Deep Earthquakes Could Reveal Secrets of the Earth’s Mantle
Published:29 Mar.2023 Source:University of Chicago
Most of the earthquakes we hear about on the news are relatively shallow, originating in the top crust of the Earth. But occasionally, there are earthquakes that originate deep within the Earth -- down to 450 miles below the surface. These earthquakes are not as well-studied as shallower ones, because they’re not as destructive to human settlements. But because they reach down into the mantle, Park thought they might offer a way to understand the behavior of the mantle.
Park and her colleagues looked at one particular such earthquake, which occurred off the coast of Fiji in 2018. The quake was magnitude 8.2, but it was so deep -- 350 miles down -- that it did not cause any major damage or deaths. However, when the scientists carefully analyzed the data from GPS sensors on several nearby islands, they found the Earth kept moving -- after the earthquake was over. The data revealed that in the months following the quake, the Earth was still moving, settling in the wake of the disturbance. Even years later, Tonga is still moving slowly down at a rate of about 1 centimeter per year. “You can think of it like a jar of honey that slowly comes back to level after you dip a spoon in it -- except this takes years instead of minutes,” said Park. This is the first solid observation of the deformation following deep quakes; the phenomenon had been observed before for shallow earthquakes, but experts thought the effect would be too small to be observable for deep earthquakes. Park and her colleagues used this observation to infer the viscosity of the mantle.