22/2/11 Earthquake M 6.3 - Christchurch

[Manukau heads obs]

the message there is that you are likely to get another 5 richter scale aftershock some time in the next 12 months
but much less likely that will be over 6

[Storm Struck]

Yeah i still believe that percentage is higher unfortunatly, even 23% is a high percentage and i think there some things that will not get passed onto the public and i believe thats a very valuable comment.
Theres something to be said about aftershocks which go on for up to 8 months and are all situated on different faults and areas.
But thats just my input even if it sounds over the top

[Richard]

And i recon if a descant sized quake is going to happen any time soon it will happen near Lees valley going by the amount of shakes thats taken place near there over the last week or so.

[Storm Struck]

Simular article on the front page of the press today, interesting to note the 25% chance of a 6.0 plus was observed at the start of February for the next year.
So this makes 23% chance of a 6.0-7.0 in the next year still sound like a strong figure.

http://nz.news.yahoo.com/a/-/top-storie ... ive-quake/

[CHCH Weather Chaser]

Below is an email reply I had from Hamish Bennett from GNS Science about some questions I asked him. His words and answers are mentioned in bold.

Hi Hamish

I have a couple of questions that I would really appreciate you answering for me in relation to earthquakes and Christchurch. I ask this on behalf on many concerned people.

It is said that Christchurch experienced one of the worlds greatest peak ground acceleration.

Yes, this is correct. At least one of GeoNet's Strong Motion Instruments recorded a vertical ground acceleration of 2.2g (2.2 times that of gravity).
However, it is important to remember that intensity drops off dramatically with distance so the most intense effects were very localised.
To put this in perspective, some residents in NW Christchurch were not aware of the Feb 22 event.

1) Does this mean Christchurch experienced the worlds biggest earthquake in terms of its 'feels like intensity'?

Maybe. As suggested above, t all depends on how you define 'Christchurch'.
You could say that a (small) part of Christchurch, located between Hallswell and Sumner, experienced some of the most intense ground shaking ever recorded in a city environment.

2) In relation to the alpine fault, I hear that it could be as low as 7.6 and could be as high as 9. Due to the longest time between known alpine events, does this mean it will be greater than the 8mags recorded in the past? (What is your gut estimate?)

I cannot answer this in a meaningful way because we simply do not have any direct experience of a major shunt on the Alpine Fault.
There may or may not be a direct relationship between magnitude of an earthquake on any one particular fault, and its recurrence time.

3) How will Christchurch withstand a mag 8+ earthquake due to its poor ground underneath? (this is a big concern of mine)

How Christchurch will behave in a magnitude 8 earthquake all depends on where the epicentre is in relation to Christchurch.
I would say that the only prospect of an earthquake of this magnitude proximal to Christchurch, would be either on the Alpine Fault system, or on the plate boundary interface somewhere between Kaikoura and East Cape.
Because Christchurch is reasonably well removed from these active tectonic entities, I think that Christchurch will be okay. You will fell it of course.
There will be ground shaking and low lying ares (eastern Christchurch in particular) may be subject to liquefaction effects comparable with what you have witnessed since September last year.
There will also be rock fall and cliff collapse etc. So, more of the same but quite possibly much less severe than what the 6.3 Feb 22 event caused right in the middlev of the city.

4) Will the distance of 90+km significantly weaken the earthquake and how do you think it will compare to the 7.1 or 6.3 in terms of its Mercalli in Christchurch?

Yes, I do. Distance from the4 picentre is all critical; it matters greatly.

5) Is christchurch at risk of Tsunami from the alpine fault and if so how far could it get into the city? (I live in Halswell)

No, I doubt it very much. Strictly speaking the Alpine Fault is on land and in order to generate a tsunami, the sea floor needs to be offset hence causing massive displacement of the water mass.

You may mean the plate interface that exists NE of the Hope Fault, offshore of Kaikoura and extending all the way up off the coast of East Cape. This is of concern in that we do not think that there we have ever recorded a major earthquake on the actual plate interface (which is subhorizontal). All recorded earthquakes (i.e over the past 80 years or so) associated with the plate boundary relate to break-up of either the over-riding plate or the down-going (subducting) plate. The magnitude 9 earthquake off Japan on March 11 was a plate interface event. We think that such an event is possible off the east coast of New Zealand, but they are rare events, occurring every 2,000-20,000 years, and as I say, we have no satisfactory record of an.....but check out Bruce McFagen's book: 'Hostile Shores' in which he identifies several major tsunami in pre-European times (with a major event notably in the late 1400s in particular) in NZ from his considerable knowledge and research of the NZ archeological record. So, there is a possibility of a major tsunami associated with such an event, and yes, we would expect coastal Canterbury to be adversely affected. You will have seen the TV footage of the Tohoku Earthquake. Tsunami travelled well inland in many places along the east coast of N Japan. The nature of tsunami impact on Christchuch all depends on how big the tsunami is, how close it is etc. So, it is very hard to paint a meaningful picture for you.

