Friday, September 10, 2010

LIQUEFACTION:  A new word in our vocabluary - when solid ground is shaken to mush...

A sand volcano after the Christchurch earthquake. Large tracts of silty, low-lying land compounded the effects of Saturday's earthquake in Canterbury, as whole streets were transformed from firm land to sludge.

In what's known as liquefaction, Christchurch's sandy soil was shaken violently, causing water to rise through its pores. Scientists compared it to jumping on wet sand at the beach - it soon turns to a murky soup.

Professor Michael Pender from the University of Auckland geology department said the Canterbury quake was one of the most significant cases of liquefaction in New Zealand history.

He said the process could affect any town or city near a river, estuary or coastline. Auckland's waterfront, built on reclaimed land from the Ferry Building up to Shortland St, would be very vulnerable to large tremors.

Coastal developments which encroached on sand dunes, such as Mt Maunganui and some North Shore beaches, were also susceptible during a major earthquake.

Large sections of Christchurch were built on soft sediments which remained saturated after a wet winter.

Roads, bridges and pipe infrastructure have been unsettled by the water squirting up through the soil during the 7.1 magnitude tremor.

As many as nine out of 10 homes on the city's flat have been damaged by the quicksand-like effect. Much of this damage was superficial rather than structural. But in Bexley, a 5-year-old subdivision near New Brighton, at least 100 new homes were left uninhabitable after silt, sewage and grey sludge cracked the road and squeezed through floorboards.

The worst-affected areas were coastal spots such as New Brighton, and suburbs that skirted the lower reaches of the Avon and Heathcote Rivers, in particular Dallington.

Homes in Kaiapoi, near the Waimakariri River, were also reported to have sunk into the soil.

Geologists said that when rising water was concentrated into small cracks it could create sand volcanoes - mounds of sand that pushed up to the surface, as seen on the streets of Avonside and Linwood.

Professor Pender was surprised that some new developments were susceptible to major damage from underground water.

"You'd expect the engineers doing the site investigation to realise that there is this loose sand present and do some remedial work before they built foundations."

New Zealand Society for Earthquake Engineering spokesman Win Clark said most of Christchurch's structures performed well under the stress of the quake. "The buildings responded as expected - most damage was to unreinforced masonry and buildings built before World War II."

I understand this phenomenon has been experienced before in California, for instance. This has encouraged scientists from the US to comever and investigate.

http:/www./   Christchurch Earthquake Appeal

Wednesday, September 08, 2010

Christchurch Central CityImage via Wikipedia Christchurch earthquake is New Zealand's Hurricane Katrina...

ANALYSIS: First published at Fortitude

by peter petterson

You might say that the Christchurch, New Zealand, earthquake, of 7.1 on the Richter Scale, that occurred at 4-35am last Sunday morning, Pacific time, is the Kiwi Hurricane Katrina. Why do I say that, you might ask? It is not the devastation, because the Christchurch earthquake is confined to an area within the city and its environs, and obviously does not compare to the widespread and intensive damage caused by the American hurricane.

Christchurch City could be a bombsite because of the comparative damage to homes, buildings and infastructure. Probably 80% of these have been affected to some degree. The earthquake is not yet over because of the continuing after-shocks; they total close to 200 already and could continue for days or even weeks. A number have been recorded in the range of up to 5.4 on the Richter Scale Christchurch residents are stressed out and becoming traumatised, especially the young and elderly.

Residents cannot assess the real damage to their propertiess because of the continuing after-shocks which are still affecting the damage. But neighbours, families and friemds have assisted each other, especially the elderly and those living alone.

It is the comparative damage within the confines of this city of 400,000 and its surrounding districts, that draws my comparison. Over a hundred unstable buildings are believed to be in danger of being condemned; two protected buildings were condemned today and will be demolished — one traces its origins back to about 1877. A majority of homes in the suburb of Bexley, for instance, are in danger of being condemned because of the nature of the ground they are built on. The suburb is a drained swamp and perhaps its origins have come back to haunt the burghers of Christchurch.

Many brick chimneys and concrete facades have already been knocked down in many parts of the city. Buiding inspections have been carried out, as have inspections of schools in area. Schools will not reopen this week. Power has been restored to about 80% of residents. Water and sewerage has progressively been restored as well.

The northern township of Kaiapoi, about twenty minutes by road, was particularly devastated with broken sewer mains and water pipes. Many expensive homes in this area will have to be demolished. Many streets and roads in Christchurch City and the Canterbury region have also been severley damaged. Other infastructure has been severly damaged as well.

There have been no deaths caused by injury, though two middle aged men in their fifties are in hospital because of injuries received in the initial earthquake — one being hit on the head by a falling chimney and still on the critical list in ICU, the other severely cut by glass is in a comfortable condition. One person died of a heart attack during the earthquake. There are many minor injuries that have not yet been collated.

The government moved very quickly on Sunday supporting the Civil Defence, Christchurch City Council and the police on Sunday — the prime minister and some of his ministers arrived very early to assess the situation. Welfare centres were set up for the homeless and those who didn't wish to return to their homes. Police secured the CBD on Sunday and were relieved by eighty soldiers from the nearby Burnham Army camp on Monday morning.

As there are so many damaged business premises, many people are unable to go to work this week and earn a living — the Government will pay a subsidy of NZ$350.00 per worker in immediate assistance for the first four weeks to businesses employing less than twenty employees.

Damage to historic buildings, including the historic and iconic Christchurch Cathedral, are of great concern. Christchurch may never be the same again.

As I wrote above, the after shocks continue and more damage is expected in coming days. Some of that damage may be hard to bear and accept. Que sera sera!

The faultline that was responsible for this earthquake had lain idle for 16,000 years and nobody knew about it. At least the cause is now known.

Earthquake Christchurch is indeed the kiwi equivalent of Hurricane Katrina, but there is a lot more to write about in coming days, months and years as New Zealand's second largest city is repaired and rebuilt. This is but an early chapter in the book of Christchurch's future. Cantabrians are like the rest of New Zealanders, tough, resilient and caring to a fault at times.

That indomitable spirit so evident in Canterbury sporting teams over many decades is so very evident in city and country township communities right now. The future looks bright amid the gloom of the present.  Christchurch Earthquake Appeal

Published on Fortitude September 08, 2010

Copyright 2010 peter petterson

Monday, September 06, 2010

An enlargeable satellite image of New ZealandImage via Wikipedia

Understanding the massive 7.0 Christchurch Earthquake...

The September 3, 2010 South Island, New Zealand earthquake occurred as a result of strike-slip faulting within the crust of the Pacific plate, near the eastern foothills of the Southern Alps at the western edge of the Canterbury Plains.

The earthquake struck approximately 50 km to the west-northwest of Christchurch, the largest population center in the region, and about 80-90 km to the south and east of the current expression of the Australia:Pacific plate boundary through the island (the Alpine and Hope Faults).

The earthquake, though removed from the plate boundary itself, likely reflects right-lateral motion on one of a number of regional faults related to the overall relative motion of these plates and may be related to the overall southern propagation of the Marlborough fault system in recent geologic time.

Today’s earthquake occurred approximately 50 km to the southeast of a M7.1, surface-rupturing event in Authur’s Pass, on March 9th, 1929, which caused 17 fatalities. More recently, two earthquakes of M6.7 and M5.9 occurred in June 1994 approximately 40 km to the northwest of today’s event, but did not cause any known fatalities or significant damage.  Christchurch  Earthquake Appeal