Mysterious deep-seat quakes: When it rumbles deep inside the earth

Mysterious deep-seat quakes: When it rumbles deep inside the earth

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Mysterious deep-seat quakes: When it rumbles deep inside the earth

E.r earthquakes that occur at great depths of up to 700 kilometers have been puzzling geoscientists since they were discovered almost a hundred years ago. How can fractures occur in the rock at a depth where the pressure and temperature are so high that the rock is plastically deformed there and does not, as in the brittle earth’s crust, break under the influence of mechanical stresses?

That this is obviously still possible is shown by the very rare but clearly measurable deep-center earthquakes, of which the strongest tremor measured so far had a magnitude of 8.3. This quake occurred in May 1995 at a depth of 610 kilometers under Bolovia on the Nazca Plate, which dips under the South American Plate along the west coast of South America. A quake off Japan on May 30, 2013 holds the record depth of 695 kilometers. A group of researchers from the Carnegie Institution in Washington has now found a possible solution to the mystery of these deep earthquakes – in diamonds that were “contaminated” with trapped minerals.

Most earthquakes occur in the earth’s crust, which is up to 70 kilometers deep. Down to such depths, the rock is consistently quite brittle and can suddenly break when exposed to mechanical forces. As the temperature rises, however, this brittleness is lost and the rock becomes plastic, similar to modeling clay. Because fractures are theoretically impossible there, geoscientists have long held phase transitions responsible as the cause of deep-seat earthquakes.

How water acts as a lubricant and promotes fractures in the rock

The process under consideration can be compared to the bursting of a water bottle left in the freezer compartment of a refrigerator for too long. Because ice is around seven percent less dense than water, it expands when it freezes. The resulting forces are large enough to cause brittle glass to burst, i.e. to break it.

A diamond from a great depth. The mineral inclusions are clearly visible.
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Image: Evan Smith / 2021 GIA

At a depth of several hundred kilometers, on the other hand, it is the transition between the two minerals olivine and spinel that leads to a significant change in volume. Although chemically identical, spinel is about seven percent denser than olivine. During the transition from olivine to spinel, there is a reduction in volume, which becomes noticeable as a sudden implosion under the great pressure in the earth’s mantle. The rock collapses suddenly and an earthquake occurs. Because all olivine has already been converted into spinel below about 700 kilometers, there are no more earthquakes at these depths.

However, this explanation has a disadvantage. With modern mathematical methods one can reconstruct the physical mechanism in an earthquake focus from seismograms of several earthquake stations. A clear distinction can be made between an implosion and other processes that lead to earthquakes, such as an explosion or a rupture. Almost all deep-center earthquakes analyzed in this way, however, show a rupture and not an implosion as the “focus mechanism”, as the phase transition hypothesis predicts.

The research group around Steven Shirey from Washington now describes Advances in the journal AGU another possible cause of deep-seat quakes. The most important role is played by water, namely water molecules that are chemically bound in rocks. At great depths, these water molecules can be loosened from the rock, making it prone to fracture. The water acts as a lubricant, so that two rocks can suddenly slide against each other, similar to a brittle fracture.

The analysis of diamonds puts the researchers on the right track

At first, this idea seemed absurd, because it was previously not considered possible that the water molecules would survive the journey within a subduction zone at such great depths. But petrologists who study diamonds from great depths contradict this. These scientists have found inclusions in many of these gemstones that are clearly derived from hydrous minerals. Shirey and his colleagues have now found out that these diamonds come from areas in which deep-hearth quakes also occur.

So if there are water-containing minerals down there, as the analysis of the diamonds has shown, then the water can also detach itself from the rock and develop the possible lubricating effect that causes earthquakes. If this new hypothesis is confirmed in further investigations, geoscientists would have come a good deal closer to solving the mystery of deep-seat earthquakes.