The Quake That Shortened Our Days

Earthquakes like the one that occurred in Chile can change the way the earth rotates.

Researchers at NASA’s Jet Propulsion Laboratory have calculated that the February 27, magnitude 8.8 earthquake in Chile actually shifted the axis of Earth’s rotation by about eight centimetres, and shortened the length of a day by 1.26 microseconds (in other words, it sped up the Earth’s rotation very slightly).

As commented by David Kerridge of the British Geological Survey in a Financial Post article, this is an interesting but rather “esoteric” finding.

In fact, any movement of mass on the planet will cause a slight shift in the way the Earth rotates, in the same way that tampering with a cricket ball (which is against the rules, by the way) can change the way it spins. The shift is so small, however, that it has no perceptible effect.

An earthquake represents a rather large and sudden movement of planetary mass, as two of the Earth’s plates slide past each other. The Chilean earthquake was particularly effective in this regard because one plate (the Nazca plate, which underlies the part of the Pacific Ocean adjacent to Chile and Peru) slid downwards underneath the South American plate. This type of motion results in vertical displacements of mass, which change the shape of the Earth and so change its rotational behaviour.

These vertical movements are also more likely to cause tsunamis because the ocean floor drops as the plates shift. The 2004 magnitude 9.1 Sumatran earthquake was caused by similar vertical plate motions, and generated a catastrophic tsunami that killed hundreds of thousands of people.

In contrast, the magnitude 7.0 earthquake in Haiti earlier this year involved the horizontal sliding of two plates past each other. This had less of an effect on the shape of the planet and did not cause a tsunami because the level of the ocean floor did not change very much. It did, however, cause much more damage and a far greater loss of life than the Chilean event, even though it was two orders of magnitude less powerful.

This was because horizontal slippage tends to cause side-to-side shaking, or shearing, which is potentially much more damaging for buildings than up-and-down motions. Exacerbating the problem was the poor quality of construction in Haiti compared to the modern buildings in Chile, which are designed to resist earthquakes. Typical building codes for earthquake prone regions (at least where people can afford to adopt such codes, something they could not do in impoverished Haiti) require the use of steel reinforcement in concrete structures, which gives normally brittle concrete some flexibility and shear strength. Such buildings might sway alarmingly in an earthquake, and may even sustain serious damage, but will resist outright collapse, thus allowing occupants time to escape.

Is it a coincidence that the Haitian and Chilean earthquakes occurred within a few weeks of each other, with a third, magnitude 6.4 event occurring in Taiwan on March 4? Or are they connected? Most likely, it is merely a coincidence, although any time the configuration of the Earth’s plates shifts in one location, it subtly changes the stress field on plates across the world. However, the Haitian and Chilean quakes were separated by several other plate boundaries, so it seems unlikely that significant stress could have been transmitted from Haiti to Chile.

Earthquakes of the magnitude of the Haitian event occur 10 to 20 times per year globally, while ones like the quake in Taiwan are even more common (over 100 per year). Even earthquakes of magnitude 8 to 9, such as the Chilean event, occur on a frequency of one every one to 20 years.

It may be the devastation and loss of life in Haiti (even though it was in fact a relatively small quake) and the large size of the Chilean earthquake have heightened media awareness of earthquakes so that even small and relatively non-destructive events like the one in Taiwan receive international media attention.