Editorial: The killer aftershock

Published in The Hindu on February 25, 2011

Barely six months after Christchurch, New Zealand’s second largest city, was struck by a 7 magnitude earthquake, the city was the target of another quake of 6.3 magnitude on Tuesday. This one, the sixth aftershock of last year’s tremor, turned out to be a killer. Prime Minister John Key has warned that the total number of deaths will almost certainly exceed 200: the confirmed death toll is 98 and 226 people are still reported missing. This makes it the country’s deadliest quake in the last 80 years. The tremor of September 4, 2010 damaged buildings but, mercifully, did not kill anybody. Though the February 22 aftershock was nearly ten times weaker, many died as the epicentre was very close to the city; last year’s was about 45 km west of Christchurch. The shallow focus of the aftershock was another reason. Occurring at a depth of about 5 km from the surface, it struck the city hard. The September quake was at a depth of about 10 km from the surface. According to Nature, the earthquake had a directional thrust towards the surface, and “the amount of acceleration felt at the epicentre was almost 1.9 times the force of gravity.”

There was another factor that played an important role in accentuating the shaking process of buildings. Unlike many other quake-prone regions around the world, Christchurch sits atop loosely packed soil. The sand, silt, and gravel that make up the soil, when saturated with groundwater, behave more like a liquid than a solid during earthquakes. The liquefaction repacks the soil to make it compact and denser, and in the process leads to subsidence in some areas. Differential subsidence at the site of buildings can adversely affect the structures, causing walls to crack and even collapse. The February 22 quake typifies the fact that magnitude alone does not determine the killer nature of a tremor. While such a lethal combination would have proved catastrophic in a developing country, buildings in Christchurch faced relatively less damage with fewer deaths. This is primarily because structures in New Zealand are designed and built in compliance with one of the best building codes in the world. Such codes have become necessary as New Zealand is located in one of the most tectonically active regions. It is at the margin of a subduction zone where the Australian plate overrides the diving Pacific plate northeast from the North Island to Samoa. It suddenly changes to become a transform fault south of the North Island where the Australian and Pacific plates move past each other without one diving under the other. Additionally, a third major fault called the Alpine Fault runs along the South Island itself. Under the circumstances, New Zealand has done well not to let Nature wreak greater devastation.