Seismologists, people who study earthquakes, estimate that more than 600,000 tremors occur annually without anyone knowing (less than 2.0 on the Richter scale).
Another 300,000 register on seismographs (2.0-2.9), but still no one feels them. Some 49,000 quakes are noticed only by people living near their epicenters, where the quakes reach the surface (3.0-3.9).
Slight, localized damage occurs in about 6,000 minor shocks every year (4.0-4.9). Moderate quakes happen an average of 1,000 times a year (5.0-5.9). About 120 large shocks, destructive only in populated areas, take place annually (6.0-6.9).
Major earthquakes, causing serious damage and recorded worldwide, happen at a rate of only 14 a year (7.0-7.9), while great earthquakes that cause complete destruction near their epicenters occur once every 5 to 10 years (8.0-8.9).
The largest earthquakes, such as the 1964 Good Friday quake in Alaska, will take place once or twice within a century (9.0 and up).