Shifting Soils

Today we are traveling to New Zealand, figuratively speaking. Before our trip, we need to have a short geography lesson. New Zealand is in the southern Hemisphere, with two islands, logically, if not creatively, named North Island and South Island. This post will concern Christchurch, a city on South Island. Christchurch is built on the sedimentary deposits of a river system that flowed eastwards from the Southern Alps, so the ground soils are sandy, silty, or made of peats or gravel. These appear in different layers in different formations throughout the city.
A diagram of the soils below Christchuch. Taken from Cubrinovski et al 2011, today's paper.
A diagram of the soils below Christchuch. Taken from Cubrinovski et al 2011, today’s paper.
You might remember from past science classes the idea of tectonic plates. These are large pieces of the Earth’s continental crust that actually move over very long time scales in a process called continental drift. Generally you don’t notice that there is any movement, but the meeting points of two plates are often prone to volcanoes or earthquakes, depending on what exactly is happening where the two plates meet. New Zealand is on the border of the Australian and Pacific plates, so it tends to be prone to earthquakes, and North Island has active volcanoes. Fractures in the Earth’s crust (which may not be visible from the surface), called faults, are where the plates may suddenly shift, creating an earthquake. The Christchurch earthquake discussed here happened on a fault line that had been previously unrecognized, taking the city (and the rest of the country) by surprise. Its epicenter, or point of origin, was shallow for an earthquake: only five kilometers (three miles) deep, and a similar distance away from Christchurch. Christchurch took heavy damage, including its Central Business District (CBD).
An image I took in February 2013 of a map of Christchurch's red zone, on a post in the city. Not shown is the red zone in the residential areas.
An image I took in February 2013 of a map of Christchurch’s red zone, on a post in the city. Not shown is the red zone in the residential areas.
I should probably say here that I spent some time in New Zealand, more specifically, in Christchurch. In 2013, I did a semester-long study abroad. I arrived in early February, and left late in June. I saw first-hand the damage two years after the earthquake. The well-known ChristChurch Cathedral, partially destroyed while some walls still stood, tucked away in Cathedral Square that was blocked off by a chain link fence. There were piles of rubble and seemingly out-of-place parking lots that I suspect were once buildings, torn down after sustaining too much damage. I went to a mall made of shipping crates to replace what had once stood there. I am not from a place prone to any natural disasters, except the occasional hurricane or blizzard. The damage remaining after two years was stunning to someone unaccustomed to notices in elevators explaining what to do if an earthquake occurred while I was there.
Images of earthquake damage, two years later. Left: a city street with rubble, blocked off. Center: details of the remains of the ChristChurch Cathedral. Top Right: Rubble piles. Bottom Right: Tramway tracks leading into the red zone. Taken by me, February 2013.
Images of earthquake damage, two years later. Left: a city street with rubble, blocked off. Center: details of the remains of the ChristChurch Cathedral. Top Right: Rubble piles. Bottom Right: Tramway tracks leading into the red zone. Taken by me, February 2013.
The article for today is Geotechnical Aspects of the 22 February 2011 Christchurch Earthquake. The list of authors are Misko Cubrinovski, Brendon Bradley, Liam Wotherspoon, Russell Green, Jonathan Bray, Clint Wood, Michael Pender, John Allen, Aaron Bradshaw, Glenn Rix, Merrick Taylor, Kelly Robinson, Duncan Henderson, Simona Giorgini, Kun Ma, Anna Winkley, Josh Zupan, Thomas O’Rourke, Greg DePascale, and Donnald Wells. I know that’s a long list to read, but I should not leave anyone out. The article appeared in the Bulletin of the New Zealand Society of Earthquake Engineering, December of 2011—less than a year after the earthquake.
As it happens, today is the fourth anniversary of the earthquake. The 6.3 magnitude quake struck at 12:51 in the afternoon, local time. Technically it was an aftershock from a 7.1 magnitude earthquake that had stuck in September of the previous year. The February 2011 earthquake had more of an effect in the city, however. Damage to infrastructure was intense, and as if that was not enough, 185 humans lost their lives in the quake. I highly suggest you click the link to the article. It has amazing pictures of some of the damage, showing the awesome power of the force of nature. Along with the pictures, the article discussed some of the effects of the earthquake, and the geological side of what happened, which is what I’ll be talking about here.
As I already mentioned, the geology of Christchurch is sedimentary, the leftovers of what rivers brought from the mountains and built over time. One item to occur was liquefaction, which can occur with wet sediments (Christchurch’s water table is generally below three meters (nine feet)).  Liquefaction is what my geology professor once described as ‘geological Jell-O’. The sediments are stable and can be built upon, much as you could balance some plastic dinosaurs on an unmoving surface of Jell-O. But if you move that Jell-O too much, your dinosaurs come tumbling down. Liquefaction has a similar result: the shaking of the ground makes the particles in the soil (whether they be silty, sandy, gravelly, or similar) want to settle down, like being sifted in a pan with water. This increases the water pressure of the water in the cracks between particles, generally pushing water towards the surface. The ground is no longer solid but more like mud, closer to the shaking Jell-O, but now instead of plastic dinosaurs, it is buildings that are falling over. During liquefaction, sand can be pushed up through cracks in the ground, creating mounds of sediment known as sand volcanoes. Cracks and fissures can be created in the ground, pavement is distorted, cracks appear in foundations and structures, and the ground surface can develop new depressions. All this happened in the Christchurch earthquake, damaging some buildings beyond repair, while others moved their positon or found themselves resting on a cracked foundation. The worst affected by liquefaction were suburbs to the east of the CBD, near the Avon River that laces through the city.
Additionally, there was lateral spreading, which is horizontal movement of the ground. This was especially noticeable near the Avon River. In places the movement was over two meters (six feet), and up to 200 meters (almost 220 yards) away from the river banks. Lateral spreading also caused significant damage to some of the bridges in Christchurch. It also damaged the pipes waste water and potable water, the problem intensified by liquefaction. Underground electrical cables also suffered from liquefaction and lateral spreading.
An image of lateral spreading towards the Avon River. Taken from Cubrinovski et al 2011, the main paper for today.
An image of lateral spreading towards the Avon River. Taken from Cubrinovski et al 2011, the main paper for today.
 On top of all of this, there were landslides in the nearby Port Hills and rock falls from the cliff faces. These destroyed or damaged many roads and buildings, and were responsible for five of the deaths.
It has been four years since all of this happened, and about a year and a half since I left Christchurch. The cordons around the CBD Red Zone have since been dropped, except in places where there is ongoing construction. The Anglican Church that owns the ChristChurch Cathedral have decided to tear down the historic building and rebuilt it with a more contemporary design. The ground shook for only one minute, but it changed a city forever, as its still in the process of rebuilding certain infrastructure, its buildings different than before. Christchurch is and will continue to rebuild, but it will never quite be the same city it once was.

