The rocks can also fracture at a depth of 700 km. – Woodland Herald

Earthquakes at depths as great as 700 kilometers have puzzled geologists since their discovery nearly a hundred years ago. How can fractures occur in rocks at a depth where the pressure and temperature are so high that the rocks are deformed there and do not break, as in the fragile crust of the Earth, under the influence of mechanical stresses?

This is clearly still possible as evidenced by the very rare but clearly measurable deep earthquakes, which were the strongest tremor measured so far with a magnitude of 8.3. This earthquake occurred in May 1995 at a depth of 610 km below Bolovia on the Nazca Plate, which dips under the South American Plate along the western coast of South America. An earthquake in Japan on May 30, 2013 recorded a record depth of 695 km. A group of researchers at the Carnegie Institution of Washington has now come up with a possible solution to the mystery of these deep earthquakes: in diamonds “contaminated” with trapped minerals.

Most earthquakes occur in the Earth’s crust up to 70 kilometers deep. Even at these depths, the rock is permanently brittle and can suddenly fracture when subjected to mechanical forces. However, as the temperature rises, this brittleness is lost and the rocks become plastic, similar to modeling clay. Because fractures are theoretically impossible there, geologists have long blamed phase shifts as the cause of deep earthquakes.

How does water act as a lubricant and promote rock fracture

The considered process can be compared with the explosion of a bottle of water left in the freezer of the refrigerator for a long time. Since the density of ice is about seven percent less than the density of water, it expands when it freezes. The resulting forces are large enough to blow fragile glass, that is, break it.

A diamond of great depth. Metallic impurities are clearly visible. Photo: Evan Smith / GIA 2021

On the other hand, at a depth of several hundred kilometers, the transition between the two minerals olivine and spinel leads to a significant change in size. Although spinel is chemically identical, it is seven percent denser than olivine. During the transition from olivine to spinel, a decrease in volume occurs, which is felt as a sudden implosion under the great pressure of the Earth’s mantle. The rock suddenly collapses and an earthquake occurs. Since all the olivine has already turned into a spinel less than 700 km away, there are no more earthquakes at these depths.

However, this interpretation has a downside. Using modern mathematical methods, the physical mechanism in seismic focus can be reconstructed from seismograms from different earthquake stations. A clear distinction can be made between an implosion and other processes that lead to earthquakes, such as an explosion or rupture. However, nearly all deep-center earthquakes analyzed in this way show a rupture rather than an implosion as the ‘focusing mechanism’, as predicted by the phase transition hypothesis.

The research group on Stephen Sherry of Washington now describes another possible cause of deep earthquakes in the journal AGU Advances. The most important role is played by water, that is, water molecules that are chemically attached to rocks. At great depths, these water molecules can separate from the rocks, making them susceptible to fracture. Water acts as a lubricant, so 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 absurd, because previously it was not possible for water molecules to survive traveling within the subduction zone at such great depths. But petroleum scientists who study diamonds from great depths contradict this. These scientists have found inclusions in many of these gemstones that are clearly derived from wet minerals. Sherry and her colleagues have now discovered that these diamonds come from regions where deep earthquakes also occur.

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Therefore, if there are minerals that contain water there, as the analysis of the diamond has shown, the water can also dislodge from the rock and create a potentially lubricating effect that causes earthquakes. If this new hypothesis is confirmed in further research, geologists will be much closer to solving the mystery of deep earthquakes.

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