The trace at the bottom is a type of a seismogram that shows the energy released versus time. The first small increase in energy is when the P-wave arrives at the station and the large jump in energy occurs when the larger amplitude S-wave arrives. You can see that there is continued shaking across Christchurch for 20-30 seconds, this late seismic energy is known as coda. It is due to the reverberations of the waves in the relatively loose soil that Christchurch is built on.
This earthquake was one of many aftershocks recorded by both the permanent New Zealand network, GeoNet, and temporary QCN sensors. Approximately 180 QCN sensors were installed following the M7.1 Darfield earthquake that occurred on September 3, 2010. Since the mainshock there have been hundreds of smaller, but damaging aftershocks as large as M6.3. The QCN network recorded events ranging from M2.6 – M6.3.