There's a 12% chance of a big earthquake hitting by midnight

The chances of a new big quake hitting today and in the next month have been calculated and it's a better bet than winning Lotto.

The government's earth scientists, GeoNet, have worked out a series of scenarios and percentage chances of big quakes following the seismic activity overnight.

The predictions come with the news that this morning's big quake was actually two earthquakes.

GeoNet's Sara McBride said quakes were followed by "spikes of activity and occasional larger earthquakes".

"We have updated our probabilities of larger or similar-sized earthquakes. We use probabilities as we cannot predict earthquakes. These probabilities describe the likely progression of the sequence within the next week, month and year."

She said some of the predictions would be troubling for people. "We recognise that while these scenarios may increase anxiety, the best thing is to be prepared.

"Remember: If you feel a long or strong earthquake and you are on the coast, evacuate immediately."

She said the chance of an earthquake of magnitude 7 or greater on the Richter scale in the next day was about 12% for an area from Christchurch north to Wellington, and west to Golden Bay.

The 2010 quake which hit Christchurch was 7.1 on the Richter scale while the 2011 quake was 6.3 on the Richter scale.

Last night's quake was measured at 7.5 on the Richter scale.

The GeoNet predictions also calculated the chance of a quake more than 7.0 on the Richter scale at 24 per cent. In the next month, the chance of such a quake rose to 32 per cent.

There was a greater chance of being hit by a slightly lower band of aftershock, from 6 to 6.9 on the Richter scale.

The chance of one hitting in the 24 hours after the original quakes was 71 per cent. During seven days, that rose to 93 per cent.

The chance of such a significant aftershock in the next month was 98 per cent.

McBride said the two earthquakes hit one after the other. It appears to explain the phenomena which left many expressing astonishment at how long the earth shook.

She said the quakes were different types of seismic shift.

"Our reports indicate that the combination of these two quakes lasted two minutes, with the most severe shaking at around 50 seconds."

One of the quakes appeared to have been a "strike-slip" event, a vertical fault which moves horizontally.

The other was a "thrust fault", where older parts of the earth's crust are shoved up through the earth on top of newer layers.

GeoNet scenario one - 80 per cent and greater

"A normal aftershock sequence that is spread over the next few months. Felt aftershocks (eg magnitude greater than 5) would occur from the 7.5-magnitude epicentre near Culverden, right up along the Kaikoura coastline to Cape Campbell over the next few weeks and months. This is the most likely scenario."

GeoNet scenario two - 60 per cent and greater

"In the next month, it would be likely that rupture of earthquakes of about a 6 magnitude in the North Canterbury and Marlborough regions will occur, as well as potentially offshore in Southern Cook Strait and offshore Kaikoura."

GeoNet scenario three - less than 40 per cent

"The least likely scenario is that in the next month, (it is unlikely but still possible) there would be rupture of longer known faults (with earthquakes of about 7 magnitude), in the Marlborough and Cook Strait regions."

It meant aftershocks would most likely hit from Christchurch north to Wellington and out to Golden Bay.

"It is important to understand that earthquakes can and do happen outside this box but the box represents the most likely area related to this sequence."