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Saturday, September 18, 2010

Earthquakes: Motion, Poverty, and Population make a Difference

In Earthquakes, Poverty, Population and Motion Matter –

There are plenty of reasons damage and deaths from the 7.1-magnitude earthquake that struck near Christchurch, New Zealand, on Saturday utterly paled compared to the absolute devastation wrought by the 7.0-magnitude quake near Port-au-Prince, Haiti. I sent a query about the different outcomes in Christchuch, Haiti (and Chile) to a half dozen engineers and geologists working on earthquake preparedness and design and you can read some of their observations below.

Here are some of the main points:

- Not all earthquakes of the same magnitude have the same destructive force. Ground motion is a critical factor. Check the two charts below for a comparison of the motion of the earth in New Zealand and Haiti. No competition.

- Communities and countries that have frequent low-level seismic activity (New Zealand, Japan, California) tend to have better construction standards and preparation than those where quakes are devastating, but rare (Haiti, the Pacific Northwest).

Below you can compare tables showing the intensity of ground motion in communities around the epicenters in New Zealand and Haiti. Click here for descriptions of the rankings. Basically red and orange are very bad. (Santiago Pujol of Purdue University sent the charts, which are from the U.S. Geological Survey.) Also look at the populations in the two quake's danger zones.

New Zealand:

Here's are several reactions from experts in earthquake risk, starting with John Mander, a Christchurch native teaching at Texas A&M University.

John Mander:

Christchurch is my home town. I received my PhD in seismic design in the 1984 from the University of Canterbury in Christchurch, NZ. I am a structural engineering professor and conduct research in earthquake engineering and other hazards.

Of any place in the world, this would probably be the best prepared. The City council many years ago in the 1970s started upgrading programs for earthquake prone buildings. In spite of that there is a shocking amount of damage, but that can generally always be attibuted to historic non-engineered construction. That is construction built by convention, was conforming to building codes of the day, but these have historically not had any earthquake restance provisions. There are reports of unliveable modern houses in a seaside suberb, Bexley. I suspect this is attibuted to the foundations and specifically liquefaction and ground failure.

I am not so familiar with the recent Chile event, but compared to Haiti, where all the construction was heavy non-engineered systems that fared somwhat better.

The lesson in all of these events is pretty much the same:

- Well engineered structures are less damage prone, but it should be noted they are NOT earthquake proof! Engineers will now take some time to inspect the high rise buildings, as they may be hiding signs of damage behind their ceilings or facades. Of particular concern will be beam-column joints. It is too soon to tell whether some of these will need repairs, even though they may be fine. Housing that consists of unreinforced masonry, particularly bricks, and any other heavy materials are more damage prone than engineered structures.

- The type of construction that has evidently suffered the most is the older commecial structures that fit between the above two categories, specifically two-storey shops that have a brick frontage, and either wood-frame brick veneer or sometimes all-brick back. The front generally has not been well tied to the back the main structure, that is why it simply peels off during shaking. Many of these structures were built in the late 1800's and up to the depression. Commonly there was retail downstairs and living upstairs. Because many of these failed were within the CBD, they are now rarely used as living accommodation. It was fortunate that the earthquake did not occur on a weekday, as it would have been more likely to get casualties. Building styles changed after WW2 due to the ned for more rapid construction, the high cost of bricks, and of course the bad experiences of brick buildings in the 1931 Napier Earthquake.

- When there are signs of large ground movement, fissures, surface water, there is a high chance the sub-surface utilities, such as water and sewerage piping may have sustained breakages. Christchurch decommissioned its old town [coal] gas several decades ago, that is why it is unlikely there would be any fire spread.

Peter Yanev:

Construction, in general, in Christchurch is very similar to California construction. Much residential wood frame. The new buildings have good quake designs, unlike absolutely no design in Haiti. The older buildings are wood frame (pretty safe) and some are, just like in California, unreinforced brick. The media is showing mostly partial collapses to those kinds of buildings. Nothing surprising there. However, at first impression, the ground motion in the city is significantly weaker than the worse parts of Port-au-Prince. That is primarily due to the distance of the faulting (weaker ground motion). Therefore one should expect much less damage proportionally on both accounts – further away from the very strong motion and better construction.

I have a feeling that as we get the strong motion records, we will see that the Haiti quake was much worse because of stronger ground motion where the people were and much weaker construction (i.e. no earthquake resistant design and generally poorer quality of construction and construction materials – no wood).

Kit Miyamoto:

We found that New Zealand was prepared for this earthquake by engineering capacity, code, and emergency response. It is definitely advantageous to have frequent earthquakes. It makes the society aware of the risk. It makes the construction and engineering practice better. On the other hand, the last major earthquake in Port au Prince was in the mid 1700's. Society forgot about the earthquake risk and was not prepared for it. Many so called engineered structures also collapsed because there were no earthquake resistive system. We saw the same thing in Sichuan, China in 2008. They thought they were immune from the earthquakes. It killed 90,000 people including thousands of students in classrooms. In Haiti, I still find the dead bodies buried in the collapsed buildings. So I think the estimation of 230,000 death is underestimated. I believe both Sichuan and Haiti will change their practices. But it is too late for these 320,000 people and children who died. There are many parts of the world like this, including many cities in the US where earthquakes are not a current concern. I think that's why it's so important for engineers to communicate and educate the public about earthquake risks.


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