KiwiRail is sending almost 300,000 litres of water to Christchurch.

KiwiRail chief executive Jim Quinn said the water would arrive aboard a train early this afternoon and was in response to the city's need for fresh water as a result of earthquake damage following yesterday's 7.1 magnitude quake.

About 15 per cent of the city remains without water, and those with it are being urged to use it sparingly. Any drinking water should be boiled for at least three minutes.

"It's a practical contribution we can make to Christchurch's emergency response and we will be looking for other ways that rail can be used to support the region's recovery," he said.

"We are grateful to Fonterra for making the tanks available for the shipment."

KiwiRail opened the railway line south of Christchurch last night but continuing aftershocks meant a speed limit of 40km/h had been imposed. That was reduced to 25km/h over bridges.

"We still have work to do repairing damage to the track and one bridge between Belfast and Rangiora. We will be road bridging freight to Christchurch from Rangiora while this work is being completed," Mr Quinn said.

Hidden danger in chimneys

Residents are being warned to treat all chimneys as dangerous until they can be inspected, as damage may be under the roofline and not visible.

Christchurch roofer Tony Stuart, who spent most of yesterday repairing roofs and inspecting and removing damaged brick chimneys following yesterday's 7.1 magnitude earthquake, said today visible damage could be only part of the picture.

Mr Stuart said he removed one chimney down to the roof line yesterday and covered the hole with roofing iron. However, below the roof line the chimney had moved on the base and further inspection was needed because it could be unstable.

Other chimneys still standing above the roof line could be fragile.

"In lots of chimneys bricks have gone from the lower part of the chimney but the top part is still there, which makes it really dangerous," he told NZPA.

"There is bugger all supporting the top of it and bricks are really heavy.

"In many chimneys, the mortar holding the bricks together is just sand and cement but over the years it has lost its capacity to hold the bricks together. The bricks are just stacked on top of each other."

People could visually inspect their chimneys from the ground but said the rest should be left to experts.

"They need to get a roofing company to check their roofs and chimneys. They shouldn't get up there themselves. It is too dangerous but they need to be checked.

"Often you can't see the cracks that have appeared around the chimneys until you get up there on the roof."

Before the earthquake he had removed many brick chimneys and in most cases they just picked the bricks off, Mr Stuart said.

"We didn't even have to knock them or do anything," he said.

Scientists seek answers

Meanwhile, teams of scientists have started arriving in Christchurch to capture seismic data from the continuing stream of aftershocks rocking Canterbury in the wake of Saturday's big quake.

Up to 13 portable seismographs will be deployed from today by GNS Science, and Stanford University in the United States is sending more recorders.

Mid-Canterbury was already heavily monitored by a permanent network of seismographs sited by researchers anticipating the "Big One" - a magnitude 8 quake which most geologists expect to occur eventually on the Alpine Fault and likely to be at least 10 times stronger than yesterday's 7.1 quake.

Seismologists study aftershock sequences to find out more about the mechanics of the main shock and rupture, and to check whether if stress in the earth's crust has been transferred to other faults in the region.

The number of recorders may eventually number 40, and data from them will help to pin down the mechanics of yesterday's quake.

One theory is that if the "directionality" of the quake was away from the city, it may have been received a lesser impact, according to Professor Euan Smith of Victoria University.

Scientists yesterday said the quake appeared not to have been on one of the many known faults in the region; eons of alluvial gravels and silt being deposited on the Canterbury Plains can make it hard to detect traces of old fault lines.

Those gravel layers are another puzzle waiting to be cracked by analysis of detailed data.

Historically, seismologists have said the cities built on former river beds or even former swamps may have earthquake shocks effectively magnified as the soils act like jelly.

But data from Saturday's quake will also allow them to probe whether in some cases the water-filled soils may also be "dampening" some of the higher frequencies that would do the most damage to squat structures such as brick houses. Such a shock-absorber mechanism would not reduce the impact on "longer" structures, such as chimneys or bridges.