In the years 1811 and 1812 three strong earthquakes, measuring approximately 8 on the Richter scale, caused parts of the Mississippi river to flow backwards, which caused the Reelfoot lake in Tennessee to form.
The strongest earthquake was recorded in Chile on May 22nd 1960. The tremors measured 9.5 points on the Richter scale and left over 2 million people homeless. During the disaster 3,000 people were injured and 1,655 people dies. The resulting tsunami reached Hawaii, Japan, the Philippines and the US West Coast causing millions in damage.
Earthquakes usually occur when the pent-up tension between tectonic plates discharges in a big tremor. Although these movements in the earth's crust are not the only cause of earthquakes: Seismographic instruments are measuring earthquakes during meteorite impacts, volcanic eruptions and missile tests. The gravitational forces of the moon and sun also cause regular earthquakes, but these are so small that we can only measure with sensitive instruments. Each year around 500,000 earthquakes are recorded, 100,000 of them can be felt, but only about 100 of those are strong enough to cause damages.