Japan jolt 8000 times more violent than Christchurch's

The Japan earthquake was 8000 times stronger than the Christchurch shake and unzipped a faultline half the size of the North Island, a NZ scientist says.

The magnitude-9 tremor near Japan's east coast on Friday occurred at the same depth as the February 22 quake near Lyttelton, but struck on a far larger fault, believed to be 350km long and 150km wide.

This meant an enormous amount of energy was created as the fault ruptured for 150 seconds.

Associate Professor John Townend, a Victoria University expert in earthquake faults, said early investigations in Japan showed a rupture half the size of the North Island.

"The larger the fault, the larger the earthquake. When you see the fault is around 45,000sq km, moving several metres, you can understand how much grunt there is."

The fault forms the boundary between the Pacific tectonic plate and the North American plate, which sits under northern Japan.

It has been locked up for decades, but slipped up to 10 metres on Friday night.

Nasa scientists calculated that the jolt tilted the Earth's axis slightly and may have shortened the day by nearly 2 millionths of a second.

The epicentre also bumped some nearby global positioning stations eastward by 4 metres.

The greatest harm to human life, however, came from the tsunami.

The quake caused the seabed to abruptly lift. This pushed up a column of water, which then collapsed on to itself, generating a wave which reached the eastern coast of Japan in 15 minutes.

Victoria University Professor Martha Savage, from the school of geography, environment and earth sciences, said that if a similar quake occurred near the coast of New Zealand, a 15-minute warning would allow many people to reach higher ground on foot.

However, Japan's coastline is flatter. Near Sendai, residents in low-lying spots had to travel 10km inland to reach higher ground. Even with an early warning system, escape was difficult.

The earthquake had also caused the coast around Sendai to drop about half a metre, meaning the tsunami was able to wash over land more easily.

The waves moved at 800km/h, rearing up to 10 metres, before slowing down.

Water has continued to slosh around the globe for days, similarly to the Boxing Day tsunami, which travelled around the world three times.

Professor Savage said Japan was the most prepared country in the world for earthquakes, and its devastation was a fright for all countries in seismically active zones.

"What struck me is that this is the most advanced country in the world for earthquake preparedness, earthquake building codes and land use. And still, they got hammered.

"Much of the problem is with land-use planning. You can only build buildings to withstand so much. Some of it is just a matter of placing where you are."