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.