Monday, March 7, 2011

Why weren't they warned?

Assuming there’s an inquiry into the damage caused by the February 22 Christchurch earthquake, it will no doubt want to consider why no warning appears to have been given, after September 4, of the likelihood of another big shake.

It’s not as if there were no precedents. The famous Napier earthquake of February 3 1931, which registered 7.8 on the Richter scale, was followed only 10 days later by another of 7.3 magnitude. And the 7.2 magnitude quake that struck Masterton on June 24 1942 was followed on August 2 by another that registered 7.0. In both cases, the second jolt was more severe in some localities than the first.

Despite this, I don’t recall any seismologist suggesting Christchurch should brace itself for a follow-up quake that could almost match the September 4 event for intensity. Certainly there were the usual warnings of aftershocks, but the advice was that these would taper off over time.

The February 22 quake caught Christchurch off-guard – but would the city have been so unprepared if people had been reminded of the Napier and Masterton double-whammies?


Jim Donovan said...

I think you may be mistaken. I recall several warnings to that effect, and that it might happen months after the initial event- but agree that we all assumed that their likelihood tapers off.

Will de Cleene said...

I also recall several warnings after the Darfield shock. I dug up one of them:

An aftershock of magnitude six was still possible, but Quigley was optimistic it might not occur.

"Significant aftershocks, such as the ones we have been feeling ... are likely to continue for weeks to months.

"I know this is distressing a lot of you, but I am hopeful that these aftershocks might be the biggest we get. However, they may not be, and we could get a bigger one months from now."

Karl du Fresne said...

Yes, general warnings were given of aftershocks, but it's my impression Christchurch wasn't prepared for another quake of that magnitude.

Brendan Pfahlert said...

I recall there being a warning that often a large earth quake of magnitude x would be followed by one of x-1.

And another warning that an earthquake of magnitude x could be followed by 10x-1's, 100x-2's, 1000x-3's etc.

Phil said...

Karl, you are definitely wrong - there were many warnings, as time went on there was a hope that it wouldn't happen.

see from 7th September

"There is however, 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," said Smith.