Priscilla Porter, of Middlemarch, asks :-

I understand the horrific 9.1-9.2M earthquake in the Indian Ocean in 2004, which killed hundreds of thousands of people in multiple countries, caused the earth to wobble on its axis. Little has ever been reported on this. Has the Earth stopped wobbling? Has it shifted on its axis? If so, has it contributed towards global warming?

Bob Hurst, a physicist who worked with the University of Canterbury's large Ring Laser, responded.

It was estimated just after that Indian Ocean quake that it might have shifted the rotation axis of the Earth, in the order of a few centimeters, and also the rotation rate might have speeded up, by a few microseconds per day. These were based on computer modeling, using the estimated redistribution of the mass of the Earth.

The calculated changes were too small to show up in short-term observations, and as time passed they soon were swamped by other more “routine� changes going on such as tidal drag (slowing the rotation) and ice melts (changing the axis a little). So I don’t believe there were any definitive measurements of these shifts.

As for “wobbling� of the axis, it depends what you mean. Certainly there was wobbling of the Earth’s crust, as happens in earthquakes, due to the seismic waves generated. For this quake the waves were detectable for many hours afterwards. The seismic waves also produce local rotational effects which change the rotation and (locally) this might be interpreted as wobbling of the axis. But taking a larger view, no, the quake didn’t wobble the axis.

Routinely, there are external forces acting on the Earth due to its oblate shape (equatorial bulge) interacting with the gravitational attractions of the moon, sun, and planets and these do indeed cause the axis to wobble. But these wobbles are fairly slow. There is a component with a period very close to 1 day, and there are other components with timescales even up to about 26000 years, rather than the much shorter timescales associated with earthquakes.

Canterbury University used to operate a large ring laser gyro in a cavern under the Port Hills (see then ringlaser, which could detect such wobbles, but unfortunately it was undergoing a rebuild at the time of the Indian Ocean quake. I don’t know of any mechanisms which would cause the quake to contribute to global climate change.

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