Guess What Santa brought us

well, not only to us but to all Christchurch residents.

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[xx(]

A very strong 4.9 aftershock, centered in Christchurch.

This big one happened at 10.30 am. We were on the roof top car park of Riccarton mall :eek: and it was a nasty big shake. The whole deck was swaying, the cars were hopping and swaying, the light poles were swaying and we were trying to remain standing. Riccartron Mall is only a hand full kms from the epicentre. Our son was inside the mall and people were screaming, glass was breaking. We were leaving (we had finished our shopping) and people were outside the shops, including some employees. the traffic lights were out, too.

During the past night we were awoken 3 times by aftershocks, 4.2 was the strongest one, all three centered under Christchurch Hospital.

Must be un-nerving so many people, these endless aftershocks. releived to hear on the news reports no injuries, take care.

I've just been reading about this - it must be very un-nerving as GK said

It was noticeable, but quite gentle, this far out - the 4.9er was under Opawa. The cluster of aftershocks all near the centre is worrying me because my older two boys are staying with their dad in the city centre and he's on the first floor of an old brick building. He said quite a few cracks had appeared in the downstairs flat when I spoke to him earlier, so I'll be a bit nervous until my boys come home in a couple of days.

Hi Seaside,

I can very well understand your worries and I hope your boys will return safely. I've been to a couple of talks about these earthquakes, held by geologists, and the only thing they are certain about is that this local fault line has spent its energy which means the 7.1 is the strongest tremor it was able to deliver. For a fault line of this length (27 km in total) a 7.1 quake is VERY strong, right at the upper end of what fault lines of this size can produce.
The aftershocks we are getting are 'adjustments' at either end of the fault line that ruptured on September 4th. I've had enough of these 'adjustments', I had calmed down and now everything has been stirred up again, especially because we've been in such a precarious place when the 4.9 happened. The 4.4 during this past night woke me up again.

Was sorry to hear you guys getting this again...my friend who lives in central city ChCh texted yesterday to say they have upped sticks and headed to Nelson for a week in the hope that they can get some more sleep, her two boys are probably wound up again and no doubt tired and fractious after months of demo work on that building opposite them that was in the news.

She said that the cluster was around the central city area and felt very strong. The office building next door that was up for repairs after the first big quake has now been cleared out while the business can still get access, and is very damaged.

Kia kaha...

The 4.9 was centred only a few blocks from here and we have suffered more damage, but no "new" damage as such. The double brick wall is further cracked and separated from the house but still hasn't collapsed, there are more cracks in plaster inside & out etc, but only a worsening of what was already there.

I can't help but be suspicious though, that almost all the latest shakes are directly under the city - does anyone else remember geologists suggesting the possibility of a separate faultline under the city shortly after the 7.1 or am I imagining things?

Jenna;351415 wrote:
I can't help but be suspicious though, that almost all the latest shakes are directly under the city - does anyone else remember geologists suggesting the possibility of a separate faultline under the city shortly after the 7.1 or am I imagining things?

No, you are not imagining things. Geologists now think that the Greendale Fault continues under Christchurch City but as it has not yet broken through the surface the exact location is still not known. Geologists now even ask themselves whether the fault continues under the sea as there was one small quake centred in Pegasus Bay.

The 7.1 quake ruptured the surface on September 4th. The following Wednesday there was a 5.6 aftershock centred under Lyttelton. Although the surface wasn't broken the land has moved - has been dislocated. The land between the Greendale Fault and the zone around Lyttelton hasn't moved as much as either of the adjacent zones so I assume that the Boxing Day aftershocks are filling in that gap.

It is amazing what tools geologists have available today. They can compare sattellite scans of the surface after the quake with scans of the same area before the quake. They make maps that show a colour coded image of how much the land has moved. Around Lyttelton the area on the map I have seen was vividly coloured, the area around Norwood - Greendale - Burnham was coloured even more vividly and around Halswell and Christchurch City there was hardly any colour. (This doesn't mean that this area wasn't shaken - it only means that there was no or only very little displacement.)

I can't help wondering what this is doing to those great Aquifers under Canterbury.

Hey guys - just know you are very much in our thoughts. I was watching it on the news last night and just so relieved that it has not happened up here but know that this feeling of personal relief is nothing compared to the heartfelt concern for all Christchurch people who have had so much to go through for so long and it is not over yet seemingly.

Thanks Simkin. I keep reminding myself that the chances of another really big one are slim (for now) and the building stood up to the original quake - though the building next door (on Worcester Boulevard) lost its wall and was demolished the weekend after the main quake. My boys were staying with their dad at the time and were evacuated at 4am because someone had set fire to the empty building. All very dramatic, but the boys are taking it in their stride - they find it all quite exciting if anything. And their dad said the cracks were only "cosmetic".

Hasbeen, I'm sure I read that Ecan is keeping an eye on the aquifers because of course the underground dynamics can easily change when the earth moves! Will be interesting to see what the effect has been. I know first hand that the movement has sent up a lot of silt, we've had to remove it from our bore filter and bore pump (in fact, our pump got munted).

The geologist professors also told their audience that in the event of a magnitude 8 quake at the Alpine fault the shaking we'll experience here in Christchurch will not be stronger than what we experienced during the September quake. However, high rise buildings will be more severely affected than single- and two-level buildings (which were most affected during the September quake) as the shaking frequency will be lower.

Earthquakes generate both low and high frequency shaking, but the lower frequency shaking doesn't dissipate as much through the ground, so is felt more strongly at a distance. This is one reason a more distant quake feels like more of a sway than a shudder.

Buildings all have a natural resonance frequency, and generally, high rise buildings resonate at lower frequencies than medium rise and low rise. The number of floors is significant. Usually buildings say from 5-13 floors are similarly affected, and 13-25 etc. So not all high rises are equally affected by the same quake. The building material also has a significant effect, but so does the type of soil. Engineers try to design buildings that will not resonate at likely frequencies. (like the infamous Tacoma Narrows Bridge )

The worst case would be:
a large earthquake occurring at a particular distance from a built up area that means most shakes there are at a particular frequency
and the soil type resonates at that frequency,
and there are local areas of rock that reflects waves backgiving the doubling of amplitude
and the waves are at the resonant frequency of the building.

This occured for some buildings in Mexico City in 1985 and in San Francisco in 1989.