BSince it was discovered about a hundred years ago, earthquakes that occurred at a depth of 700 kilometers have puzzled geoscientists. How can cracks occur in the rock at a depth where the pressure and temperature are so high that the rock undergoes plastic deformation and does not break under the influence of mechanical stress like a brittle crust does?
Very rare but clearly measurable deep earthquakes indicate that this is clearly still possible, with the strongest magnitude 8.3 measured so far. The earthquake occurred in May 1995 with a focal depth of 610 kilometers, located below Borovia on the Nazca Plate, which sinks under the South American Plate along the west coast of South America. On May 30, 2013, an earthquake occurred in Japan with a depth of 695 kilometers. A team of researchers at the Carnegie Institution in Washington has now found a possible solution to the mystery of these deep earthquakes-diamonds are “contaminated” by trapped minerals.
most earthquake Occurs in the earth’s crust, which is 70 kilometers deep. At such a deep place, the rock is always very brittle and may suddenly crack when subjected to mechanical force. However, as the temperature increases, this brittleness disappears and the rock becomes plastic, similar to modeling clay. Since cracks are theoretically impossible, geoscientists have long believed that phase transitions are the cause of deep earthquakes.
How water acts as a lubricant and promotes rock fracture
The process under consideration can be compared to the bursting of a water bottle left in the freezer of a refrigerator for too long. Because the density of ice is 7% less than that of water, it expands when it freezes. The force generated is large enough to cause the brittle glass to burst, that is, to break it.
Diamonds from the deep sea. The mineral inclusions are clearly visible.
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Image: Evan Smith/2021 GIA
On the other hand, at depths of hundreds of kilometers, the transition between the two minerals olivine and spinel causes a significant change in volume. Although the chemical properties are the same, the density of spinel is about 7% higher than that of olivine. During the transition from olivine to spinel, the volume will decrease and it will suddenly implode under the tremendous pressure of the mantle, which will become obvious. The rock collapsed suddenly and an earthquake occurred. Because all olivine has been converted to spinel below about 700 kilometers, no more earthquakes will occur at these depths.
However, this interpretation has a drawback. Using modern mathematical methods, one can reconstruct the physical mechanism of the seismic source from the seismograms of multiple seismic stations. A clear distinction can be made between implosion and other processes that cause earthquakes, such as explosions or ruptures. However, almost all deep earthquakes analyzed in this way show rupture rather than implosion as a “focusing mechanism”, as predicted by the phase transition hypothesis.
The research team around Steven Shirey in Washington now describes “progress” in AGU Magazine Another possible cause of deep earthquakes. The most important role is water, that is, water molecules that chemically bind to the rock. In very deep places, these water molecules can loosen from the rock, making it easy to break. The water acts as a lubricant, so that two rocks can suddenly slide against each other, similar to a brittle fracture.
Diamond analysis puts researchers on the right track
At first, this idea seemed ridiculous, because it was previously thought that water molecules could not survive such a deep subduction zone. But petrologists who study diamonds from the depths contradict this. These scientists found inclusions in many of these gemstones that were clearly derived from hydrous minerals. Shirey and his colleagues have now discovered that these diamonds come from areas where deep furnace earthquakes have also occurred.
Therefore, if there are water-containing minerals underneath, as the analysis of diamonds shows, the water will also separate from the rock and produce lubrication that may cause earthquakes. If this new hypothesis is confirmed in further investigations, geoscientists will be one step closer to solving the mystery of deep earthquakes.
