What is the main cause of …

Four main causes of an earthquake are:

Folding and Faulting: A fault is a fracture or a crack in the rock surface where already a great deal of displacement has taken place. If a fault is located in a zone, where the plates moves against each other, the chances of earthquakes occurring are more. San Andreas Fault of California is one such example.

Plate Tectonics: The lithosphere is broken into a number of plates called the lithospheric plates. When these plates collide with each other or slip past each other, their edges results in the creation of faults along the lines of weakness. Many earthquakes take place because of the movements of these plates.
Volcanoes: Most volcanic eruptions are accompanied by earthquakes. However, many active volcanoes are located far away from the plate margins.
Isostatic Disturbances: Isostatic disturbances may cause disturbance equilibrium of the earth’s crust. These disturbances may be produced by the deposition of sediments brought down by the rivers and glaciers on the ocean floor. As asthenosphere is in a semi molten state, the disturbance in the equilibrium may result in an earthquake.
Human Made Causes: Nuclear explosions, or the building of a large dams or water reservoirs near a fault zone may cause earthquakes in the region. One such example was the Koyna earthquake in Maharashtra which was caused due to the construction of the Koyna dam in 1957.