still occur when high pressures push fluid-filled pores in rocks, forcing the contained fluids to escape. Under these conditions, the rocks can also be subject to breakage similar to those that occur more on the surface.
Indeed, even before the last observed phenomenon, there had been earthquakes in the lower mantle, up to 670 kilometers deep. Earthquakes long considered mysterious by experts, because at those depths all the water should have been eliminated and therefore it should be complex to trigger ruptures of the kind we are used to within a hundred kilometers. An explanation could be found in the olivine group, a set of neosilicates of magnesium, manganese and iron that make up a large part of the mantle of our planet and take on different structures from transformation to transformation. These structures, from the upper mantle to the lower one, would seem less and less tending to break and therefore to create faults that trigger the release of energy, finally earthquakes.
Geologists are still not in agreement on the reasons why earthquakes occur in the upper mantle, an area on which they found a common point of fall only during the eighties linked to the passages and transformations of minerals, precisely of the olivine that jumping one stage and subjected to a certain pressure it could break. Giving rise to an earthquake in those transition areas under 400 kilometers. But precisely the new Japanese earthquake identified by the latest survey would have occurred even more in depth. One possible explanation is that the boundary between the upper and lower mantle is “not exactly where seismologists expect it to be in the Bonin region,” says ScienceAlert, quoting Heidi Houston, a geophysicist at the University of Southern California. Or. again due to an anomalous behavior of the minerals of the olivine group, which in fact do not conclude or even begin the transition of state due to elements that disturb those passages, such as colder continental crust plates that have overlapped in particular terrestrial geological regions, deeper than where they should be. That way that lower mantle layer would not be as hot as scholars would expect.
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