23 February 2011

On the causes of the high levels of loss in the Christchurch earthquake

Posted by Dave Petley

The level of damage from the Christchurch earthquake is exceptionally high, as the various galleries of images demonstrate.  From a global insurance perspective this is likely to be one of the most expensive earthquakes of all time, and it is likely to rank as the first or second worst natural disaster in the history of New Zealand in terms of loss of life. It is very interesting to note that the Mw = 7.1 earthquake in September caused no loss of life, where as this Mw = 6.3 event may have killed 300 people.  Clearly magnitude is not the only indicator of mortality risk.

Inevitably there is considerable, sometimes somewhat ill-informed, speculation about why the losses are so high. The key reasons are likely to be associated primarily with the exceptionally high peak accelerations, the location of the earthquake, the nature of the buildings, ground conditions, and the timing of the earthquake.  So let’s take a look at those in turn:

1.  The peak ground accelerations

Geonet has a map online of the measured peak ground accelerations from the earthquake.  The unit here is in % g (gravity) – i.e. 100% g is the same as the acceleration due to gravity:

Although provisional, these are exceptionally high accelerations.  At Heathcoat Valley primary school a peak acceleration of 220% g was recorded, and in the suburbs of the city 188 % g was recorded at Pages Road Pumping Station.  These accelerations are likely to have exceeded the design strength of many buildings, even with the strict building code in place in New Zealand.  Of course, the question then needs to be asked as to why a mw = 6.3 earthquake has generated such high levels of peak ground acceleration.  There are probably several reasons for this:

a. The earthquake had a reverse fault mechanism (the September event at Darfield was a strike-slip event).  This may have generated higher levels of shaking;

b. The earthquake is very shallow (5 km).  Shallow earthquakes tend to generate very intense shaking over smaller areas, whereas deep earthquakes generate less intense shaking over larger areas;

c. The shaking may have been amplified by the ground conditions (see 5. below).

2. The location of the earthquake

As I noted yesterday, this earthquake is extraordinary in that it is in effect a direct hit on a major city.  The map above shows that the area with high levels of shaking is quite small, but it so happens that Christchurch occupies this space.  This is exquisitely bad fortune.  That modern buildings such as this one (source) have collapsed so catastrophically is an indication of this:

3. The nature of the buildings

New Zealand has a very strong and intensely-imposed earthquake building code.  Many structures have performed well given the very high levels of shaking.  However, Christchurch is a city with many older masonry buildings, which were certainly not designed to be resistant to earthquakes in most cases.  Building codes do require progressive retrofitting of older buildings, but this is often a slow and expensive business.  Finally, it must be noted that building codes are designed to improve resistance to earthquake shaking, and unfortunately the magnitude of this improvement is finite.  In this case the shaking may well have exceeded the design specifications in many cases.

It is notable that there have been no reports of children trapped in schools, in contrast to both the Wenchuan and Kashmir earthquakes.  Schools have been targeted for seismic strengthening in New Zealand – for obvious reasons and because in many cases schools become the local centre for relief efforts.  This appears to have been successful, which is to the credit of the authorities.

4. The ground conditions

The growing availability of imagery from the earthquake indicates that ground conditions may have played a substantial role.  Particularly startling is this gallery from NZ Herald.  The image below shows the stadium in Christchurch – note the many large pools of water:

This is indicative of liquefaction – i.e. the intense weakening of the sediments underlying the city.  Liquefaction was a huge problem in the Christchurch in the Darfield event and appears to have been even more intense this time around.  There will be huge damage to infrastructure (gas pipes, water pipes, etc) as a result.

Soft sediments can also tend to amplify shaking, so it may well be that a part of the  reason for the intense shaking was a site effect associated with the sediments under the city.  This is going to be a key issue in the aftermath of the earthquake as plans are drawn up to rebuild the CBD.

Given that this is a landslide blog, I cannot resist the temptation to include these three NZ Herald images of what appear to be mass movements of various types.  I hope that more information will emerge about the landslides associated with this earthquake over the next few days:

5. The timing of the earthquake

Sadly, the timing of the earthquake may also have been a significant factor.  The Darfield earthquake occurred at the ideal time of day from the perspective of minimising loss of life – i.e. very late at night when most people are off the streets.  In this case the opposite is true – the earthquake occurred during lunchtime, when many people are out on the streets buying lunch.  The collapse of many buildings into the street will have been a terrible hazard for those people, as the picture below (source) graphically illustrates:

Postscript – paying for the damage

A couple of people have asked about the how the clean up is funded.  For residential property New Zealand has a residential earthquake pool called the Earthquake Commission (EQC).  This is in effect a government-backed insurance fund financed through a levy on insurance premiums.  In the event of an earthquake (or indeed a landslide) EQC will pay up to NZ100,000 for property damage and NZ$20,000 for possessions, with the remainder of the costs being covered by the insurance companies, so long as the house owner had insurance.  For commercial property, insurance will cover the costs.  Infrastructure costs are more complex – in many cases the government will foot the bill from reserves, but it may be that some infrastructure is privately only, in which case the picture is more complex.  Of course the effect of all of this is to drive a huge amount of spending into the economy, which ironically can act as an economic stimulus in the long term.