Earthquakes in 2010 and 2011 -- in Chile and in northeast Japan -- shortened the length of an Earth day fractionally by changing the distribution of the planet’s mass, in much the same way that a figure skater speeds up a spin by drawing her arms in closer to her body. Both quakes caused the Earth to rotate more quickly, trimming a total of 3.06 microseconds off our 24-hour day.
The 8.9 magnitude event in Japan and the 8.8 magnitude temblor in Chile were both thrust earthquakes. This type of event creates inward motion, which can shorten days. Other types of earthquakes -- such as a horizontal strike-slip quake in which two plates slide horizontally past one another -- have no effect on Earth's rotation.
Hold on a microsecond:
- A microsecond is a millionth of a second.
- One Earth day is about 86,400 seconds long. In a year, its length varies by about 1,000 microseconds, depending on seasonal variations such as the shift in the jet stream.
- The measurement of a day is accurate only to about 20 millionths of a second. So the effect of an earthquake can be estimated, but not accurately measured.