The rotation speed and shape of Earth's deepest layer has been experiencing some surprising changes.
At the center of our planet is a very hot rotating ball of mostly iron and nickel (sorry Journey to the Center of the Earth fans). Scientists infer features of this innermost geographic layerâ€"like the fact that it's divided into a liquid outer core and a solid inner coreâ€"by analyzing how it distorts seismic waves caused by earthquakes.
Using this method, seismologists from the U.S. and China have built on previous studies to suggest that the last two decades may have seen changes in both the inner core’s shape and rate of rotation. Their work, detailed in a study published today in the journal Nature Geoscience, sheds light on previously unknown dynamics taking place 1,802 miles (2,900 kilometers) beneath our feet.
Repeating earthquakes produce similar seismic waves at different times. By analyzing seismic waves from repeating earthquakes that pass through Earth's core, scientists can track changes in the core over time. Previous research using this method suggested that around 2010, the inner core's rotation slowed to a rate below that of the rest of the planetâ€"after previously spinning fasterâ€"according to researchers.
In the recent study, seismologist John E. Vidale from the University of Southern California, Los Angeles, and his colleagues analyzed pairs of repeating earthquakes from before and after 2010. They focused on cases where, by the time the second earthquake struck, the inner core’s rotation had returned to the same position as it was during the first earthquake. This meant that any difference between the seismic waves of the two earthquakes couldn’t have been caused by the rate of rotation.
The team then discovered that the seismic wave pairs that crossed shallow regions of the inner coreâ€"regions close to its boundary with the outer coreâ€"showed surprising differences. The researchers suggest that these differences could point to changes in the shape of our planet's inner core.
"The changes near the inner-core boundary most likely result from viscous deformation driven by coupling between boundary topography and mantle density anomalies or traction on the inner core from outer-core convection," the researchers explained in the study. In other words, the inner core's shape could be changing over time because of complex interactions among the Earth’s layers.
The researchers found that changes in seismic waves traveling through the inner core are likely caused by both its rotation and activity near its boundary, helping to settle a long-standing debate.
"Previous research has proposed that the inner core has undergone either rotational or shape changes through time, but not both simultaneously," the researchers further explained in a statement.
While these results might not have an obvious impact on the average person's day-to-day life, they provide insight into the obscure mechanisms that run this planet we call home.