Frictions and pressure between the tectonic plates increase and if it becomes too strong, a sudden shift of the plates along the already existing fault line leads to an abrupt jerk. In the case of substantial shifts, the Earth's surface trembles noticeably.

What causes the earth to shake?

The tectonic plates are always slowly moving, but they get stuck at their edges due to friction. When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth's crust and cause the shaking that we feel.

Is the sudden shaking of the earth?

An earthquake is the sudden, rapid shaking of the earth, caused by the breaking and shifting of subterranean rock.