September 30 2014, updated 10:38pm

The science behind the shakes

PAUL GORMAN - SCIENCE REPORTER
Last updated 16:39 26/12/2010
The 2.07am quake is centre left, two quakes close to each other. The earlier quake is the one in Gisborne. The big 10.30am shock is bottom left, and it shows the ground has barely stopped shaking since.
Geonet

HIGHS AND LOWS: The 2.07am quake is centre left, two quakes close to each other. The earlier quake is the one in Gisborne. The big 10.30am shock is bottom left, and it shows the ground has barely stopped shaking since.

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The ground directly below Christchurch has been kicked into life by this morning's shallow magnitude 4.2 earthquake.

The 2.07am quake appears to have ushered in a new series of shallow events - what seismologists refer to as an aftershock cluster or swarm.

Since the magnitude 4.9 shake at 10.30am that caused some damage in the city, cut power and alarmed many residents, the ground has barely stopped shaking for more than a few minutes at a time.

All of the quakes are still technically aftershocks of the 7.1 magnitude shock on September 4.

While the frequency and size of the quakes has declined in the last month or so, seismic activity is still higher than would be expected compared with long-term earthquake averages for Canterbury.

GNS Science duty seismologist John Ristau said such a cluster was not unexpected although what had caused it was unknown.

The quakes were being felt so strongly because they were all close the centre of the city and were being generated by rocks breaking in the first 5km below the surface.

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