How does an earthquake occur?

Hamlet.

Huge underground plates (known as faults) move... Jan 03 04, 11:53 PM

DogRL

Breaking rock (two skills of antiquarian horology). http://www.geo.mtu.edu/UPSeis/why.html Earthquakes are usually caused when rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves that make the ground shake. When two blocks of rock or two plates are rubbing against each other, they stick a little. They don't just slide smoothly; the rocks catch on each other. The rocks are still pushing against each other, but not moving. After a while, the rocks break because of all the pressure that's built up. When the rocks break, the earthquake occurs. During the earthquake and afterward, the plates or blocks of rock start moving, and they continue to move until they get stuck again. The spot underground where the rock breaks is called the focus of the earthquake. The place right above the focus (on top of the ground) is called the epicenter of the earthquake. Jan 04 04, 1:29 AM

Baloo55th

Hamlet's right about the plates, but the faults are where edges of plates meet in an unfriendly manner. In the Atlantic, the plates are moving apart because new seabed is coming up gently. The San Andreas fault is where the Californian coast (which doesn't really belong to the rest of the USA geologically) is trying to move along its own way. I was reading on a fundamentalist Christian website that the fossils found up on Everest were evidence for the Flood. They didn't seem to realise that Everest is there because India is still moving northwards into Asia and that has pushed up the Himalayas. And caused strain on the Iranian and Arabian plates into the bargain. Jan 04 04, 1:04 PM

TheAlphaWolf

If you are like me that wants every detail and doesn't like the short answer, here it goes. The core of the earth is hot. VERY hot. So the heat produced by the core (the molten lava moves with the heat just like water) of the earth rises and as it rises it begans cooling since it's further from the core. Since the hot magma is going up, the cooler magma replaces it. It goes down where it heats again and the cycle starts over. (got it?) So that movement, called convection currents, pushes the crust (the hard part we live on.) The movement of the crust is called plate tectonics. Sometimes two currents are pushing the crust in the same direction so one piece of the crust has to go under (always the crust under the sea.) That creates a lot of tension because these giant plates are moving around and sometimes pressure builds up because one plate doesn't want to move (it gets stuck or whatever) and when the pressure is released the earth moves and creates earthquakes. I hope I explained it well although I know I didn't because I'm so incoherent. Jan 05 04, 4:18 PM