Fresh Eye on the Radio: How earthquakes affect the day

Earthquakes alter planetary speed in two ways. Shifting plates rearrange the distribution of the Earth's mass, causing it to bulge imperceptibly in spots it didn't bulge before and contract in others. That rearrangement should further shift the Earth's inclination, or figure axis (the axis around which the Earth's mass is balanced, which is slightly different from the north-south axis around which the Earth rotates) -- in the case of the Chile earthquake, by about 3 inches. The law of conservation of angular momentum, however, requires that even under these exigent circumstances, the Earth's angular momentum stays constant, which means the planet must step on the gas (or the brake) to accommodate shifting mass. The same thing happened in 2004 with the 9.1 Sumatran earthquake that triggered the tsunami. That earthquake should have shifted the Earth's figure axis by 2.76 inches and shortened its day by 6.8 millionths of a second, according to computer models.

They also found the earthquake decreased the length of day by 2.68 microseconds. Physically this is like a spinning skater drawing arms closer to the body resulting in a faster spin. The quake also affected the Earth's shape. They found Earth's oblateness (flattening on the top and bulging at the equator) decreased by a small amount. It decreased about one part in 10 billion, continuing the trend of earthquakes making Earth less oblate.