Earthquake is caused by the sudden movement of the earth! (Answer: 1. Earthquake)
A sudden movement of the earth can be considered as the sudden shift in the tectonic plates under the surface of earth. This movement generally causes earthquakes.
Some information on Earthquakes:
According to Wikipedia, An earthquake can be described as the shaking of the Earth’s surface due to a sudden release of energy in the Earth’s lithosphere that creates seismic waves.
Earthquake or its intensity is measured on the Richter scale. Theoretically, it doesn’t have any upper limit but the highest we measured till now is 8.6 and that was measured in an earthquake in Chile in 1960.