A day on Earth is commonly understood to last 24 hours. However, the time Earth takes to complete one rotation can change, even if the difference is extremely small.
The earthquake that struck Aceh on December 26, 2004, is one example. According to NASA calculations, the quake made Earth rotate slightly faster, shortening the length of a day by a few microseconds, or millionths of a second.
NASA's initial calculation in 2005 estimated a reduction of 2.68 microseconds. A calculation revisited in 2011 put the figure at 6.8 microseconds. These numbers came from different calculations, not direct measurements of changes in the length of a day following the earthquake.
So, how could an earthquake beneath the ocean affect the rotation of an entire planet?
The Aceh Earthquake Shifted the Seafloor on a Massive Scale
On December 26, 2004, a magnitude 9.1 earthquake struck off the northwestern coast of Sumatra. Known as the Sumatra-Andaman earthquake, it was one of the largest earthquakes ever recorded.
The earthquake caused massive shifts in the seafloor. According to NOAA, a rupture along the fault extended at least 1,200 kilometers northward in about eight minutes. In some areas, the Earth's crust shifted by an average of up to 15 meters.
This movement displaced large volumes of water and triggered a tsunami that swept across the Indian Ocean region. Waves reached coastal areas in several countries, including Indonesia, Sri Lanka, India, and Thailand.
A 2014 update from NOAA recorded 227,898 people dead or missing as a result of the disaster. Indonesia was the hardest-hit country, with 167,540 victims. Other sources reported different death tolls, ranging from more than 230,000 to 283,000 people.
In Aceh, tsunami heights reached 20 to 40 meters at several locations along the northwestern coast. Field surveys recorded a maximum height of 51 meters.
However, the earthquake's effects went beyond shifting the seafloor and triggering a tsunami. The movement of such large masses of rock could also influence how Earth rotates.
How Can an Earthquake Make a Day Shorter?
To understand this, imagine Earth as a giant ball spinning around an axis. One complete rotation provides the basis for measuring the length of a day. If Earth spins slightly faster, it takes less time to complete that rotation.
The speed of Earth's rotation is related to how mass is distributed throughout the planet. When an earthquake shifts large amounts of rock, the distribution of mass relative to Earth's rotation axis changes. This can make the planet spin slightly faster or slower.
A simple example can be seen in a figure skater spinning on ice. When the skater pulls their arms closer to their body, they spin faster. Changing the position of their body's mass affects the speed of their rotation.
A similar principle applies to Earth, although the processes involved are much more complex. The Aceh earthquake shifted mass within the planet, leading scientists to estimate that Earth would take slightly less time to complete one rotation.
On January 10, 2005, Richard Gross of NASA's Jet Propulsion Laboratory (JPL) and Benjamin Fong Chao of NASA's Goddard Space Flight Center published calculations of the earthquake's effects. Their results indicated that the Aceh earthquake shortened the length of a day by an estimated 2.68 microseconds.
For context, one microsecond is one-millionth of a second. The change calculated by NASA was far too small for people to notice in everyday life.
The Earthquake Also Changed Earth's Axis and Shape
NASA's calculations also suggested that the Aceh earthquake affected Earth's rotation axis and shape. The rotation axis is an imaginary line running through the North and South Poles around which the planet spins.
The 2005 calculation estimated that the average rotation pole shifted by about 2.5 centimeters toward 145 degrees east longitude. It also suggested that Earth's shape changed slightly, making the planet marginally rounder.
This does not mean Earth suddenly shifted out of place or changed shape in a way visible to the naked eye. These changes occurred on an extremely small scale and were calculated using scientific models based on the earthquake's characteristics.
This distinction between calculation and direct measurement is important. NASA did not claim that the change in the length of the day had been directly observed or measured when the earthquake occurred. Instead, the figures estimated how the redistribution of mass caused by the quake could affect Earth.
Why Did NASA's Calculations Produce Different Numbers?
In 2011, NASA revisited the effects of the Aceh earthquake in calculations that also compared it with the 2010 Chile earthquake and the 2011 Japan earthquake. This time, the Aceh earthquake was estimated to have shortened the day by 6.8 microseconds and shifted Earth's figure axis by about 7 centimeters.
The difference between 2.68 and 6.8 microseconds does not mean that the length of a day changed from the first figure to the second over time. Both numbers were estimates derived from different calculations of the same earthquake.
The figures for axis displacement also refer to different things. The 2005 estimate of about 2.5 centimeters refers to the shift in the average rotation pole. The later estimate of about 7 centimeters refers to Earth's figure axis, which is related to the planet's shape and mass distribution.
Therefore, the 2.5-centimeter and 7-centimeter figures cannot be compared directly because they describe different measurements. NASA's calculations also show that major earthquakes can affect Earth's rotation and axes in different ways, depending on where they occur and how the ground moves.
For comparison, the magnitude 8.8 Chile earthquake in 2010 was estimated to have shortened a day by 1.26 microseconds and shifted Earth's figure axis by about 8 centimeters. The 2011 Japan earthquake was estimated to have shortened a day by 1.8 microseconds and shifted the figure axis by about 17 centimeters.
Although the Chile earthquake was smaller than the Aceh earthquake, its estimated axis displacement was greater. This is because an earthquake's effects depend on more than its magnitude. Its location and the direction of ground movement also influence how much mass shifts and how that movement affects Earth's axis.
Tiny Changes That Scientists Can Calculate
Although these changes can be calculated, that does not mean they are easy to confirm through direct observation. Earth is constantly moving and rotating, while the atmosphere, ocean currents, and changes in water stored on land also affect the planet's motion. Scientists must account for these influences to distinguish small changes caused by earthquakes from those produced by other natural processes.
According to JPL's 2011 release, Earth's figure axis could be measured with an accuracy of about 5 centimeters. This meant that the estimated 17-centimeter shift caused by the Japan earthquake could potentially be detected.
However, scientists would first need to separate the effects of the atmosphere and oceans from the observational data to determine the change associated with the earthquake.
The Aceh earthquake shows that a major event in one part of Earth can have effects far beyond what is visible at the surface. Beyond triggering a tsunami and reshaping coastlines, the movement of mass caused by an earthquake can also affect the planet's rotation, even if the change amounts to only a few microseconds.
