Port Hills half a metre taller after Christchurch earthquake

PAUL GORMAN
Last updated 05:00 02/03/2011

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Christchurch's Port Hills are nearly half a metre taller in places as a result of the colossal forces unleashed by last week's earthquake.

Satellite analysis by GNS Science shows the top of the roughly east-west buried fault responsible for the February 22 magnitude-6.3 quake lies between one kilometre and 2km below the southern edge of the Avon-Heathcote Estuary.

Land on either side of the fault has slipped horizontally as well as vertically, causing the Port Hills to rise by about 40 centimetres and land just south of the Avon-Heathcote Estuary to shift to the west by a few tens of centimetres.

Land immediately north of the estuary, on the other side of the fault, has also moved tens of centimetres, but to the east.

GNS natural hazards manager Kelvin Berryman said the estuary and land just west of it had sunk by about 10cm, although ground compaction from strong shaking had led to more substantial sinking in places.

Lyttelton was now several centimetres closer to Christchurch.

GNS analysis indicated the fault rupture began shortly before 12.51pm and 50 seconds, with small movement between the two sides of the fault about 7km underground.

"Over the next few seconds, the rupture propagated upwards and towards the northwest, with the amount of slip increasing with time," Berryman said.

"The largest amount of slip – up to about 1.5 metres – occurred between about 3km and 5km depth.

"The [direction] of the rupture – up and towards the north-west – focused the energy of the earthquake towards Christchurch and helps explain the severe damage in the city.

The suburbs of Heathcote and Redcliffs lie directly above the top of the fault and this helps explain the damage and rockfall in this area."

The fault was angled so that most of it lay beneath the Port Hills.

The quake released about 42,000 tonnes of energy.

According to the United States Geological Survey, the first, primary or "P" waves, from the quake reached Wellington 44 seconds after the rupture, Brisbane about five minutes later and London about 20 minutes later.

Berryman said the faults associated with the September 4 and February 22 quakes were separated by what may be at least two more hidden faults underneath Christchurch.

Aftershock patterns suggested at least two north-east-south-west trending faults lay between the east-west Greendale Fault exposed at the surface in September last year and the fault under the Port Hills.

Scientists originally thought there had been 17km of subsurface rupture between Halswell and Taylors Mistake.

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However, that had been refined to movement over an 8km-wide and 8km-long area.

"It's not a big fault. It hasn't had a lot of movement on it in the past.

"The quake occurred at the periphery of the September 4 aftershock cloud.

"The stress front has arrived there and found a piece of the crust that was primed and ready to go."There was no indication how long it was since the Port Hills fault last moved, Berryman said.

"It has not been determined before now in any geological maps or geophysics work.

"It could have moved before, but it might be the first time it ruptured in the last 100 million years," Berryman said.

- © Fairfax NZ News

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