[Geysers] New Zealand

[Ron Keam]

Further to my earlier e-mail about the Christchurch earthquake:

Ron Keam

	Why Was New Zealand's Latest Earthquake So Deadly?

			Christchurch copes with a tragedy it did not see coming
Scientific American
By <http://www.scientificamerican.com/author.cfm?id=1324>Larry Greenemeier  
February 22, 
2011 <http://www.scientificamerican.com/article.cfm?id=new-zealand-earthquake-christchurch#comments>2


STILL STANDING: Christchurch's Pico Wholefood building following this 
week's aftershock. Image: COURTESY OF SCHWEDE66, VIA WIKIMEDIA COMMONS

New Zealanders living in the nation's second-largest city, 
 <http://en.wikipedia.org/wiki/Christchurch%2C%20New%20Zealand>Christchurch 
(population approximately 377,000) on the South Island's 
 <http://en.wikipedia.org/wiki/Canterbury%20Plains>Canterbury Plains 
were 
<http://www.scientificamerican.com/article.cfm?id=new-zealand-quake-kills-at-lea>hit 
hard Tuesday by magnitude 6.3 earthquake, an aftershock from 
September's magnitude 7.1 tremor.  Prior to these two seismic events, 
Canterbury Plains likely had not experienced an earthquake in 
thousands of years.  In fact, scientists did not even know there was 
a geologic fault there until it ruptured last year.

The latest Christchurch tremors were not as strong as the original 
earthquake, but they have caused considerably more damage and claimed 
dozens of lives.  (No one died during the September quake).  The 
Christchurch epicenter was only 10 kilometers outside of the city, 
whereas the 2010 event took place about 40 kilometers to the west, in 
an area that is mostly farmland.  Adding to Christchurch's 
misfortune, the aftershock struck only about four kilometers in depth 
below the city, whereas September's temblor originated about 10 
kilometers deep.  Compounding these problems, Tuesday's quake hit 
during lunchtime when the city was buzzing with activity, whereas the 
earlier disaster occurred during early morning hours.

Scientific American spoke with 
<http://oregonstate.edu/marketing/robert-yeats>Robert Yeats, a 
professor emeritus of geology at Oregon State University in 
Corvallis, about why earthquakes are so difficult to predict and what 
is being done to lessen the odds of surprise temblors.

[An edited transcript of the interview follows.]

The 7.1-magnitude earthquake in September caught the locals 
completely off guard. Why was that earthquake as well as Tuesday's 
aftershock such a surprise?
The earthquakes struck an area of New Zealand's South Island where 
sediments are deposited from the Southern Alps and from the nearby 
rivers. The sediment deposits reach all the way to the east coast on 
what is called the Canterbury Plains. The fault that ruptured in 
September had not done so in thousands of years, during which 
sediments had been deposited on top. I've been out there, and it's 
like driving anyplace where it's all flat. There are farms, but 
there's nothing that says, "Here's a fault." So, when the earthquake 
struck in September they were totally surprised. On the South Island, 
the  <http://en.wikipedia.org/wiki/Hope%20Fault>Hope Fault and 
 <http://en.wikipedia.org/wiki/Marlborough%20Fault%20System>Marlborough 
Fault System are better known-there had been 
 <http://en.wikipedia.org/wiki/1888%20North%20Canterbury%20earthquake>an 
earthquake in 1888 along the Hope Fault. Much less was known about 
faulting on the Canterbury Plains because no earthquake had happened 
in their historical record.

What defines an "aftershock" as opposed to an earthquake? Does a 
certain amount of time have to elapse for a seismic event to be 
considered an earthquake?
It takes many years before seismic activity can be considered an 
earthquake rather than an aftershock of a previous earthquake. That's 
a point of debate among seismologists. If you look at a map of 
southern California, you'll see quite a few little earthquakes south 
of Bakersfield in the San Joaquin Valley. Some people regard those as 
still 
 <http://en.wikipedia.org/wiki/Bakersfield%2C%20California>aftershocks 
from the earthquake of 1952, which measured 7.3. That's not the 
consensus but it indicates that these aftershocks go on for decades. 
It takes quite a long time for everything to become quiet again. Now 
the San Andreas, on the other hand, had a large 
<http://earthquake.usgs.gov/earthquakes/states/events/1857_01_09.php>earthquake 
in 1857, just west of San Joaquin Valley, and it's quiet as could be. 
Same with the area of the 
 <http://en.wikipedia.org/wiki/1906%20San%20Francisco%20earthquake>1906 
San Francisco earthquake along the San Andreas. You can't paint all 
aftershock series with the same brush.

