How long can these earthquakes go on? That is the most important question for Christchurch in the wake of another "aftershock" as big as the one that devastated the city in February. If residents knew how long this might last, they could see an end to their ordeal.

If the insurance industry knew how long this might last it could see an end to its ultimate liability. If the Government knew, it could answer most of the questions it is plainly ducking.

The agency they all look to for an answer is GNS Science, the Crown's geological and nuclear research centre in Lower Hutt. When the first earthquake struck Canterbury in September, GNS said aftershocks could occur for up to a year.

After the February 22 shock, the clock was reset. Yesterday, after Monday's 6.3 magnitude event, GNS estimated a 30 per cent likelihood of another 6 to 6.9 earthquake in the region within the next 12 months.

In other words, there is no end in sight. Seismologists admit the large number of aftershocks since September's 7.1 quake puts them in "unknown territory".

Sequences like this have happened in other parts of the world, they say, but not in New Zealand and certainly not in Canterbury, which for thousands of years has been so quiet seismically that they are only now mapping faults under the plains and the sea.

An interval of thousands of years is a mixed blessing, they explain. It is in these sorts of places, where the underlying crust is particularly strong, that sequences such as Canterbury is suffering can happen when tectonic stress is eventually released.

New Zealand lies on a plate boundary; no part of the country is very far from tectonic stress. Regions that have minor earthquakes fairly frequently might be fortunate.

None of this is much help to Christchurch citizens shovelling the stinking muck of liquefaction from their property for the third time and pleading, again, for some decisions about whether their land and suburbs are going to be made liveable again.

The minister in charge of earthquake recovery, Gerry Brownlee, says it is "blindingly obvious" some parts of the city will have to be abandoned but he cannot be more definite, he says, until insurance liabilities are sorted.

The rising cost of Christchurch is said to be of increasing concern to international insurers. They have been given an estimate that Monday's shakes have added as much as $6 billion to a bill already assessed at $15 billion to $20 billion. The additional sum seems extraordinary for property that must have been severely damaged by the earlier quakes.

New Zealand covers natural disasters with a dual system of public insurance, to a value of $100,000, and private insurance for the remainder.

Disputes between them are bound to arise and one has just been taken to court. But it is unlikely insurance issues can be settled until the earth itself settles. The question, how long will the quakes go on, underlies everything.

If the Earthquake Commission and household insurers are liable for new payouts after each big aftershock, the costs of reinsurance are going to rise for the whole country.

The Government has already warned that the rise is going to be substantial. All insured property owners in New Zealand have an interest in knowing how long these earthquakes could last, and when the picture might clear sufficiently for insurance to be safely paid out.

In the meantime, life is hard for many in Christchurch. Geologists and engineers will be doing their best to find the answer they need. We can only ask that the answer, no matter how alarming, is made public as soon as the Government has it. People are resilient; they can deal with the truth and they deserve it.