Further to my earlier post on the apparent failure to warn Christchurch of the possibility of a further quake almost equal in intensity to the one of September 4:
On September 10, GNS issued a statement headlined: Textbook aftershock sequence, seismologist says. It quoted GNS seismologist Warwick Smith as saying the number and size of aftershocks at that point, six days after the 7.1 magnitude quake, was in line with expectations. The frequency was already declining rapidly but they were likely to continue for some time yet.
“What we are seeing in Canterbury is pretty much a textbook aftershock sequence. They won’t get smaller in a hurry, but they are already getting much less frequent.” (Christchurch residents, having endured thousands of aftershocks in the following months, might quibble with that assessment.)
Dr Smith did say there was still a possibility of an aftershock larger than those experienced so far, but the chances of this happening were decreasing by the day.
In his celebrated interview last week with “moon man” Ken Ring, TV3’s John Campbell referred to a statement by GNS back in September that mentioned the possibility of a magnitude 6 aftershock. A reader named Phil, who commented on my post below, mentions a similar statement, made by the same Warwick Smith to TVNZ on September 7, in which Dr Smith said: “There is … kind of a rough rule that the biggest aftershock is something like one magnitude unit less than the main shock. So we could be looking at a magnitude six I’m afraid.”
So where does this leave us? I suggested in my earlier post that the big aftershocks that followed the Napier and Masterton earthquakes of 1931 and 1942 should have alerted experts to the possibility of another upheaval almost equal in intensity to the September 4 event.
To be fair, the 7.3 Napier aftershock happened only 10 days after the primary quake, while the 7.0 Masterton aftershock followed the main event by five weeks. In the case of Christchurch, the time difference was far greater: five and a half months. So it was possibly well outside the period when previous experience suggested we could expect a severe aftershock.
Still, you have to wonder whether the official statement from GNS, which said aftershocks would continue but emphasised that they were likely to decline, was both premature and a little too reassuring in its tone. Even Dr Smith’s statement about the possibility of a magnitude 6 aftershock was buried well down in the TVNZ story – admittedly not Dr Smith’s fault, but I still can’t help thinking the experts could have been more forthright in warning people of the risk. It almost looks as if they were playing it down while still covering themselves by not entirely ruling it out.
I followed the media closely in the weeks following the September 4 quake and can recall no statements that would have braced Christchurch residents for what they experienced on February 22. Perhaps the authorities didn’t want people panicking unduly.
I’d be interested in the views of Christchurch people. Did they feel the official warnings were adequate? Or did they prefer not to know what was theoretically possible, realising the result could have been complete paralysis?
I guess what it all adds up to is that seismology remains an inexact science, that events don’t necessarily comply with the textbook, and that with each major quake scientific understanding advances just a little bit further. At least one hopes so.
It's important to acknowledge that it's easy to be wise after the event. Nonetheless, I still think the inquiry will be justified in asking why no more explicit warnings were given about the possibility of a really severe event – rather than just an unnerving one – following September 4.
7 comments:
Hi,
I live in CHCH and this, I think, is a fair representations of my circle of acquaintances view.
We were always told that a 6.x was probably, that was the 'normal' pattern, and as time went by we were told the probability was decreasing.
We were amazed that we walked away from September with no deaths thanking our luck that it occurred at 4am and not 4pm.
So I would be amazed to hear of any Cantabrian that was surprised that the 2nd big one came, although we were all surprised to hear it was only 6.3 as it did so much damage.
(as an aside, we are becoming quite adept at guessing the size of the aftershock, if you want to know the size before geonet publishes then follow twitter http://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=10710367)
To be honest I am not sure what else could have been said/done.
Life is full of risk, expecting to fully eliminate all risk is delusional.
Outside the CBD the areas affected are different to those in September.
So without abandoning the whole city I think reasonable, practical, PROPORTIONAL steps were taken to mitigate the risks, or steps were under way.
If you chose to live in NZ then you have to accept that it is part of the Pacific Circle of Fire.
That said, the deaths were predominantly due to falling masonry from heritage buildings, perhaps a more vigorous pre-emptive demolition program could have been effected - that was certainly discussed locally without widespread support.
The loss of heritage buildings was considered too big a 'social' cost compared with their risk. Of course now the risk is further reduced there is now talk of ripping it all down :-(
The other big killer was the collapse of CTV and PGG, both of which were judged to be safe. Perhaps there will be an enquiry to figure out what went wrong there. I'm sure that it will find that no reasonable person seriously expected them to fall in another quake.
Following the jist of your article I pose the following question - the experts know there will be a big one in Wellington, they know it is imminent in geological terms, but can't predict exactly when - why isn't Wellington being relocated somewhere 'safer'?
On a separate note most think that (I refuse to mention his name) is bonkers and think it is irresponsible to give him and his 'predictions' the oxygen of publicity. For many people these are desperate times and he is 'preying' on their fears, scaring the shit out of them.
Thank you Phil.
There's a small problem with your premise.
The Feb' 22 quake was not an aftershock. It was a new quake on a completely different faultline.
Are you suggesting it was totally unrelated to the September 4 event? Hmmm ....
The direct response to your initial questions Karl, and please note I am NOT a CHchian...
"Did they feel the official warnings were adequate?"
YES!!
"Or did they prefer not to know what was theoretically possible, realising the result could have been complete paralysis?"
The implied is "No", but then the "they" creeps in ... only joking.
I have in the back of my mind, someone "in the news" after the Sept earthquake saying that after-shocks could continue for years.
You said the truth - we really do not know beyond making (hopefully) educated guesses.
R-inanity. Interesting thought, but remember that fault lines are not two dimensional like a line on paper. They are in fact three dimensional, and the plane of the fault can be bent, twisted, curved... As good as the Chch quake map site is, it is a pity that no one (to my knowledge) has plotted from September in those three dimensions. The data is there, but it is beyond my immediate capabilities. What an opportunity!
Obviously there are different perceptions in Christchurch as to the adequacy of the warnings following September 4. But in support of my contention that people didn't expect a follow-up shake as big as that of February 22, I note that Bob Parker was reported yesterday (Radio NZ) as warning against complacency over the prospect of yet more quakes, saying Christchurch didn't want to get taken by surprise again. I interpret that as meaning Parker thinks the warnings were not adequate.
Fair enough.
I thought Parker's comments were a warning alright - against repeating the level of complacency that had set in before the March biggie.
That does not indicate the warnings were inadequate. One might argue with equakl validity that there were too many warnings and people were ignoring them.
Fact is that people did get complacent. That is understandable given that events were following the pattern of "slowly reducing over time".
It highlights the inadequacy of our science more than that of the warnings.
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