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Seismologists studying the devastating Christchurch earthquake have discovered aftershocks "buried in the signal" of other tremors.

GNS Science seismologists have been analysing data from the February 22 quake in detail and have pinpointed the size of the initial 12.51pm quake to magnitude 6.343.

They have also upgraded two strong aftershocks that came within hours of the devastating tremor.

A magnitude 5.7 quake at 1.04pm has been upgraded to 5.845, while a magnitude 5.5 quake at 2.50pm has been pushed up to 5.91.

GeoNet project director Ken Gledhill said the only really significant upgrade was the magnitude 0.4 increase to the 2.50pm quake, with the others still within magnitude 0.1 of initial estimates.

The research would help scientists better understand the quakes, Dr Gledhill told Radio New Zealand.

"What we're doing, and it's a part of our routine processing, is going through and analysing all the data in detail, right from the first 6.3 and right through that whole sequence.

"So what we're finding is a lot of earthquakes buried in the signal of other earthquakes."

A magnitude 5 aftershock was still a possibility, but estimates of what was likely had not changed.

"You're going to still be getting an odd magnitude 4 aftershock in a year's time. That's the reality - this has got a long tail, it's going to go on for a fairly long time.

"In the next year there could be another one, two, three magnitude 5s."

- NZPA