What causes Earthquakes with the greatest magnitudes?

Earthquakes with the greatest magnitudes are primarily caused by the movement of tectonic plates, particularly at convergent or transform boundaries where stress accumulates over time. When the accumulated stress exceeds the strength of rocks, it results in a sudden release of energy, leading to a significant seismic event. Additionally, subduction zones, where one plate is forced beneath another, can generate extraordinarily powerful earthquakes. Other factors can include volcanic activity, but tectonic processes are responsible for the majority of high-magnitude earthquakes.