What causes earthquakes?

 

You might think Earth is a giant lump of rock, but you'd be wrong—it's more like a freshly boiled egg: there's a hot, molten core bubbling away inside a surprisingly thin outer crust. The countries we live in feel like they're safely anchored on solid rocky foundations, but really they're fixed to enormous rocky slabs called tectonic plates that can slide around on the mol...ten rock beneath. Imagine living your life on aneggshell!
Earthquakes happen at places called faults (or fault lines ) where the jagged edges of two tectonic plates grind against one another. Most earthquake activity happensin the middles of the oceans where plates are pushing apart on the floor of the sea. Some of the most violent earthquakes happen around the edges of a huge tectonic plate in the Pacific Ocean, forming an intense area of activity known as the Ring of Fire (so-called because there are many active volcanoes there too).
Tectonic plates are constantly moving—in incredibly slow motion—and we don't evennotice most of the time. But every once in a while two grinding plates will suddenly jolt into a new position. The energy released by this movement creates an earthquake. It starts at a point inside Earthcalled the focus where the moving plates are in contact, then travels through the ground as very low-frequency sounds called shock waves or seismic waves . The greatest damage happens at a place called the epicenter , which is the point on Earth'ssurface above the focus. Earthquakes continue until all the energy released at the focus has been safely dissipated. Even then, there's still a chance that further earthquakes, known as aftershocks , will happen for some hours or even days afterward.
Seismic waves travel in two very different ways. Some of them, known as primary waves (or p-waves), vibrate the ground in the direction in which the waves themselves are moving. They travel in a similar way to ordinary sound waves by alternately squeezing and stretching the ground in patterns known as compressions  and rarefactions. Waves like this are called longitudinal waves and travel at incredible speeds of around 25,000 km/h (15,500 mph). There's another kind of seismic wave known as a secondary wave (s-wave) that travels only half as fast. Unlike p-waves, s-waves travel by making the ground vibrate up and down as they move forward. It's because seismic waves travel at such amazing speeds—broadly speaking, as fast as a rocket taking off—that we get so little time to avoid quakes. Earth's diameter is a little under 13,000 km (8,000 miles) at the equator, so a really fast p-wave can theoretically shootfrom one side of the planet to the other in less than half an hour!
Artwork: As s-waves travel forward, they shake the Earth up and down or from side to side (at right angles to the direction of motion). P-waves shake the Earth back andforth in the same direction in which they'removing. An s-wave is an example of a transverse wave; a p-wave is an example of a longitudinal or compression wave.