Better place for cathedral closer to Alpine Fault
Updated 4 September 2011
The Alpine Fault doesn’t move 30 millimetres every year. It does on average, but the fault hasn’t ruptured since 1717.
If goes anytime soon, nine metres of pent up horizontal movement could be released in an earthquake of a magnitude of more than eight.
While that might not sound formidably large, only six earthquakes have ever been recorded at greater than eight. And if it were an 8.5 magnitude event, it would unleash two thousand times the energy of the 22 February 2011 Christchurch Earthquake.
The previous Alpine Fault events were in 1615, 1410 and 1230. Along with the last one, which was 294 years ago, this is an average repeat time of 160 years. As geologist Dr Chris Rowan puts it:
The math is hardly reassuring, is it?
In contrast, if the Wellington Fault ruptured tomorrow, it could be considered 600 years early. It is now understood to produce a major event every 900 years, and to have last ruptured 300 years ago. That is not to suggest fault-straddling Wellingtonians need not be vigilant—even a minor jolt could prove deadly.
The shallowness of Tuesday’s killer quake, and it being centred so close to Christchurch’s central business district was just one of the reasons 181 people died: Liquefaction was a major factor.
It used to be a point of pride that the city had been built where once was swamp—maps highlighting Canterbury soils prone to liquefaction are pretty much a map of Christchurch. Although liquefaction is most visible in the suburbs, it doesn’t need to bubble to the surface to have a devastating impact on buildings. Liquefaction can magnify the shake of an earthquake, by momentarily turning the ground under central city buildings to jelly. The city could anticipate a better future if its business district was re-built closer to the Alpine Fault.
There is, of course, an even more compelling imperative for the future business district of Christchurch to be located well west of the soils prone to liquefaction. By the end of this century, the probable multi-metre rise in sea level will likely have Cathedral Square awash, and be rising at the rate of five metres per decade.
This suggests that the business district should be relocated about 10 kilometres west of the airport, which itself may not be inundated until the 2130s.
The vast majority of the world’s cities began life as, or near, seaports. These now all need to migrate inland. Christchurch at least has readily available options. In contrast, about the same time Christchurch Airport is awash, the Tamaki isthmus will have morphed into the Makaurau Strait, dotted with the islands Motukiekie, Motuwhau, Motureipae, Motumangere and others, once-named One Tree Hill, Mount Eden, Mount Wellington and Mangere Mountain.
The cost of moving the metropolis will dwarf the $20 billion job of rebuilding Christchurch. However, a considerable difference is that Makaurau, by prioritising low-lying areas such as the waterfront and Mangere—both the suburb and international airport—can span its relocation over the course of a century.
While the New Zealand Transport Agency no doubt continues to sink money into planning a Pūhoi–Wellsford motorway, the cost of rebuilding Christchurch will likely ensure that physical work on the antediluvian project never begins. Aside from the self-evident need for the agency to heed its mission of building a better transport system for New Zealanders, the cost of staying ahead of sea level rise will consume an ever-increasing proportion of its budget.
It would be wholly unwise to rebuild the cathedral and business district on shaky ground, being made increasingly shaky by the inexorably rising water table—ahead of wholesale sea level rise.
It is natural to treat the carnage in Christchurch as a test of character, but it is also a test of wisdom.
Warming last straw To the list of factors involved in the fiendishly complex science of earthquake prediction, anthropogenic global warming can now be added. Ice sheet melt reduces pressure on some parts of the earth’s crust, while sea level rise increases it in other places—both will trigger seismic action. Researchers have linked the El Niño –induced rise in local sea level, of a few hundred millimetres, with increased 4–6 magnitude earthquakes in the Easter Island microplate. As Bill McGuire of University College London told the New Scientist:
You don’t need huge changes to trigger responses from the crust—the changes can be tiny.