The earthquake in New Zealand last month that killed dozens of people and toppled buildings in Christchurch has implications for future disasters in the United States, according to a Cornell University earthquake expert.
I spoke last week with Thomas O'Rourke, an engineering professor who studies earthquake effects. O'Rourke, who lived and worked in Christchurch, explained why an earthquake with a moderate intensity level caused such destruction -- and what lessons we should take away from the disaster.
The earthquake that caused the most damage in Christchurch was actually an aftershock?
The fault that ruptured on Sept. 4 is called the Greendale Fault. It transferred stress to other locally weak parts of the Earth's crust in and around the Christchurch area. Apparently, there's a subsidiary or related fault. That is the rupture mechanism that led to the affects we see now in Christchurch associated with that Feb. 22 earthquake.
They just corrected the magnitude on that. Instead of being a 6.3 magnitude, which is very moderate, it's actually gone down further to a 6.1. It serves as a great example that earthquakes don't necessarily correlate with magnitude. You could have a 9 magnitude earthquake in the middle of the ocean and by the time the waves get to an island, they could be attenuated to the point that there was only a minor shaking there. We always distinguish between magnitude and intensity. Also, we distinguish between magnitude in terms of the energy that's released and the magnitude of the disaster.
There's another scale that's going on here and that's infrastructure. The infrastructure can respond very poorly because the intensity is great in terms of the level of shaking and the ground motion that is occurring within that infrastructure system. It also can respond poorly because it has weak elements, such as vulnerable buildings and particular characteristics of its infrastructure that make it much more susceptible to damage.
Since this was a moderate earthquake, how do you explain the level of damage?
The New Zealanders are among some of the best seismic engineers in the world. They have one of the oldest codes. It's certainly as old as the code in the United States for earthquakes. They also have a very up-to-date approach to structural design, disaster preparedness, understanding the effects of earthquakes on their built infrastructure and undertaking measures to ensure as good a performance as possible from their lifeline networks, such as transportation, water supply, electric power, telecommunications. They had a 7.1 magnitude earthquake on Sept. 4, 2010. The epicenter of this earthquake was located about 12 to 15 kilometers to the west of Christchurch. It's very close and they did sustain significant levels of acceleration and other types of ground motion and they had massive liquefaction during that event. Yet not a single person died. That reflects more than luck. It reflects many of the codes they had in place for their more modern buildings and the steps they'd taken to ensure that there would be a reliable response from their infrastructure. Their electric power worked very well during that main shock. [For comparison] in January 2010, we had a magnitude 7 earthquake in Haiti which killed [thousands of] people and completely destroyed the infrastructure of Port Au Prince.
The second event [in New Zealand] is really extraordinary in the fact that it was a rupture that occurred underneath the port of Lyttelton and a little north of Lyttelton, very close to the business district of Christchurch. It generated some significant dynamic movements or strong ground motions and it had a very significant impact on the built environment. The central business district of Christchurch has a lot of unreinforced masonry structures. Their iconic structure had been the cathedral. Of course, the spire of the cathedral has collapsed. Many, if not the grand majority, of the unreinforced masonry structures have either collapsed or sustained serious damage. They were weakened by the first event, but the second event caused a very serious response.
The soil liquefaction during the second event was more extensive. It covered about 40 percent of the area. What happens is seismic shaking drives up water pressure so firm soil loses its strength and turns into liquid. If it flows toward a river channel or low point in the topography, [it disrupts] the buried infrastructure. That's what happened in the second event.
The type of motion we're seeing from this small magnitude earthquake, because of its proximity to the central business district, is very similar to the ground shaking one might [see] in earthquakes in the United States. [This is in places] where the infrastructure and ground behavior has certain very similar characteristics to what we see in Christchurch. In 1812, there were three earthquakes in the Mississippi River area. There is historical evidence for soil liquefaction. If you want a good likeness to what would happen if we had a severe earthquake in the New Madrid zone, all you'd have to do is look at the central business district of Christchurch. We have even a greater concentration of unreinforced masonry structures in places like Memphis and St. Louis that would be vulnerable to this type of event. We know that recent sediments from the Mississippi River are very similar to those of sediments in New Zealand. So when you look at the massive liquefaction in Christchurch, you don't have to have a lot of imagination to understand what would actually happen in a large and severe Midwestern earthquake.
What can be done to prepare for that?
One of the things to do is have building codes in place, which they do. People are trying to improve the standard of the building codes all the time. As new structures are put up, the older ones which are vulnerable to earthquake effects are replaced by newer ones that have more modern and more explicitly-designed reinforcement for earthquake effects. The recurrence interval for a large, severe earthquake in the central part of the United States might be on the order of 500 years. For a smaller magnitude earthquake, that recurrence interval could be a couple of hundred years or even 100 years, similar to earthquakes in California. That's fortunate because it gives you time over a couple of generations to replace vulnerable parts of your infrastructure with parts which are much better designed for what is an inevitable event.
Talk about the time you spent in Christchurch and the work you did there.
I lived there in 1999. I was on sabbatical at the University of Canterbury in Christchurch. I was there to lecture and travel and work with some of the faculty on problems related to lifeline networks. We were very concerned about the behavior of water supply and about liquefaction effects on buried infrastructure. At the time, they were developing a regional plan for what the vulnerabilities were to their lifeline systems. In 2007, I was with the office of the prime minister and cabinet in Wellington. I was there provide input for policy formulation with respect to the vulnerability of their critical infrastructure and to natural hazards, of which they have many.
Do you have anything else to add?
This is a very severe earthquake. It shows the vulnerable nature of modern communities. Christchurch, in terms of the way it is set up, is very similar to modern urban centers in the United States. This earthquake has shut down the port, so they're not getting oil and gas into Christchurch. There is a real obstacle now to getting food in. [After the initial shock in September], they were in a very serious situation with regard to food supply because in many of their warehouses, the racks that contained the food collapsed. You ended up having food rotting in the warehouses. That was a really important lesson for lots of other places around the world. Many times we focus on the outer structure. But you have secondary structures inside, like these large food racks, and when they fail you can't get to the food. The loss of the port really ramps up the significant economic consequences of this event. They've developed a number of coal fields and they transport the coal by rail to the port and ship it around the world. With the port closed, that also has a dramatic impact on the national economics.
If you want to look for a living laboratory of how a modern urban center responds to a severe earthquake, then this is the place you'd look. We all know that can occur in the United States. We have so many other news events going on that this particular event has really gotten put on pages two and three. I'm amazed that something that would normally be what people would focus on a lot more intensely and trying to learn from has really been eclipsed. This is a really interesting event. It has tremendous implications for urban environments, especially urban environments in the United States.
Photo: Thomas O'Rourke