 

Bibliography
Sources I used information from directly in this post. I’d also like to give a shout out to three of my friends in New Zealand, who informed me of what was going on now and directed me to a few sources.
Backhouse, Matthew. “Christchurch Red Zone Cordon Finally Lifted.” New Zealand Herald 30 June 2013. Web. http://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=10893922
News article on the Red Zone being lifted.
Cubrinovski, Misko, et al. “Geotechnical aspects of the 22 February 2011 Christchurch earthquake.” Bulletin of the New Zealand Society of Earthquake Engineering (2011).
Identifier: http://hdl.handle.net/10092/6393
The main paper for today. Also the source of the diagram for Christchurch’s soils and image of lateral spreading.
GeoNet. “M 6.3, Christchurch, 22 February 2011 – Earthquake.” N.p., n.d. Web. 16 Feb. 2015. http://info.geonet.org.nz/display/quake/M+6.3,+Christchurch,+22+February+2011
GeoNet’s page on the earthquake. Used for details of the epicenter, magnitude, and a couple of other details.
Marshak, Stephen. Earth: Portrait of a Planet. 3rd ed. W.W. Norton & Compant. Print.
My geology textbook, used to refresh my memory on plate tectonics, and a bit of reading on liquefaction.
O’Connor, Sarah-Jane. “Court Paves Way for Cathedral Demolition.” Stuff.co.nz 30 May 2015. Web. http://www.stuff.co.nz/national/10104246/Court-paves-way-for-cathedral-demolition
A short article explaining the plans for the cathedral, and the courts involved.
“Re:START Mall – Christchurch, New Zealand.” New Zealand on the Web. N.p., n.d. Web. http://www.christchurch.nz.com/re-start.aspx
Aimed at tourists, a page about the shopping mall made of chipping crates. Having been there in person, I already knew of its existence, but needed more information, and to check that it went over the location of the former mall.
“‘We May Be Witnessing New Zealand’s Darkest Day’: PM Says 65 Killed in Quake.” Sydney Morning Herald. 22 February 2011. Web. http://www.smh.com.au/world/we-may-be-witnessing-new-zealands-darkest-day-pm-says-65-killed-in-quake-20110222-1b356.html
A news article used for the duration of the shaking, written the day of the quake.
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