Earthquakes occur where two tectonic plates meet. These tectonic plates, which make up the Earth’s crust, are moving. Very, very slowly. In some places, that movement is fairly smooth. In others, bits of crust stick. Energy builds up. When the bits unstick, the energy is released. It moves outward from that spot in waves. Those waves shake the ground.
Not all earthquakes start that way, though. Quakes also can be caused by human activity. They can occur where oil and gas operations inject large amounts of wastewater deep underground. This causes the pressure of fluids under the ground to increase. And in some areas of the world, such as Oklahoma, this has triggered quakes.
Every day, somewhere in the world, the earth shakes. In fact, this happens dozens of times daily. In a year, there are some 12,000 to 14,000 earthquakes worldwide. The vast majority are so small that no one feels them. Their shaking is recorded only with seismic equipment. There are also tremors that cause shaking we can feel, but that cause no damage.
Once a month or so, though, there is a major earthquake somewhere on the planet. This is a quake that measures 7 or more on the Richter scale. And once a year, there will be a great earthquake of magnitude 8 or more. These types of earthquakes can be devastating. They may level cities and kill thousands.
Want to know more? We’ve got some stories to get your started:
Three things scientists want to know after California’s July 2019 earthquakes: One of them — is tectonic activity slowly shifting away from the San Andreas fault? (July 24, 2019) Readability: 7.4
A million tiny quakes shook Southern California — and no one knew: Data on lots of little quakes can help scientists learn about what triggers the big ones (5/31/2019) Readability: 7.8
Nepal earthquake offers hints of worse to come: Even larger quakes could strike among the Himalayas (4/29/2015) Readability: 6.6
The quake that shook up geology: A huge earthquake in Alaska in March 1964 triggered a shift in what geologists know about Earth (3/26/2014) Readability: 6.4
See how the size of a building and the materials it’s made from can change the frequency of vibrations during shaking with this activity from the Exploratorium.