3. Why do earthquakes occur?

Plate tectonics, a scientific theory created in the second half of the 20th century, states that the rigid earth’s crust (also called lithosphere) is broken up into “plates” moving on a low-viscous layer with a fluid mechanic behaviour (also called the asthenosphere). The very cause of earthquakes is thus the movement of such large plates which compose the outer shell of the Earth (earth’s crust).

When tectonic plates move under, over or slide past each other, earthquakes happen. Such motion can be either of millimeters, producing small shakings, or of meters, producing very violent earthquakes. Heat from the layers below the litosphere and the difference between the light density of lithosphere and heavy density of the underlying asthenosphere, explain such movements and are viewed as the most important source of energy driving plate tectonics. Difference in density also allows the asthenosphere to sink into the deep mantle at subduction zones.

Different types of Convergent Boundaries and subduction zones. Source: Wikipedia.

Tectonic plates can include continental crust, oceanic crust or both. Their movement is mostly due to the relative high density of lithosphere and the relative fluid mechanic behaviour of the asthenosphere.

This map by National American Space Administration shows the world areas in which tectonic late motion is more active. Consequently, in such areas Convergent, Divergent and Transform boundaries may be found where the main seismic and volcanic activity is registered. Source: Wikipedia, NASA

As it has been indicated in question 2, small-magnitude earthquakes can be also induced by human activities (ref. to Question 2 for the most common types), although no validated data is available so far to the scientific community, to identify some kind of correlation between those activities and big-magnitude quakes.