STILL STANDING: Christchurch's Pico Wholefood building following this week's aftershock.
Image: COURTESY OF SCHWEDE66, VIA WIKIMEDIA COMMONS
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New Zealanders living in the nation's second-largest city, Christchurch (population approximately 377,000) on the South Island's Canterbury Plains were hit hard Tuesday by magnitude 6.3 earthquake, an aftershock from September's magnitude 7.1 tremor. Prior to these two seismic events, Canterbury Plains likely had not experienced an earthquake in thousands of years. In fact, scientists did not even know there was a geologic fault there until it ruptured last year.
The latest Christchurch tremors were not as strong as the original earthquake, but they have caused considerably more damage and claimed dozens of lives. (No one died during the September quake). The Christchurch epicenter was only 10 kilometers outside of the city, whereas the 2010 event took place about 40 kilometers to the west, in an area that is mostly farmland. Adding to Christchurch's misfortune, the aftershock struck only about four kilometers in depth below the city, whereas September's temblor originated about 10 kilometers deep. Compounding these problems, Tuesday's quake hit during lunchtime when the city was buzzing with activity, whereas the earlier disaster occurred during early morning hours.
Scientific American spoke with Robert Yeats, a professor emeritus of geology at Oregon State University in Corvallis, about why earthquakes are so difficult to predict and what is being done to lessen the odds of surprise temblors.
[An edited transcript of the interview follows.]
The 7.1-magnitude earthquake in September caught the locals completely off guard. Why was that earthquake as well as Tuesday's aftershock such a surprise?
The earthquakes struck an area of New Zealand's South Island where sediments are deposited from the Southern Alps and from the nearby rivers. The sediment deposits reach all the way to the east coast on what is called the Canterbury Plains. The fault that ruptured in September had not done so in thousands of years, during which sediments had been deposited on top. I've been out there, and it's like driving anyplace where it's all flat. There are farms, but there's nothing that says, "Here's a fault." So, when the earthquake struck in September they were totally surprised. On the South Island, the Hope Fault and Marlborough Fault System are better known—there had been an earthquake in 1888 along the Hope Fault. Much less was known about faulting on the Canterbury Plains because no earthquake had happened in their historical record.
What defines an "aftershock" as opposed to an earthquake? Does a certain amount of time have to elapse for a seismic event to be considered an earthquake?
It takes many years before seismic activity can be considered an earthquake rather than an aftershock of a previous earthquake. That's a point of debate among seismologists. If you look at a map of southern California, you'll see quite a few little earthquakes south of Bakersfield in the San Joaquin Valley. Some people regard those as still aftershocks from the earthquake of 1952, which measured 7.3. That's not the consensus but it indicates that these aftershocks go on for decades. It takes quite a long time for everything to become quiet again. Now the San Andreas, on the other hand, had a large earthquake in 1857, just west of San Joaquin Valley, and it's quiet as could be. Same with the area of the 1906 San Francisco earthquake along the San Andreas. You can't paint all aftershock series with the same brush.
You mentioned that the aftershock was "shallow". What is the difference between an earthquake that takes place four kilometers below ground and one that takes place 10 kilometers below the surface, as the September earthquake did?
It's like how close you are to a bomb going off. If you're within a couple of kilometers, you're likely to get injured. If you're three or four times that distance, you not likely to. The waves are attenuating, or propagating, toward the surface. Christchurch was a very shallow earthquake, and that's a reason why the damage was much worse than the earlier one.
What do you look for when you investigate seismic activity in a particular area?
If I study a particular fault, I like to know its slip rate, how fast it's moving, whether it's a millimeter per year or a centimeter per year. As plates move, they're building up strain, and I estimate how much strain can build up before there is a rupture. New Zealanders have been good about trenching faults (digging trenches along fault lines to study previous seismic activity). The problem with this fault was that they didn't even know it was there. That tells me it's a pretty slow-moving fault but, nonetheless, when it builds up toward an earthquake of magnitude 7, then that's going to continue to produce aftershocks for a long time. It's not an exact science.
I'm working with the New Zealand Institute of Geological and Nuclear Sciences on a project funded by the Global Earthquake Model to map all of the active faults on Earth. This Global Faulted Earth project will include a global active fault and seismic source database, along with a book I'm writing. At some point in the not too distant future, if you hear about an earthquake in a place like Christchurch, you will be able to click on this database to find out what is known about it.
What can be learned from this week's aftershock in New Zealand?
We can map faults, and that's what we do, and we can use what we learn about those faults to establish some probability of an earthquake happening—but you can't map all faults. In the case of Christchurch, I'm not sure what they could have done differently. They could have said, let's do a seismic survey of the whole Christchurch metropolitan area just to be sure there's nothing going on underneath the city. But it was unlikely for the faults to extend as far south from the original Darfield earthquake site at as they did, so I can't fault them for not doing that.
You have to realize that New Zealand has some of the strongest building codes in the world, and those building codes are respected. That means you have loss of life, but it's in the dozens or maybe 100 or 200. If the same earthquake were to happen under a city of that size in a developing country, the number of deaths would be in the thousands, if not tens of thousands. Turkey, for example, had great building codes but that didn't keep tens of thousands of people from getting killed in the 1999 Izmit 7.6-magnitude earthquake because they weren't paying attention to those codes.
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6 Comments
Add Comment"The waves are attenuating or propagating toward the surface. "
Reply | Report Abuse | Link to thisWhile there is sure to be some attenuation, or loss of energy, isn't the major component in the relative strength the greater surface area given the same energy? The energy propagates in a sphere, area being 4*Pi*r^2, or the inverse square law for a point source of electromagnetic radiation. Thus doubling the distance would reduce the energy per area to 25%, or E1 = E0/4.
