When the 2011 Japan earthquake shoved a tectonic plate deep into the mantle, Earth's mass shifted inward. To conserve angular momentum, the planet spun faster, shortening our day.
The same physical law that makes a figure skater spin faster when they pull their arms in governs the Earth. Our world is not a perfectly rigid, solid sphere; it is a dynamic, squishy planet that constantly rearranges its mass through geological and atmospheric processes. When that mass shifts closer to or farther from the planet's axis of rotation, the Earth must speed up or slow down to keep its total angular momentum constant.
Here is how this mass redistribution changes the length of our day:
Melting Polar IceGlobal climate change is currently melting vast ice sheets in Greenland and Antarctica. These ice masses are located near the poles—right on Earth’s axis of rotation. As the ice melts, the resulting water flows into the global oceans and bulges around the equator. In the figure skater analogy, this is the Earth throwing its arms out. Moving that mass away from the rotation axis increases Earth's moment of inertia, forcing the planet to spin slower. Because of this, our days are currently getting longer by tiny fractions of a millisecond.
Glacial Rebound
During the last Ice Age, heavy ice sheets covered regions like Canada and Northern Europe, pressing down on the Earth's crust and squeezing mantle material outward. When the ice melted, that immense weight vanished. Ever since, the depressed land has been slowly rebounding, pulling dense, deep mantle material back up toward the poles. Because this dense material is moving closer to the Earth's axis of rotation, it acts like a skater pulling their arms in. For thousands of years, this post-glacial rebound has been steadily speeding up Earth's rotation.
Massive Earthquakes
When tectonic plates violently shift during huge earthquakes, they can reorganize vast sections of the crust in an instant. As seen in the 2011 Japan quake, when a dense plate dives deeper toward the center of the Earth through subduction, mass moves closer to the rotational axis. This sudden inward shift acts like a skater sharply pulling their arms in, speeding up the planet's rotation by microscopic but measurable amounts, such as the 1.8 microseconds shaved off the day in 2011.