Peak Ground Acceleration
The Mercalli Intensity Scale is a measure of Peak Ground Acceleration (PGA), which can exceed 1g in the most severe earthquakes. When the upward forces exerted by an earthquake exceed the gravitational force that normally keeps things grounded, structures, cars and people can get tossed into the air.
The Haitian earthquake produced maximum ground shaking intensities of around MM X, with MM IX producing most of the damage in Port-au-Prince. Similarly, early indications from the Canterbury event suggest maximum ground shaking intensities of around MM IX, with some areas possibly experiencing MM X. In both cases the earthquake was centred on land, so there was no accompanying tsunami, although there may have been a localised tsunami caused by a submarine landslide during the Haiti event.
Clearly, the two earthquakes must have had similar destructive power. So why, incredibly, was no one killed in Canterbury, while 230,000 people lost their lives in Haiti?
Better Building Standards
Learning from past earthquakes (especially the magnitude 7.8 Napier earthquake in 1931), New Zealand has implemented stringent building codes. Modern homes are generally of timber-frame construction, which flex and absorb the energy of an earthquake. Modern commercial and office buildings are generally constructed with isolated foundations, while many historic buildings have been retrofitted with earthquake dampening devices. New Zealand is now a world leader in earthquake engineering.
Still, there was significant damage in Christchurch, most often to older un-reinforced brick structures, and in areas where liquefaction amplified the ground shaking. And of course, there was major damage to the water and sewerage infrastructure, and disruptions to power supply and transportation networks.
The rebuilding cost in Canterbury is currently estimated at over @2 billion (NZ), compared to over $20 billion for the rebuilding efforts in Haiti.
Haiti is one of the poorest countries in the world, and does not benefit from stringent building codes. Construction practices are substandard and earthquake-proof buildings are few. An estimated 250,000 residences were destroyed or severely damaged in Haiti, leaving nearly 1 million people homeless. Even such important buildings as the Presidential Palace and National Assembly did not withstand the severity of the shaking. The collapse of buildings in Haiti led to tens of thousands of people being buried under rubble, or trapped inside unsafe structures.
Essential services were decimated. Infrastructure vital to the disaster response was severely damaged, meaning that people could not get the help that they needed in time. The loss of hospitals, major roads, rail links, harbours, and communication networks severely hampered rescue and relief efforts. Without sufficient aid, thirst, famine, looting, and eventually disease took a terrible toll.
Not a time for complacency
The relatively small amount of damage in Christchurch (at least compared to Haiti) allowed emergency services to mobilise and respond quickly to the earthquake. Hospitals remained operational throughout. Some essential services were damaged, but the Christchurch City Council responded quickly to water and sewerage disruptions, while electricity and communications providers worked effectively to get their systems back online. Most of Christchurch already has running water, working sewers, electricity and communications restored. This is a tribute to the preparation of the region, and reflects New Zealand's strong commitment to disaster planning and preparedness.
However, it must be acknowledged that luck played a part here as well. If the Canterbury earthquake had occurred at 4:53pm, as it did in Haiti, the number of deaths and serious injuries would surely be much higher. People may have been crushed or trapped by collapsing buildings, chimneys and brick facades. Traffic accidents would have occurred all over the city, as traffic lights stopped working, fissures opened up in road surfaces, and various structures collapsed onto busy streets.
Cantabrians can be proud of how their region and communities have coped so far with this disaster. But we must not lapse into complacency. We have the best possible scenario here; a major disaster, but with minimal impact, that we can learn from, in order to better prepare ourselves for the next one. As seen in Haiti, it could have been so much worse. And let us not forget, the Alpine Fault is still there, and the elastic band is stretching a bit further each day, storing up energy.
Jesse Dykstra is a PhD student in the Natural Hazards Research Centre at Canterbury University. View his work and that of 30 other scientists and science writers at Sciblogs, New Zealand's largest science blogging network.
