By Gayathri Vaidyanathan

A magnitude-6.3 earthquake that hit Christchurch, New Zealand, at 12:51 p.m. local time today is the country's deadliest in 80 years. Details are still trickling in, and the numbers of dead and injured are expected to rise following what Prime Minister John Key has said "may well be New Zealand's darkest day."

But this earthquake is just one of a series that has shaken the area during the past six months. A magnitude-7.0 earthquake on September 3, 2010, caused an estimated NZ$1.4 billion (US$1 billion) of damage, but no deaths. Nature examines whether the earthquakes are linked and what caused them.

How much damage did the earthquake do?

So far, 65 deaths have been recorded and emergency teams are working to save those trapped in fallen buildings. The other aftershocks associated with the previous, less damaging, earthquake cost the government NZ$2 billion just in compensation for those affected. Presumably, the economic costs of this disaster will be much greater. The mayor of Christchurch has already declared a state of emergency.

The force of shaking was so great that about 30 million tonnes of ice calved off the basal and terminal face of the Tasman Glacier and plunged into Terminal Lake, according to a New Zealand news channel.

What triggered it?

This earthquake is another aftershock resulting from last September's event. Since that earthquake, there have been six aftershocks of a magnitude greater than 5.0 including this one. Tuesday's aftershock is the largest so far.

What triggered the September earthquake?

New Zealand lies in a geologically active area. To the east of North Island, the Pacific plate is being dragged under the Australian plate in a process called subduction. There are a number of volcanoes and other faults in the region, forming part of what is known as the Pacific Ring of Fire. To the south of the country's South Island is another subduction zone, with the Australian plate being pulled under the Pacific plate.

Shakemap of New Zealand's South Island: https://www.nature.com/news/2011/110222/full/news.2011.117/box/1.html

There is a third major fault in the region, called the Alpine Fault, which runs along the South Island itself. This fault consists of two plates sliding past each other, a phenomenon known as a strike-slip fault. This area is thought to rupture every 200-400 years, producing earthquakes of around magnitude 8. Last year's earthquake occurred in the region where the subduction zone turns into the strike-slip fault, says John McCloskey, a geophysicist at the University of Ulster in Coleraine, Northern Ireland.

Why did the 6.3-magnitude aftershock cause more damage than the 7.1-magnitude earthquake?

The epicenter of this earthquake was very close to the South Island city of Christchurch; the previous earthquake's epicenter was about 45 kilometers west of the city, according to data from the U.S. Geological Survey. "What determines the deadliness of an earthquake is how close it is happening to population," says Andreas Rietbrock, a seismologist at the University of Liverpool, UK. He gives the example of recent earthquakes in Haiti and Chile: whereas the 8.8-magnitude earthquake in Chile caused about 500 deaths, the 7.0-magnitude one in Haiti resulted in more than 300,000 deaths.

Today's earthquake was also as close to the surface as the September quake, at a depth of 5 kilometers. This is almost as shallow as an earthquake can be, says McCloskey. It had a directional thrust towards the surface, and the amount of acceleration felt at the epicenter was almost 1.90 times the force of gravity.

The earthquake also released water from the ground, suggesting that sediments below the surface are loosely packed, says McCloskey. This tends to amplify shaking. "The thing was shallow, it had a thrust mechanism, it was under an area with some or lots of soft sediment. This makes shaking worse than average for this type of earthquake," he says.

What does this mean for future earthquake risk in the region?

No one can predict an earthquake, but it is possible to say that aftershocks are likely, say experts. Tuesday's earthquake has already spawned four aftershocks of magnitude greater than 5.

"The magnitude-7 earthquake triggered this one, and this one triggered others, and others trigger something else so there's a cascade, where the effect of the first one triggers all the other ones," McCloskey says. Recent research in his lab, not yet published, on the 1992 Landers earthquake in California suggests that aftershocks can occur for up to 8 years after an earthquake, he says.

And although the trigger for this aftershock was the September 2010 earthquake, its energy came from elsewhere, McCloskey says. The energy had already been building up, and the previous event probably increased stress, eventually triggering the quake.

Have research institutions been affected?

The University of Canterbury, located in Christchurch, is closed until further notice, but no serious injuries or fatalities have been reported. Lincoln University, also in the city, is closed for the day tomorrow and its campus seems to have escaped damage. "Please be assured that students and staff are safe and well," says vice-chancellor Roger J. Field in an online update.