The total energy would be the same for the entire surface but per unit of area, less.
Also the soil condition have an effect. Sediment is more likely to liquify resulting in loss of structural foundation than solid rock.
It would be very useful to compare the levels of strong ground shaking for the main earthquake event and the aftershock. I would expect that there were strong motion recorders installed in Christchurch, even if it was considered to have no active faulting in the area.
Reply | Report Abuse | Link to thisAlso, newspaper accounts state that sand and water were expelled from the ground, indicating that liquefaction occured. Liquefaction may help to explain the failure of modern buildings built to the otherwise competent New Zealand seismic structural building code.
Liquefaction occurred. Enormous amounts of water and silt rose through the ground. Cars were half buried in it in places. Some of it formed a great whirlpool, shown on TV. Students have been out with barrows shovelling it up, today. There are also huge cracks in the ground.
Reply | Report Abuse | Link to this98 people have been found dead so far, and unfortunately many more may be found as they search. Search teams have arrived from all over the world, assisted by sniffer dogs. But as time goes on, chances of the buried surviving grow less.
The material damage is enormous, but there is a great spirit among the people.
The location of the earthquake surprises us, probably because we haven’t understood the mechanism of this natural phenomenon. I am saying that after reading a most interesting and ground-breaking analysis of the greek author and gnosiologist Mr. Ioannis G. Tsatsaris, who indicates that earthquakes are not only caused by inner Earth activity, but also by electromagnetic fields and atmospheric pressure. This is a new perspective worth of careful examination by scientists on the field and I quote a excerpt of his seminal work for your information:
Reply | Report Abuse | Link to this“… Earth’s material body enacts its presence in its organic totality and from there the various organic convulsive contractions-and-expansions take place, on the basis of atmospheric density and heat in its internal organism, which are immediately correlated with Earth’s outer atmosphere.
In that geological and atmospheric rule, we have an absolute formation of electromagnetic fields, which have an immediate relation with the Earth’s interior circulatory, as well as with its atmospheric, systems. Thus when we have a large expulsion of steam and other geological elements in a molten state in the planet’s interior, and from the other various projections of increased electromagnetic external fields which are produced by the disturbed atmosphere as residues, pressure is formed on the planet which takes place as follows: atmospheric pressure accumulates its residues on Earth’s surface and thus forms an enclosure to various geological passages in the earthy sphere from which small outlets of pressurized inner geological steam flowed. There, the internal state of the steam together with the other geological elements seeks its outlet. It determines its outlet where it finds the softest geological layers. And thus an earthquake manifests itself. For this is how it should occur…
Here we should say something else: electromagnetic fields define the Earth’s geological dimension as pressured to receive steam states of the atmosphere which, on the basis of the positions of hydrogen, oxygen and nitrogen, are pressured to descend to an underground geological domain, to be mobilized there in the recycling of the process and to be released into the atmospheric dimension as an etheric state of pure steam which will greatly reinforce the animate element of the combinations of the planet’s natural position…”
(Ioannis G. Tsatsaris, The Next Step of Creation, The Revelation, Vantage Press, NY, p.231)
Cleo Politof-Katsivela, Civil Law Notary, LLLM in Medical Law.
Your article claims: The 7.1-magnitude earthquake in September caught the locals completely off guard. Why was that earthquake as well as Tuesday's aftershock such a surprise?
Reply | Report Abuse | Link to thisBoth statements are false. Seldom does Scientific American do an article that is so misleading.
1. Locals were surprised by the intensity of the 22 February earthquake, not its occurrence. It caused so much damage because the shaking was very intense. Peak Ground Acceleration (PGA) was up to 2.2 times g. Most cities in the world would be totally flattened by such acceleration.
Reference from the authoritative GNS is: ‘Shaking intensity in the city was much greater for this earthquake than the magnitude 7.1 earthquake for any of its other aftershocks. This is due to the proximity of the epicentre to the city and its shallow depth. The highest shaking was recorded at Heathcote Valley Primary School at 220% g, with readings of 188 %g at Pages Road Pumping Station and 107 %g at Hulverstone Drive Pumping Station.’
(Source: http://www.geonet.org.nz/news/feb-2011-christchurch-badly-damaged-by-magnitude-6-3-earthquake.html)
2. Earthquakes near or under Christchurch were not unexpected. A research paper commissioned by the NZ EQC (Earthquake Commission) in 1991 predicted ‘moderate earthquakes under the Canterbury Plains and Christchurch itself.’ (Source: http://eqc.govt.nz/research/researchpapers/p_105.aspx
3. Liquefaction was also predicted (Source: ibid)
The people surprised by the event were misled by the popular scientific press which attributes earthquake risk almost exclusively to known fault-lines.
Hi from Christchurch,
Reply | Report Abuse | Link to thisI am a commercial property owner of 100year old brick buildings in the CBD of Christchurch I have been involved in the design and installation of earthquake strengthening. Yes New Zealand has very good Building Standards but the reality is that very building were in fact up to the Seismic Codes. The Codes are upgraded about every 10 years or so, and this comes about from the shared information from earthquakes around the globe and from scientific research in universities in Japan, New Zealand, USA, and Chile.
The latest Code change came out in 2008, so infact only a very few of the building in the CBD were up to anything like the Code. interestingly many of the old unreinforced brick building have withstood the force remarkably well although they have sustain quite some damage.
Steel framed building have sustained very little damage as thet flex and absorb the energy released.
The buildings that have faired worst in my opinion are the Reinforced concrete buildings and most of the deaths occurred in these buildings and the concrete buildings that have survived simply are so damaged and cannot be repaired and are being demolished resulting in a huge loss of property in the CBD.