Because they are detected by seismometers.
If you have ever felt an earthquake you will know they exist. Billions of people have felt earthquakes and many millions of people have been killed by earthquakes. Buildings collapse in many earthquakes. Earthquakes leave plenty of evidence and witnesses behind them that they happened.
27 earthquakes have happened from 1995 to 2008.
the earthquakes that happened
seismic gap
seismic gap
Earthquakes happen when the Earth's crust shifts violently. So if we know that an earthquake has happened in California, then scientist know that the crust under California is unstable, therefore making earthquakes more likely to happen there. Earthquakes can happen anywhere, but the tend to happen repeatedly in certain spots
Earthquakes happen every day, all around the world. On March 20, 2014, a total of 38,289 earthquakes had happened in the past year.
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Earthquakes are like a shock
it is more likely to have a strong earthquake where few recent earthquakes have happened
about how many earthquakes were in north America
The earthquakes.