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The USGS estimates that several million earthquakes occur in the world each year. Many go undetected because they hit remote areas or have very small magnitudes. The NEIC now locates about 50 earthquakes each day, or about 20,000 a year. As more and more seismographs are installed in the world, more earthquakes can be and have been located.
However, the number of large earthquakes (magnitude 6.0 and greater) has stayed relatively constant. See: Are Earthquakes Really on the Increase?
The USGS says the quake followed four others that all measured more than 5.0, over a two-hour period leading up to the big quake. It says such quakes are uncommon in the region. Only 15 quakes of more than magnitude 5.0 had been measured in the region in the previous 40 years.