The earthquake that devastated Christchurch is the product of a newly discovered fault line in the Earth's crust.
New Zealanders know that their country is vulnerable to tectonic movements. Fault lines dissect the nation, snaking up the western coast of the South Island before splitting in two just south of the capital, Wellington.
For seismologists, it was always Wellington that created the most concern, because of a fault running through the city centre that is expected to produce a major earthquake in the next few decades. "Wellington has always been considered much more at risk because it straddles the plate boundary," explained Kevin McCue, director of the Australian Seismological Centre.
By comparison, until about six months ago Christchurch was thought to lie more than 80 miles away from the nearest known fault line. But the two earthquakes and hundreds of aftershocks that have battered the city in the past five months have turned such thinking on its head. Scientists now believe that a fault which might have lain dormant for thousands of years has sprung back into life.
"It's not a new fault, in the sense that it has only just been created, but it is a fault that has only just been discovered," said Dr Roger Musson, head of seismic hazards and archives at the British Geological Society.
"Some fault lines are very easy to see but the one under Christchurch is covered by sediment and would have been invisible without thorough geophysical searches."
Since an earlier tremor in September, which was stronger than Tuesday's but did less damage, geologists have been trying to map the new fault, which is thought to branch out from the main line running up the west coast of South Island and run beneath the plain on which Christchurch was built.
Such mapping has provided a frightening window into the future. Most seismologists are now convinced that major earthquakes are all but inevitable over the next century in such densely populated places as as Istanbul, Los Angeles, Tehran, Karachi, Kathmandu and Lima, all of which have active fault lines running through or close to them. Even capital cities such as New York and Beijing are thought to be vulnerable to seismic shifts.
New Zealand will now be keen make sure that any rebuilding is of a quality that can withstand similar quakes. But nature is difficult to predict and few can say whether the fault running under the city will continue to cause catastrophes or return to its dormant state.