Tracing the shake: Big quake assessed

The gravitational force of Sunday's earthquake has been estimated at 0.16G in Wellington and Porirua - about the same as a passenger feels when taking off in a Boeing 747 aircraft.

But the swiftly changing direction would have made it much more noticeable. "The difference in an earthquake is that you feel it back and forth," GNS seismologist Anna Kaiser said.

People in Ward, near Seddon, would have felt the strongest G-force, at about 0.2G.

A University of California Berkeley report has said that "between 0.1G and 0.2G, most people will have difficulty keeping their footing and sickness symptoms may be induced".

The National Aeronautics and Space Administration estimates that the amusement park ride in which thrill-seekers are held in a spinning disc by gravity as the floor drops away uses a gravitational pull of about 3G. The acceleration felt during a rocket launch is up to 3.4G, while re-entry is about 1.4G.

The February 2011 quake in Christchurch was about 2G, likened to the force felt by astronauts or racing car drivers.

The proximities to the epicentres mostly explained why the Christchurch quake felt much stronger, despite its smaller magnitude than Sunday night's 6.5 quake, Dr Kaiser said.

While Sunday's tremor was in Cook Strait, Christchurch's was close to the city centre, and the geology of the faultline directed energy at the city, increasing the G-force.

However, G-force was only one way of measuring how shakes were felt, she said. The feeling would vary depending on the type of ground, the direction of the shaking and the frequency of the seismic waves.

The force was also measured at ground level, so it would have felt different for people on higher levels of tall buildings.

GNS is continuing its work to establish on which fault the swarm of quakes has been occurring. Seismologist John Ristau said that would probably take two more weeks as staff awaited data from sensors deployed to pinpoint aftershock locations.

The Dominion Post