Twisted logic

Researchers have succeeded in measuring the twisting effects of earthquakes for the first time using ring laser technology, allowing structural engineers to calculate how buildings move during shocks.

Researchers in New Zealand have succeeded in measuring the twisting effects of earthquakes for the first time using ring laser technology.

The discovery will allow structural engineers to calculate precisely how buildings move during shocks, and allow the formulation of new technologies and rules that will ensure structures are able to withstand the twisting forces.

Buildings are created to withstand sideways and vertical movements during an earthquake.

But researchers from Canterbury University, Christchurch, say twisting may shake a 10-storey building by as much as 10cm at its top.

‘The twisting motion of structures is like doing the twist dance,’ said Prof John Mander, chair of structural and earthquake engineering at the University of Canterbury. ‘Building columns tend to do this naturally if they are not loaded symmetrically.’

Construction codes include margins to account for rotation, but calculations are imprecise and earthquakes’ effects are sometimes greater than expected owing to twist.

The ring laser project is the result of a collaboration between the university and companies and universities in the US and Germany.

The team built the world’s largest and most precise ring laser gyroscope at a site called Cashmere Cavern, a World War II bunker 30ft below ground.

With better mirrors and larger dimensions, the device becomes increasingly sensitive to rotation but also much harder to construct and operate.

The stable temperature and the solid volcanic rock of the cavern offer ideal conditions for ring laser operations.

Two concentrated and amplified laser beams were fired in opposing directions around a rectangle 21m long and 17.5m wide with highly reflective mirrors placed at each corner.

The beam travelling in the same direction as the Earth’s rotation moved slightly faster than the other, allowing researchers to calculate the speed of the Earth’s movement by noting the difference in velocity between them.

The Cashmere Cavern system can provide data to a scale of parts per billion of the Earth’s rotation.

This rate varies each day according to patterns governed by factors such as the pull of the Moon’s gravity and its effect on water pressure on the sea bed, as well as earthquake forces.

By placing four seismometers used to measure sideways and vertical movement caused by earthquakes in a seven km radius around the cavern, and a fifth one right next to the laser, the group can detect teleseismic waves in the Earth’s crust caused by quakes thousands of miles away.

By comparing these to rotational changes observed by the laser at certain points, the team can see the amount of rotation caused by quakes of different intensities, taking expected rotational variation into account.

The machine can also measure Earth tides, where rocks are stretched backwards and forwards slightly each day.

The team now plans to put a new ring laser on an upper floor of the university’s physics building to measure the effects of twisting from remote earthquakes in an actual structure.

The ring will be a prototype for a German-funded project which now aims to develop modular ring lasers commercially for civil engineers studying building response in earthquake prone regions of the world such as California and Japan.

Their findings could then be used to develop improved guidelines for construction in such zones based on rotation behaviour data.