There are probably several million earthquakes in the world every year, but the majority of these are too weak to be detected with current instruments. Some earthquakes which are large enough to be detected may also go unnoticed because they occur in isolated areas where there are no seismographs to pick up the tremors.
For example, while it is estimated by the US Geological Survey that there are 1.3 million earthquakes worldwide each year registering between 2 and 2.9 on the Richter Scale, only 3643 of these were detected in 2011. In total the National Earthquake Information Center reports about 20,000 earthquakes per year.
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