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Monday, March 11, 2013
Terremoto de Nuevo Zealand 2010-11
4 September 2010, at 4:35am NZ time, a Mw 7.1 earthquake rocked the Canterbury region, South Island, New Zealand. I personally raced under the door frame to be greeted by my frightened flatmates (one of whom was convinced it was a demonic possession – turns out she’d been watching too much ‘Supernatural’).
The earthquake produced a 22km-long surface rupture and up to 4m of horizontal displacement within alluvial terraces. These terraces were deposited about 16,000 years ago at the end of the last glaciation. The epicentre was in Darfield (mid-Canterbury), with a focal depth of 10 km; the Maximum Intensity on the Mercalli Scale was MM 9.
In New Zealand, the Australian and Pacific plates push against each other along a curving boundary. At the southern end of the South Island, the Australian Plate subducts below the Pacific Plate whilst in the North Island the opposite situation occurs with the Pacific Plate being pushed under by the Australian Plate. In between, through most of the South Island, the two plates grind past each other along the Alpine Fault.
The Student Volunteer army was set up via Facebook, to clean up the silt from the liquefaction around the city and surrounds. Farmers in the region set up the Farmy Army, to use their equipment to move large amounts of the silt. A baking Army was set up to feed all these volunteers – and of course the real Army was involved in establishing a curfew and cordoning off the central city. It was rather strange to say the least to see army tanks rolling through the central city streets.
Once it had been established that despite the damage and the liquefaction nobody had died, several of us rather excitedly went and checked out the fault trace. Many of my friends were involved in research into the fault and the earthquake, and I was involved in research into the sand volcanoes that littered the landscape around the eastern suburbs and some northern towns. People played ‘earthquake bingo’, where whenever there was an aftershock people would guess magnitude, depth and distance before Geonet posted the facts. For those whose sewerage had been cut off and who needed to make their own longdrops, there was a competition for the best one. A Christchurch guy set up ‘You Know You’re From Christchurch When’ to share quake humour; the book published from the site became a bestseller (proceeds went to charity).
Several different ‘theories’ emerged about the cause of the earthquake. Some guy who wrote a book about how to read cat’s paws claimed he had predicted the quake. A hardcore fundamentalist Christian group blamed the quake and the aftershocks on Christchurch’s ‘tolerance of gays’.
The worst was yet to come however. The aftershocks were frequent, and severe. Then, February 22 2011, at 12:51pm, Christchurch was rocked by a very shallow and close earthquake. The damage was considerable because the earthquake originated only six kilometres from Christchurch’s population centre and parts of Christchurch’s urban area were as close as one kilometre from the fault rupture. Its relatively shallow depth (only 5 kilometres beneath Earth’s surface) produced extraordinarily strong shaking at the surface; 2.2g. This earthquake was caused by the rupture of a 15-kilometre-long fault along the southern edge of the city, from Cashmere to the Avon-Heathcote estuary. The epicentre was located in Lyttelton, had a ML of 6.3 and a Maximum Intensity of MM9. The death toll numbered 185.
I was much better off than most, though the water cylinder in my flat burst, parts of the ceiling caved and cracks appeared everywhere, meaning I had to move. While trying to sleep at night, I couldn’t help thinking of all the people trapped in buildings while the aftershocks continued, and the rain.
The army took control of the city again, and police and Urban Search and Rescue (USAR) teams came from all over NZ. Australia immediately donated $5 million and sent teams of USAR and police officers to help out. Japan, China, Taiwan, Singapore, the UK and the Los Angeles County Fire Department sent their own USAR teams. It was NZ's first national emergency.
June 13, 2011 saw another large aftershock of Mw 6.3 at a depth of 6 km shake the region, causing more widespread liquefaction. December 23, 2011 a magnitude 5.8 earthquake at 1:58 pm struck east of Christchurch just off the coast of New Brighton. As with other earthquakes of this shaking intensity, liquefaction occurred in the eastern suburbs of Christchurch. This new sequence of earthquakes was further east again from the June earthquake, and triggered its own aftershocks, the strongest of which was a Mw 6.0.
As of September 2012, the Canterbury region has had over 12,000 magnitude 1.1 or higher quakes since the magnitude-7.1 quake of September 4, 2010. Most residents however have had the hardest struggle with sorting insurance claims.
-TEL
To view all the earthquakes in the Canterbury earthquake sequence, check out this map: http://www.christchurchquakemap.co.nz/
http://www.geonet.org.nz/earthquake/historic-earthquakes/top-nz/quake-13.html
For more on liquefaction and paleoliquefaction, see this post: https://www.facebook.com/photo.php?fbid=358309350896782&set=a.352867368107647.80532.352857924775258&type=3&theater
For more info on the Canterbury earthquake sequence, visit this page: http://www.gns.cri.nz/Home/News-and-Events/Media-Releases/Most-damaging-quake-since-1931/Canterbury-quake
Photo: https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg6Mnj36f1fOa7a_A9YSqFgVYrvhPrT18uZlS-pSzZQp_8adWANX5PiZ3fFoTWp0WFpzMRMs6q2-GpnnIBi7AKCkDMqB5bzxJtcTJ8hhK70a9oo5tDN5T5O4vs0TUG3h_9DMXsx-wmnkuA/s1600/nzearthquake.jpg
For photos spanning the entire earthquake sequence: https://www.facebook.com/CHCH.EQ.Photos
You Know You’re From Christchurch When: https://www.facebook.com/ykyfcw
Student Volunteer Army: https://www.facebook.com/StudentVolunteerArmy
First flyover of the fault trace from September 2010: http://www.youtube.com/watch?v=Npqx3WmNkv4
For a run down of all the conspiracy theories from September 2010: http://all-embracing.episto.org/2010/09/21/the-christchurch-quake-conspiracy-plural-–-part-four-some-other-theories/
Saturday, March 9, 2013
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