Aftershocks 'exactly what we expected'
SHAKY CITY: Since the September 4 earthquake, Cantabrians have experienced hundreds of felt earthquakes within a total of thousands of aftershocks following the main event.
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OPINION: Professor KEVIN P FURLONG, a geoscientist, asks: What's the deal with all these aftershocks?
Cantabrians have become experts in detecting the subtle differences among earthquakes. But such expertise does not bring with it much comfort in light of continuing events.
Since the September 4 earthquake, we have experienced hundreds of felt earthquakes within a total of thousands of aftershocks following the main event.
Throughout this entire episode, the earthquake-science community continues to reassure the public that there is nothing strange or out of the ordinary about the aftershocks. But for many, aftershocks remain a real source of personal concern.
Perhaps it is now useful to step back and explain why aftershocks occur, what controls their locations and sizes, and why the current spate of events is not a cause for alarm, but rather a reminder of the value in maintaining vigilance and preparedness.
Aftershocks are just earthquakes; they obey the same laws of physics as other earthquakes.
We use the term aftershocks to differentiate them, since we consider them to have been caused or encouraged by the initial main- shock earthquake.
In other words, if the main shock hadn't occurred we wouldn't be getting the aftershocks.
But not all aftershocks are the same. In general there are two main categories of aftershocks - ones that occur on or very near the fault plane of the main earthquake, and others that occur in areas where earthquake-caused crustal stress changes help trigger these small earthquakes.
The main magnitude 7.1 earthquake occurred primarily on a nearly-vertical fault plane extending about 10 to 15 kilometres downward into the crust and perhaps as much as 50km along its east-west length.
About 30km of this fault length ruptured all of the way to the surface, producing the impressive fault we see extending from west of Greendale to the western outskirts of Rolleston.
That 30km-long surface fault is only part of the story, and geophysical evidence suggests that the fault continues in bedrock beyond the ends of the surface rupture.
This continuation of the fault at depth is typical of many earthquake ruptures; in fact in the Haiti earthquake there was little or no surface rupture.
During our main earthquake, there was slip along the fault that averaged about 2.5 to 3 metres, but the slip during the earthquake was different at different places along the fault, so there were patches along the fault that were not in equilibrium with nearby fault patches.
Many of the aftershocks, particularly those occurring during the first few weeks after the main earthquake and that were situated on or near the main fault, result from the processes by which these slip differences are reduced or smoothed.
Their occurrence near the mapped fault trace and in regions such as the swath from Rolleston to Lincoln record these fault equilibration processes at work.
The size or magnitude of an earthquake is proportional to the product of the fault area multiplied by the distance it slipped.
The main magnitude 7.1 earthquake with a fault area of about 500 square km (50km length x 10km depth extent) and an average slip of 2.5m to 3m dwarfs a magnitude 5 aftershock with slip of about 10cm on a fault patch about 3km by 3km, or a magnitude 4 aftershock rupturing a 1 square km patch with slip of only 5cm.
Most of the aftershocks this past month record the second category of earthquakes triggered by stress changes.
These earthquakes, including the notorious Boxing Day event and the 5.1 on January 20 did not occur on the September 4 fault; rather they are within the crust of the surrounding region where stresses changed enough to cause earthquakes on other small faults.
The spatial pattern of these triggered aftershocks is not random.
Models of the stress changes caused by the main earthquake can be used to identify where changes are sufficient to either favour or inhibit aftershocks.
These stress models show that the occurrence of aftershocks in the region of Christchurch city and east of the CBD, north of the main fault trace near Darfield and further north, and west of Horarata are all in places where stress changes favour increased aftershocks.
These triggered aftershocks occur on existing structures or zones of weakness in the crust, and are often aligned along linear trends.
Maps of Canterbury earthquakes since September 4 show several such trends. In this way the aftershocks help scientists to map the fabric of the crust and identify trends and locations that merit further study.
The why, where, and when of aftershocks since the September 4, 2010, earthquake is consistent with the expectations and experiences of earthquake scientists.
Most fall neatly into the two likely causes - either adjustments on and near the main fault rupture or small triggered events in regions where stresses increased slightly as a result of the main earthquake.
Therefore when a small cluster of events occurs in any particular location, we understand their origin in relation to the stress resulting from the main event and they are unlikely to indicate that another large event on the Greendale Fault is pending.
The patterns of these aftershock earthquakes do, however, help to improve our understanding of fault rupture during the main event, and to identify other active structures within the Canterbury crust.
Perhaps most importantly, they remind us that preparations and continued vigilance are an important responsibility of living in earthquake country.
* Kevin P Furlong is a Visiting Erskine Fellow at the University of Canterbury and Professor of Geosciences at Pennsylvania State University in the United States.
