There are about 500,000 earthquakes a year around the world, as detected by sensitive instruments. About 100,000 of those can be felt, and 100 or so cause damage each year. Each year the southern California area alone experiences about 10,000 earthquakes, most of them not felt by people, according to Live Science.
The sun and moon cause tremors. It’s long been known that they create tides in the planet’s crust, very minor versions of ocean tides. Now researchers say the tug of the sun and moon on the San Andreas Fault stimulates tremors deep underground.
Earth has been more seismologically active in the past 15 years or so, says Stephen S. Gao, a geophysicist at Missouri University of Science & Technology. Not all seismologist agree, however, according to Live Science.
For more information about earthquakes click here: Live Science