Why shifting rock can change Earth’s spin
An earthquake does more than shake the ground: when a fault slips, it changes where some of Earth’s mass sits. Rock may move horizontally, sink, or rise, slightly altering the planet’s shape and the distribution of mass inside it. The change is tiny, but Earth’s rotation responds to it.
The key quantity is the moment of inertia: how strongly a rotating object resists a change in its spin, based on how far its mass lies from its axis. Mass closer to the axis contributes less resistance. If an earthquake shifts mass inward overall, Earth’s moment of inertia falls. With angular momentum—the rotational motion Earth carries—nearly conserved, the planet spins a little faster, much as a skater speeds up by drawing in their arms.
A change far below everyday notice
For the earthquake described here, the estimated change in the length of a day was about 1.8 microseconds. A microsecond is one-millionth of a second, so this is far too small to notice in daily life. It is a change in Earth’s rotation rate, not a sudden jump in the time shown on ordinary clocks.
Earth’s mass is enormous, and the rearrangement caused by even a major earthquake is minuscule beside it. The effect depends on exactly where rock moves relative to the rotation axis; not every earthquake shortens the day. But the mechanism offers a striking reminder that a day’s length is not perfectly fixed: movements deep in the planet can make Earth turn, imperceptibly, at a slightly different pace.
