Scientists have discovered the most profound earthquake ever detected, 467 miles beneath the Earth’s surface. At that deep, the quake occurred in the lower mantle, where seismologists considered earthquakes to be impossible. This is because, under tremendous pressures, rocks are more prone to flex and deform rather than break with a quick release of energy.
However, minerals do not always perform exactly as predicted, according to Pamela Burnley, a geomaterials professor at the University of Nevada, who didn’t participate in the research. Even under pressures that should cause them to convert into other, less quake-prone forms, they may persist in previous configurations.
The quake, which was initially published in the journal Geophysical Research Letters in June, was a modest aftershock following a 7.9-magnitude earthquake that hit the Bonin Islands off the coast of Japan in 2015. Using Japan’s Hi-net array of seismic sensors, researchers led by University of Arizona seismologist Eric Kiser discovered the quake. According to John Vidale, a seismologist at the University of Southern California who was not involved in the study, the array is the most powerful technology for detecting earthquakes in use today. Because the quake was so tiny and couldn’t be felt on the surface, sensitive sensors were required to locate it.
This raises some questions about the quake. The great majority of earthquakes are shallow, occurring within the first 62 miles of the Earth’s crust and upper mantle. The rocks in the crust are cold and brittle, extending down only about 12 miles on average. According to Burnley, when these rocks are stressed, they can only flex a bit before breaking, releasing energy like a coiled spring. The stones are hotter and under higher pressures deeper in the crust and lower mantle, making them less prone to breaking. However, earthquakes can occur at this depth when intense pressures force fluid-filled holes in the rocks, forcing the fluids out.
The issue with earthquakes that are deeper than around 249 miles is how minerals respond under pressure. A large portion of the planet’s mantle is composed of olivine, a bright, green mineral. The forces drove the atoms of olivine to rearrange into a new structure, a blue-ish mineral known as wadsleyite, some 249 miles underground. Wadsleyite reorganizes into ringwoodite another 62 miles deeper. Finally, 423 miles down in the mantle, ringwoodite decomposes into two minerals: bridgmanite and periclase. Of course, geoscientists cannot physically delve so deep into the Earth, but they may use laboratory equipment to simulate severe pressures and induce these changes.
Of course, geoscientists cannot physically delve so far into the Earth, but they may use laboratory equipment to simulate severe pressures and induce similar changes at the surface. Because seismic waves behave differently in various mineral stages, geophysicists may detect these changes by examining vibrations caused by significant earthquakes.
That last transition denotes the end of the upper mantle and the start of the lower cover. What distinguishes these mineral phases is not their names but rather how they act. According to Burnley, it is comparable to graphite and diamonds. Both are carbon but in different configurations. The form that is stable near the Earth’s surface is graphite, whereas the format that is stable deep in the mantle is diamond. And they have pretty distinct properties: graphite is soft, grey, and slick, whereas diamonds are exceedingly complex and precise. As olivine transforms into higher-pressure phases, it becomes more prone to bending and less prone to breaking in a way that causes earthquakes.