You mentioned that the aftershock was "shallow". What is the 
difference between an earthquake that takes place four kilometers 
below ground and one that takes place 10 kilometers below the 
surface, as the September earthquake did?
It's like how close you are to a bomb going off. If you're within a 
couple of kilometers, you're likely to get injured. If you're three 
or four times that distance, you not likely to. The waves are 
attenuating, or propagating, toward the surface. Christchurch was a 
very shallow earthquake, and that's a reason why the damage was much 
worse than the earlier one.

What do you look for when you investigate seismic activity in a 
particular area?
If I study a particular fault, I like to know its 
<http://www.data.scec.org/Module/sec1pg17.html>slip rate, how fast 
it's moving, whether it's a millimeter per year or a centimeter per 
year. As plates move, they're building up strain, and I estimate how 
much strain can build up before there is a rupture. New Zealanders 
have been good about 
<http://www.geo.arizona.edu/%7Emsarca/Trenching.html>trenching faults 
(digging trenches along fault lines to study previous seismic 
activity). The problem with this fault was that they didn't even know 
it was there. That tells me it's a pretty slow-moving fault but, 
nonetheless, when it builds up toward an earthquake of magnitude 7, 
then that's going to continue to produce aftershocks for a long time. 
It's not an exact science.

I'm working with the New Zealand Institute of Geological and Nuclear 
Sciences on a project funded by the 
 <http://en.wikipedia.org/wiki/Global%20Earthquake%20Model>Global 
Earthquake Model to map all of the active faults on Earth. This 
<http://www.globalquakemodel.org/hazard-global-components/active-faults>Global 
Faulted Earth project will include a global active fault and seismic 
source database, along with a book I'm writing. At some point in the 
not too distant future, if you hear about an earthquake in a place 
like Christchurch, you will be able to click on this database to find 
out what is known about it.

What can be learned from this week's aftershock in New Zealand?
We can map faults, and that's what we do, and we can use what we 
learn about those faults to establish some probability of an 
earthquake happening-but you can't map all faults. In the case of 
Christchurch, I'm not sure what they could have done differently. 
They could have said, let's do a seismic survey of the whole 
Christchurch metropolitan area just to be sure there's nothing going 
on underneath the city.  But it was unlikely for the faults to extend 
as far south from the original Darfield earthquake site at as they 
did, so I can't fault them for not doing that.

You have to realize that New Zealand has some of the strongest 
building codes in the world, and those building codes are respected. 
That means you have loss of life, but it's in the dozens or maybe 100 
or 200. If the same earthquake were to happen under a city of that 
size in a developing country, the number of deaths would be in the 
thousands, if not tens of thousands. Turkey, for example, had great 
building codes but that didn't keep tens of thousands of people from 
getting killed in the 
<http://en.wikipedia.org/wiki/1999%20%C4%B0zmit%20earthquake>1999 
Izmit 7.6-magnitude earthquake because they weren't paying attention 
to those codes.


>Hoping this doesn't sound inappropriately callous, but are our friends 
>like Ron Keam down under doing OK? And (this was the really callous
>question), has this second big quake nuked the thermal features? Or
>might we see Waimangu sort out its plumbing system and erupt again? 
>Hearing from lots of people (fortunately, most are OK) but was hoping 
>the lack of info on the NZ thermal features doesn't mean our geyser 
>friends (carbon and silicon-based) have been harmed.
>Karen Webb
>_______________________________________________
>Geysers mailing list
>Geysers at lists.wallawalla.edu
>

-- 


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Ron Keam
The Physics Department
The University of Auckland
Private Bag 92-019
Auckland
New Zealand
Phone +64 9 373-7599 extension 87931
FAX +64 9 373-7445
EMail r.keam at auckland.ac.nz
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