EARTHQUAKE

Geoscience Australia earthquake seismologist Dr Trevor Allen answers ABC News Online's questions about how earthquakes work and why they are causing so much chaos in the New Zealand city of Christchurch.

On February 22, 2011 a shallow 6.3-magnitude quake ripped through Christchurch, killing at least 75 people and leaving as many as 300 others missing. The city was only just getting back on its feet after experiencing an even stronger 7.0-magnitude quake on September 4, 2010.
Q. What is an earthquake and how does it occur?
A. What is happening under the Earth's crust is that the tectonic plates are moving continuously. What this does is it actually puts stress on the rocks, both on the boundaries of these tectonic plates and internally within the plates themselves. So once the stress becomes so great in the rocks, the rocks actually break and essentially that's what causes an earthquake. They tend to break along the weakest part of the rocks, which is generally a pre-existing fault line.
Q. Are there different types of earthquakes?
A. The one that happened yesterday beneath Christchurch (February 22, 2011) is what we call a shallow crustal earthquake. Those earthquakes generally occur beneath land and are generally quite shallow, less than 20 kilometres deep. They're the ones that produce the biggest impact on society and the ones that we are worried most about. Haiti was also one.
Other earthquakes we have are subduction zone earthquakes. This is where one tectonic plate subducts beneath another tectonic plate. That is actually the case at the start of the North Island in New Zealand and also in the South Island, there are two subduction zones there. They generate the largest earthquakes that we see and that is also similar to the 2004 Boxing Day tsunami, where we had a magnitude 9 earthquake off Sumatra. Recently, New Zealand has experienced much larger earthquakes but they've been in the subduction zones which are generally offshore. So because they're offshore, they're a reasonable distance from population so they tend not to do as much damage to built environment.
Within those earthquakes we have plate boundary earthquakes and we also have intraplate earthquakes, which are the ones that occur in Australia. They're earthquakes that are generally less frequent, but places like Australia still do generate these large earthquakes because there are internal forces at work inside the tectonic plates. These quakes are much less frequent than the ones on plate boundaries such as New Zealand.
Q. What are the different causes?
A. The ones in New Zealand are due to plate boundary forces. You can also get earthquakes that are generated by volcanic activity.
Q. What is the difference between an earthquake and an aftershock?
A. There is no difference. Probably the only difference is that aftershocks generally tend to occur on a faulted surface that has ruptured recently. All of the aftershocks that are occurring at the moment in Christchurch are essentially occurring on the fault surface that ruptured yesterday, so it's generally not rupturing new rock or new pieces of rock. They're more adjustment earthquakes, so the Earth is essentially resettling into its preferred state. As it resettles it is causing these smaller earthquakes.
We don't think yesterday's earthquake in New Zealand is an aftershock from September's in that it looks to have ruptured a different section of either the same fault that ruptured in September or a fault plane that is adjacent, but very near to the one that ruptured in September. The two earthquakes are definitely related, but this is a new rupture on a different part of the fault.
Q. How long do aftershocks last for?
A. Every earthquake is different. Some earthquakes you may not get any aftershocks, but from the September earthquake we noticed there was a very long aftershock sequence and they will probably still continue for up to another 12 months. We would expect to see a similar sort of pattern emerging from the earthquake we saw yesterday. But they will diminish with the magnitude and frequency of occurrence over time.
Q. What are some common misconceptions people have about earthquakes?
A. One is that they only occur on plate boundaries. We know that in Australia we are certainly not located on a plate boundary, yet we have observed earthquakes as large or larger than the one that occurred in Christchurch yesterday. In 1988 we had three large earthquakes in one day just near Tennant Creek in the Northern Territory. They ranged from magnitude 6.1, 6.3 and 6.7. Thankfully there was no large population exposure or no large communities nearby, so essentially there was no damage from this earthquake, but it demonstrates that we can have large earthquakes in Australia.
Another misnomer being perpetuated at the moment is that a lot of people think that because of all these large earthquakes that have been occurring lately, particularly in 2010, that we are seeing more frequent large earthquakes. But the truth is that these are just part of the earth's natural processes. It just so happens that they seem to be falling in places where there are lots of people. We might see spikes in the earthquake activity over particular periods, but we might also see times of quiescence where we don't see much activity.
Q. Do earthquakes happen around the world on a daily basis?
A. Yes they do. On average you're looking at at least five to six earthquakes greater than magnitude 5 a day, and we generally see two to three earthquakes magnitude 6 or greater per week. A lot of these are offshore in subduction zones and because they're a reasonable distance away from populated areas they don't get the news coverage that the ones such as Christchurch get.
Q. What made the February 22 Christchurch quake so destructive?
A. We all know that the magnitude of the September quake was much larger. We have been asked a lot why that earthquake didn't generate as much damage. The thing is that because that quake was much larger, the area of ground shaking was also much larger. However, it occurred in a relatively remote area, not a lot of people living in the area where it occurred, not very dense exposure I would say, whereas for yesterday's earthquake the magnitude was smaller, the area of maximum ground shaking was also smaller, but that area of maximum ground shaking was almost a direct hit on the city of Christchurch, so that's why we are seeing so much damage. It is really a function of the earthquake's location.
A lot of the buildings in Christchurch also had pre-existing damage from September's earthquake and I guess because it is such a large job to go around and fix all of these damages, a lot of the buildings were more vulnerable to the ground shaking form yesterday's earthquake.
Q. How does it compare to previous quakes in the region?
A. I suppose it is still unravelling in terms of how bad this quake actually is. The most deadly earthquake historically in New Zealand actually occurred in 1931, that killed about 230 people in the city of Napier. Economically speaking it probably is the worst earthquake that has struck the region, but in terms of fatalities it is yet to be seen.
Q. Is there any way of knowing whether this pattern in Christchurch will continue?
A. New Zealand overall does experience frequent large earthquakes and I guess as a seismologist it is our job to try and understand what the likelihood of particular faults are to rupture. Unfortunately the tectonics of New Zealand are so complex that we just can't identify every single fault. For example the one that occurred in September, we didn't know the fault existed prior to the earthquake, so it makes the job very difficult.
In the Christchurch area we will expect to see aftershocks from this earthquake. There is maybe a small likelihood that another large earthquake could be generated. It's not unusual that following a lager earthquake it might trigger another quake on an adjacent or a joining fault line. By moving one piece of rock you're putting pressure on other bits of rock further away.
Q. How long will it take to rebuild from an earthquake of this scale?
A. I think Kiwis in general are fairly resilient and I think they'll bounce back from this. In terms of how long it will take them to rebuild the city ... it's going be at least 12 months before the city is functioning as it was. There are going to be ongoing issues with the soil liquefaction, so you can design very strong buildings to withstand earthquakes but it is only as strong as the foundation it is built on.
Q. What are the chances of a similar series of quakes striking Australia?
A. The chances of something of that scale occurring in Australia are much, much lower than they are in New Zealand. Because we are sitting in the middle of a tectonic plate we don't have those plate boundary forces putting stress on the rocks within the continent. So we generally don't experience as frequent earthquake activity, but it is a very large country and we essentially could have an earthquake of that magnitude anywhere in the country.

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