Canterbury earthquakes broke the rules

[Mark]

The Canterbury earthquakes packed such a punch they shattered rocks deep beneath the Earth's surface, breaking all the rules of aftershocks, a public lecture has heard.

GNS Science principal scientist Martin Reyners spoke to a 100-strong crowd at Canterbury University on Wednesday night.

Canterbury's quakes broke all known rules of  ''aftershock decay,'' he said.

After the September 2010 magnitude-7.1 quake, Reyners said, he noticed the aftershocks were ''dying away too quickly'' compared with aftershock decay from other modern quakes.

Stress and pressure had been building up beneath Canterbury for thousands of years.

Reyners said Christchurch experienced more aftershocks because of the nature of rock beneath the city.

Christchurch sits on a brittle layer of fractured basalt and schist, topped by a thinner layer of greywacke.

''Earthquakes will only occur where rocks are brittle,'' Reyners said.

But the question of whether Christchruch would experience any more quakes was not easy to answer.

''We live on a plate boundary and we should all expect earthquakes. Maybe in 3000 or 4000 years, we will have another one.''

Reyners ended his talk on a positive note, saying ''the system seems to be sorting itself out''.

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