What happened underground to cause the February 22nd aftershock in Christchurch 2011?

The aftershock on February 22, 2011, in Christchurch, New Zealand, was caused by tectonic activity related to the complex interactions along the boundary between the Pacific and Australian tectonic plates. This region is characterized by numerous fault lines, and the aftershock was a result of the release of accumulated stress along these faults following the major earthquake that had occurred earlier in the month. The aftershock's magnitude and shallow depth contributed to its destructive impact on the city and its infrastructure.