GNS Science seismologist Bill Fry says Christchurch is now experiencing a sizable tremor every 30 to 60 minutes, following Tuesday's earthquake, but the frequency should drop off.
There have been at least 50 aftershocks since the magnitude 6.3 quake that struck at 12.51pm on Tuesday - mostly of magnitude 4 or less, Mr Fry says, but a few more magnitude 5 ones can be expected.
He says there's always a high activity level for a day or two after an initial tremor.
Another GNS Science seismologist, Tony Hurst, says Tuesday's quake was technically an aftershock of the 7.1 quake on 4 September.
Mr Hurst says it's clear from the pattern of aftershocks since September that a lot of the stress which has built up has been centred under Christchurch city and Banks Peninsula.