- © Fairfax NZ News
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Paul, every step of a scientific process is verifiable to you, but science takes place as part of a social process as well. You don't verify, rather challenge - try to prove your own theories wrong, and if your beliefs are confirmed 'great', but only for the present. I think you have misunderstood what some of the other people were getting at in their comments. Best to let it rest perhaps, for now.
A clarification of my previous comment. I know science can never find all the answers because we are not omnipotent like God. We have just barely scratched the surface of the vast treasure called knowledge. Science is about asking questions and seeking the answers. It is done in such a way that every step of the process is transparent and verifiable. A question is asked, a hypothesis is formed and research is done to test whether the hypothesis is correct or not. If the research results supports the hypothesis then it is accepted. If the evidence does not support the hypothesis then the hypothesis is rejected and a new hypothesis formed based upon the research results. Any method where the results or evidence is / are altered to suit or fit the hypothesis is bad science and not acceptable as such
This follows on to to my comment about asking questions. No matter how good and brilliant the scientist, like the rest of us they are still human and make mistakes. Hence the peer review system for published research. Ask questions and if you don't understand the answers ask more questions. That is the core of science - the asking and answering of questions. Science is not dogmatic like, for example, religion (I am Roman Catholic) or politics.
I will apologise for my spelling error in my comment (Paul #11) and my bad englsh. The term I used: "They are two totally different animals." is a colloquial term and shouldn't be taken literally. I never claimed I or science knew all the answers, because we don't. I personally believe we never will but that should not deter us from searching for answers. At our present level of knowledge we cannot forecast or predict when earthquakes will occur but we know that in NZ's case they will. With regard to Martins #12 comment about the UCLA research on tidal correlations with earthquakes, I have not read the papers so I cannot comment upon them. @ Malthouse #18, I am not saying don't question somebody who has been trained on the subject, in fact do question them because it is how we all learn. In Christchurch's case I am of the view that if people can understand something of the science of earthquakes, it might help them to understand quakes a bit more and hopefully help, in some small way, deal with the emotions & distress that the quakes have caused.
blar...blar...blar
30 Cumecs of water disappears from the Rakia River (equiv to RDR flows) and it must go somewhere. Quakes on high tides is mentioned. If it gets any worse seems to me this is about water and we could have a "landslip" taking place. A ferry may be required to get to the island of Christchurch one day.
LMAO I laughed at Erins comments about the getting drunk part it is like that here in nz southisland I am afraid to say,now do you scientests know whats going to happen next are we gonna expect the big one???!!!??,and do you know when that will be so we can all get out of bed in our pjs and rescue one another....pretty scary thing to be thinking about we know,is it spliting the southisland up with these earthquakes sorry aftershocks what is gonna happen,no scientests you cannot hide now can you tell us more.....we are getting sea sick down here.....or is it the ends times as we all see it???.Have a great day...
Can everyone just stop and think for a minute? We do not get a magnitude 7 earthquake every time there is a king tide. We do not get massive floods every time there is a full moon. These events (Australia floods and Canterbury EQ) happen far less often than the events that Moon Man thinks are causing them. If tides have any effect on disasters such as these you would still need the perfect storm to be brewing already or a fault to be within minutes of reaching the limits of its strength for this to make a difference and that difference is likely to be a matter of minutes not months. Trying to predict them by these methods is entirely useless.
Let's play earthquake comment bingo! Every time someone makes one of the following fallacious claims, take a drink.
- it's the tides - it's the moon - it's the tides AND the moon - why won't smart people open their little minds and see the truth! - earth scientists don't know anything (yet we trust them to come up with codes to help zone or reinforce our buildings against earthquakes, dig tunnels or build dams, study avalanches, and generally do interesting things of national worth) - the scientists know something, but they're not telling us, for no particular reason - maybe they think it's funny? - the University has SOMETHING TO HIDE, why won't those big wigs tell the truth - the Moon Man told us this was coming (yet, noone's banging on his door asking him to pay for their earthquake damage from Sept 4 for not giving them adequate warning) - there is more to life than what science can tell us (but I will keep benefitting from years of modern medicine, engineering, and general R&D thanks very much) - this is all totally abnormal and perhaps you should read xyz's easily dismissable quack website about it
... and now I'm drunk.
@ G#9 Nowhere in this article does it claim that there is only 1 fault! There are faults all over the Canterbury plains, from little to big ones. Earthquakes cannot be predicted in the same way that the order of apples falling from a tree cannot be preidcted. You can have a good idea of where they might fall but that's pretty much it. I thought the University has and still is doing a fantastic job of informing the public. Right from the start they were saying that the pattern of aftershocks was normal and will reduce with time - and it has!
Paul #11 had it pretty much spot on. Why would anyone question what is said by someone who has been trained in the sunbject?
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Can anyone tell me why aftershocks don't occur in the same place ? :)