The 2011 Japan Earthquake Shortened Earth's Day by 1.8 Microseconds

On March 11, 2011, the Tohoku earthquake ruptured a 500-kilometer stretch of seafloor off Japan's northeast coast. The magnitude 9.0 quake moved so much mass — redistributing oceanic crust and mantle material — that it slightly changed Earth's moment of inertia, the rotational equivalent of mass.

When mass moves closer to a rotating body's axis, rotation speeds up — the same principle that makes a figure skater spin faster when they pull in their arms. Richard Gross at NASA's Jet Propulsion Laboratory calculated that the quake shifted enough mass toward Earth's axis to shorten the length of a day by approximately 1.8 microseconds.

That's about 0.0000018 seconds. Not perceptible, not even measurable at the precision available to most instruments, but calculable and real. The earthquake also shifted Earth's figure axis — the axis around which Earth's mass is balanced — by roughly 17 centimeters.

The 2004 Indian Ocean earthquake, at magnitude 9.1, had a larger effect: NASA estimated it shortened the day by about 6.8 microseconds and shifted the figure axis by nearly 7 centimeters. Large earthquakes routinely nudge these values. The planet's length of day fluctuates by up to a millisecond over years anyway, driven by atmospheric pressure changes and ocean currents — the earthquake effect is a small perturbation on an already variable system.