What caused the Christchurch earthquakes of 2010–11 to be so destructive?

The destructiveness of the Christchurch earthquakes of 2010–11 was driven by several geological and structural factors:

• Proximity and Depth: The February 22, 2011, aftershock was particularly devastating because its epicentre was very close to Christchurch and its focus was shallow—only 3 miles (5 km) beneath the surface. This proximity and shallow depth resulted in intense shaking.1
• Soil Liquefaction: The seismic waves caused widespread liquefaction, where water-saturated, unconsolidated soil lost its strength and behaved like a liquid. This caused the ground to lose its ability to support building foundations, leading to significant structural failure, tilting, and the expulsion of silt and water from the ground.12
• Cumulative Damage: The region had already been weakened by the initial September 2010 earthquake and thousands of subsequent aftershocks. By the time the February 2011 event struck, many buildings and infrastructure systems were already compromised, making them far more susceptible to collapse.1
• Tectonic Setting: The events were caused by deformation along the boundary where the Pacific and Indo-Australian tectonic plates collide, triggering movement along previously unknown faults.1