I sleep well here in Wellington because I am c. 150 metres above sea-level, in Ngaio, where the hill slopes show no signs of lansliding.
My house is built on solid rock, and is located some 2.9 km away from the Wellington Fault and 2.3 km from the Ohariu Fault.

[Orion]

Most interesting. Glad you asked the questions. A fascinating part is the mention that some residents in NW Christchurch were not aware of the February 22 event: extraordinary!

[Storm Struck]

Interesting reading Leighton very good questions too, nice to see them taking the time to answer questions properly unlike that of what is answered on Geo Net's facebook page.

[snowstormwatcher]

A fascinating part is the mention that some residents in NW Christchurch were not aware of the February 22 event: extraordinary!

I know someone who lives in Avonhead who said The shaking didnt feel all that strong there during feb 22nd and there was little damage. Amazing how one part of the city can be so badly damaged yet other parts hardly damaged.

[CHCH Weather Chaser]

A fascinating part is the mention that some residents in NW Christchurch were not aware of the February 22 event: extraordinary!

I know someone who lives in Avonhead who said The shaking didnt feel all that strong there during feb 22nd and there was little damage. Amazing how one part of the city can be so badly damaged yet other parts hardly damaged.

Yes that the most amazing thing about it.(the bit about the NW CHCH) I was in the CBD at the Convention Centre at the time and the shaking was so intense I cant believe it stayed standing. Family in Westport said their kids were made to go under desks there..

[Richard]

And me as well,i find that interesting that some ares of CHCH were not aware of the February 22, i'm at 120km away and it was still was very noticeable,even some vineyard workers i know who were sitting down outside having lunch recon it was still quite strong shake.

[Razor]

Work colleagues in Riccarton felt it but didn;t think it was big. Two kms away in Cashell St we thought the world was ending. So yes, some staggering local differences to that quake

[Storm Struck]

Although i wasnt here for the february quake, it wasnt as a bad in kaiapoi as september's quake.
There was only a little bit of liquifaction in the same areas, and abit more fell from Blackwells other than that nothing else oh and the channeys onramp to the northern motorway suffered damage along with the bridge it runs under.
But thats because they did a patch job and didnt bother fixing the land from septembers quake.
I was expecting the likes of the Kaiapoi Tavern Pub to be collapsed after all it was sinking and on a big lean.
We had very little damage inside the house i think thats because we had blue tacked everything down etc, but despite that the shelves in the loungue still flew out from the cabinet and ended up with some broken things.
Outside we had new cracks in the driveway all over the place and in the footpaths.
Ive found the jolts that come from the Greendale are more stronger for us here than those from the city or Port Hills, just depends the angle the ground acceleration travels at i guess and the obstacles such as rivers slow down ground movement.

[Razor]

Simular article on the front page of the press today, interesting to note the 25% chance of a 6.0 plus was observed at the start of February for the next year.
So this makes 23% chance of a 6.0-7.0 in the next year still sound like a strong figure.

http://nz.news.yahoo.com/a/-/top-storie ... ive-quake/

Lazy journalism and sensationlist bullshit, really really poor of the media not to actually read what GNS have said.

And I quote" These longer range forecasts are where the figures in todays news came from. And as far as today's news, the figures for Christchurch metropolitan area are roughly one quarter of the figures given to CERA. So 23% probability for Canterbury becomes 6% probability for Christchurch.

In rough terms, it means the quake probability for Christchurch has become similar to many other parts of NZ such as Wellington, Hawke's Bay, Wanganui, and Poverty Bay where quakes are more frequent"

Full publication here:
http://geonet-shakennotstirred.blogspot.com/

[Brendon]

New faultlines found in Canterbury


Several new active faultlines have been discovered in Canterbury - including one under Barbadoes St and one out to sea.

Scientists, speaking at a press conference in Christchurch this afternoon, said survey work in the region had uncovered new faultlines.

One fault is at sea and is considered the greatest risk for a magnitude-6.0 to 7.0 earthquake. However, it is not considered a tsunami risk.

