What is the primary cause of earthquakes?

The primary cause of earthquakes is the elastic rebound theory, which explains how stress builds up in the Earth's lithosphere due to the movement of tectonic plates. As these tectonic plates interact at their boundaries—by colliding, sliding past each other, or pulling apart—they become locked due to friction. Over time, the stress from tectonic forces continues to accumulate in the rocks. Once the stress exceeds the strength of the rocks, it is released suddenly in the form of seismic waves when the rocks break or slip along a fault line, resulting in an earthquake. This release causes the ground to shake as the stored elastic energy is converted into motion.

While the Richter scale measures the magnitude of earthquakes and is a valuable tool for quantifying their size, it does not explain their cause. Frictional heating can contribute to the processes along fault lines but is not the primary cause of earthquakes themselves. Similarly, the release of heat may occur during geological processes, yet it does not directly cause the seismic activity that we associate with earthquakes. Thus, the elastic rebound theory succinctly encapsulates the mechanism behind why earthquakes occur, making it the correct choice.