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12 July 2011

Christchurch rocked by more than 7000 aftershocks

There have been more than 7400 aftershocks in Christchurch in the 10 months since September's magnitude-7.1 quake.

They've sent people running, brought buildings down and left 181 dead.

Many people had been injured, left without water and power - and some have had to abandon their homes.

The aftershocks include the February 22 magnitude 6.3 and the 6.3 tremor which shook the city once again on June 13.

They contribute to this year's total of more than 11,000 quakes felt throughout the country.

Four earthquakes larger than magnitude 6 have already hit New Zealand this year, compared to just three occurring for all of 2010.

Two of them were in Christchurch, one was off the Coromandel coast on January 28 and the largest, a magnitude 6.5, occurred in Taupo last week and was felt throughout the country.

Records show that New Zealand experiences about three earthquakes over magnitude 6 each year, one over magnitude 7 each decade and a magnitude-8 tremor once a century, according to GNS scientists.

Seismologist Brian Ferris says this year's record of magnitude-6 earthquakes was an "anomaly" and the two Christchurch quakes were related to September's magnitude 7.1.

Last year's 7.1 quake wasn't the only one over magnitude 7 in the past decade. A 7.8 tremor shook Dusky Sound, near Fiordland, in 2009.

GeoNet records tens of thousands of earthquakes every year but only adds the ones that have been felt to its database.

GNS Science communications manager John Callan says most of the quakes that have been felt are over magnitude 2.

In the past decade, numbers ranged from 11,521 for all of 2002 to 24,487 last year.

The amount of earthquakes felt each year had increased since 2005 but Callan says that doesn't necessarily mean that there had been more each year, but that the measuring equipment had become more sophisticated.

"Most likely this is due to more instruments being added to the network progressively since 2001. Also, analysis systems have improved, especially in the last decade."

Callan says the Dusky Sound tremor produced several thousand aftershocks and there had been 7400 aftershocks since Christchurch's 7.1 shake.

New Zealand started recording earthquakes via instruments in the 1930s and scientists had to estimate the size of quakes by observing data prior to that.

Callan says the GeoNet geophysical network was a dense network designed to monitor the whole country. It comprises of 250 strong-motion sensors, around 160 seismographs and 150 GPS instruments and was mainly funded by the Earthquake Commission.

"It is world-class in its design and operation," he said.

"Several countries have tapped into our knowledge and experience and, being good global citizens, we are happy to share our knowledge in this way."

There had been several quakes larger than magnitude 7 near the Kermadec Islands in the past decade. Callan says this was probably due to the location on the Kermadec Trench, where the Pacific Plate subducts under the Australian plate, causing many deep, large earthquakes.

There had also been several large quakes over magnitude 7 off the southwest tip of New Zealand in the last decade. Callan says this is also an area of reasonably high quake activity because of tectonic plate subduction.

- Stuff