The other fault is under Barbadoes St in central Christchurch and may have given the Boxing Day quake a home.

However, the GNS Science research confirms that the Canterbury region has a low fault risk in the long term, with the September and February quakes considered "very rare" occurrences.

The current risk from another large quake in Christchurch is similar to the risk of a quake striking Wellington.

GNS natural hazards research platform manager Kelvin Berryman said it was widely known before the latest investigation that there were several dozen active faults within an 80-kilometre radius of Christchurch.

However, preliminary studies of Canterbury's two major quakes confirmed there was a complex arrangement of faults in the bedrock under Canterbury and Pegasus Bay.

An offshore seismic survey of southern Pegasus Bay revealed a previously unknown offshore fault about 25km long - similar in length to the Greendale Fault, meaning it could potentially generate an earthquake of magnitude 6.0 to 7.0.

"For an earthquake of magnitude 6 or greater to be generated, a fault of 10km or more in length is needed," Berryman said.

Most of the faultlines in Canterbury had experienced their major period of movement more than 50 million years ago, he said.

National Institute of Water & Atmospheric Research principal scientist Philip Barnes said younger active faults were reactivating the older faults.

"However, at present there is no strong evidence that aftershocks are migrating offshore on to any of the fault structures in Pegasus Bay," he said.

University of Canterbury Professor Jarg Pettinga, who led the onshore seismic surveys, said the data had revealed several previously unknown fault structures around Christchurch and Canterbury, most of which ran northeast to southwest.

Scientists may have identified the bedrock structures on which the Boxing Day and February 22 quakes occurred.

"They appear to be very old faults in the basement rock that may have been weakly reactivated,'' he said.

"Cantabrians live in a seismically active area and finding previously unknown faults that seem to have very long intervals between ruptures won't have significantly increased the level of earthquake hazard in the region."

http://www.stuff.co.nz/national/christc ... Canterbury

[Orion]

The telling sentence is the concluding one, I think.

[NZ Thunderstorm Soc]

Nice little earthquake at 1107am this morning. First one felt here for ages, I think, since the 5.3M near Rolleston.
No rumble but the French doors rattled

JohnGaul
NZThS

[Storm Struck]

Got the same here john, just a quick short sharp jolt no rumble and was a 4.1 at rolleston.
I felt the 3.6 on wednesday morning though, in abit of a quiet phase at the moment which can be abit concerning i dont think its tapering off just yet still think we have a long way to go yet.

[Brassnz]

Very interesting email from Hamish Bennett (31/5). Thank you for posting.

I was able to visit my old stamping ground in Chch last weekend, for the first time since the Feb 'quake. Was amazed at the damage but in some ways even more so at the work that's been done in the cleanup. If media reports were taken at face value, the impression from up here is that very little has been done since 22/2 and that the city is in a holding pattern. This could not be further from the truth (notwithstanding some notable examples) and a huge amount of work has been done. I was fortunate to spend a couple of hours seeing the after effects first-hand, and it was an afternoon of very mixed emotions; sadness on many levels, and pride.

Mostly pride.

[CHCH Weather Chaser]

Just had a big aftershock here that lasted around 15-20seconds or so. Guessing at least a 5.0. Had to hold up TV and surround system. Some new liquefaction reports also.

[CHCH Weather Chaser]

5.5 in Rolleston. Only 8km away from that one!

[Richard]

Sent our place rocking too,rang sister inlaw in Philpstown and she reckoned it felt like a 4.5,i said no way its got to be a 5+ before we feel them.

[CHCH Weather Chaser]

Sent our place rocking too,rang sister inlaw in Philpstown and she reckoned it felt like a 4.5,i said no way its got to be a 5+ before we feel them.

Yeah you usually know by the length that is a decent one above 5 if its lasting more than 12-15seconds. I probably should have gone under a door but its common place to just stand and ride it out now...not a good habit.

[Michael]

Hopefully it was deep so not much damage......They said on the radio a few days ago be another chance of more.

[CHCH Weather Chaser]

Yeah it fell into the 90% bracket. It came as no surprise to me. I accept that there will be 5's every now and then. I would not be so forgiving for a 6+ though! 3 strikes and many people here I dont think will cope well at all....to go through a new aftershock sequence would be pure disaster.

[Dale]

Yeah, just this one alone would have rattled a few more nerves.. felt it clearly over here in the Grey, quite different to previous aftershocks, it felt to rotate around X & Y axis. Very odd. Has to be a mimimum of a mag 5 for me to feel it, went on for quite some time too.