Christchurch had its largest earthquake in a week early today.

The magnitude-3.9 quake at 3.44am was centred 10km east of the city at a depth of 10km.

By about 7.30am GeoNet had received reports from 142 people who felt the quake, including two who reported moderate shaking.

It was the same size as a quake, centred 25km west of the city, last Saturday, and smaller than the magnitude-4.3 quake the day before that.

Christchurch has had 58 earthquakes over magnitude-5.0 since the September 4, 2010 Darfield quake, and GNS Science warned there was a 72 per cent probability of an earthquake between 5.0 and 5.4 in the next 12 months.

It also said there was a 31 per cent probability of a 5.5 to 5.9 earthquake in the wider Christchurch area over the next 12 months, although only a three per cent chance of an earthquake over 6.5.

Martin Reyners, principal scientist at GNS Science in Wellington, is to give a public lecture on earthquakes at the University of Canterbury on Wednesday. His topics will include the reason for the long series of aftershocks in the region.

The Canterbury earthquakes raised important questions regarding the seismic hazard of the Christchurch region, he told the university.

''Such as why has there been a concentration of large aftershocks in the Christchurch area, at the eastern end of the magnitude 7.1 Darfield earthquake rupture? Why has the pattern of faulting been so complex? Why has the aftershock sequence been so prolonged? And why has the shaking from the Darfield earthquake and its larger aftershocks been so strong, compared to most earthquakes of similar magnitude worldwide?''

Dr Reyners will look at the unusual structure of the Earth's crust beneath Canterbury, and the response of the crust to the current convergence of the Pacific and Australian tectonic plates.

- © Fairfax NZ News

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