Short answers, pulled from the story.
The 1556 Shaanxi earthquake, which struck on the 23rd of January 1556 in Shaanxi, China, killed more than 100,000 people, and the region lost up to 730,000 people afterward through emigration, plague, and famine. The 1976 Tangshan earthquake, which killed between 240,000 and 655,000 people, was the deadliest of the 20th century.
The 1960 Chilean earthquake is the largest measured on a seismograph, reaching 9.5 magnitude on the 22nd of May 1960 with its epicenter near Cañete, Chile. It released roughly twice the energy of the next most powerful quake, the Good Friday earthquake of the 27th of March 1964 in Prince William Sound, Alaska.
Earthquakes are caused mostly by the rupture of geological faults, as the elastic-rebound theory describes, when stored strain energy releases suddenly along a fault. They can also be caused by volcanic activity, landslides, and human activities such as mining, fracking, and nuclear weapons testing.
Charles Francis Richter developed the first magnitude scale in 1935, in which each unit represents a ten-fold difference in shaking amplitude and a 32-fold difference in energy. Most seismological authorities now use the moment magnitude scale, which accounts for total rupture area, average slip, and the rigidity of the rock.
Around 500,000 earthquakes occur each year that current instruments can detect, and about 100,000 of these can be felt. Since 1900, the United States Geological Survey estimates an average of 18 major earthquakes of magnitude 7.0-7.9 and one great earthquake of 8.0 or greater per year.
Most of the world's earthquakes, 90 percent and 81 percent of the largest, occur in the circum-Pacific seismic belt, a 40,000 km horseshoe-shaped zone known as the Pacific Ring of Fire. Many quakes also occur along other plate boundaries, such as along the Himalayan Mountains.
From Anaxagoras in the 5th century BCE to the 14th century CE, earthquakes were usually attributed to air or vapors in the cavities of the Earth, and Pliny the Elder called them underground thunderstorms. In Norse mythology they were the struggle of Loki, in Greek mythology Poseidon caused them, and Japanese mythology blamed the giant catfish Namazu.