Why did the people of Christchurch hear a loud rumble before the shaking began?

The loud rumble heard by the people of Christchurch before the shaking began was likely due to the seismic waves generated by the earthquake. These waves travel through the ground and can produce low-frequency sounds, often referred to as "earthquake noise." The rumble serves as a precursor to the more intense shaking that follows, alerting people to the impending quake. Such sounds result from the rapid release of energy within the Earth's crust as tectonic plates shift.