Between 1976 and 2020, nearly 57,000 earthquakes rattled our planet. The bulk of them were shallow, and only a mere four percent occurred beyond 186 miles deep, which was thought to be the maximum depth for what scientists call "deep earthquakes," reports Maya Wei-Haas for National Geographic.
Now, a team of researchers has zeroed in on what could be the deepest earthquake ever detected, shaking up scientists' understanding of them. In 2015, a 7.9 magnitude earthquake struck beneath Japan's Bonin Islands. One of the aftershocks occurred deeper than the original earthquake itself, at 467 miles. It's so deep that it nears the layer of Earth known as the lower mantle, reports Andrei Ionescu for Earth.com.
"This is by far the best evidence for an earthquake in the lower mantle," Douglas Wiens, a seismologist at Washington University in St. Louis who was not involved in the study, tells National Geographic.
The study, published in the journal Geophysical Research Letters, used measurements collected by the High Sensitivity Seismograph Network, a string of stations across Japan that record seismic data. They were able to trace the origin of the seismic waves produced by the 7.9 magnitude earthquake and its aftershocks, according to a press release.
But what puzzled this team is that the shock erupted in the lower mantle, closer to Earth's core. There, temperatures can exceed 6,000 degrees Fahrenheit and the pressure is 1.3 million times the atmospheric pressure.
Deep earthquakes occur at subduction zones, where two tectonic plates collide and one is forced below the other, sending shockwaves through the Earth, National Geographic reports. But in such intense elements, rock tends to bend instead of break, begging the question: How did this earthquake even happen?
The researchers introduced a few possibilities. First, the molecular structure of minerals becomes unstable as pressure increases further into the mantle. That deformation could leave weak spots in the rock, causing earthquakes. Another theory is that the larger earthquake caused a torn slab of the seafloor to shift, and even a miniscule shift is enough to cause an earthquake, reports National Geographic.
This discovery throws a wrench in what geologists thought they knew about earthquakes in the lower mantle. They were surprised that one could occur so deep in the Earth, raising questions about the mechanisms at play beneath our feet.