Why earthquakes occur for kids

It's great for kids, because they get to see how it actually happens! What is a fault? A fault is an area of stress in the earth where broken rocks slide past each other, causing a crack in the Earth's surface.

There are the major types of faults: dip-slip normal, dip-slip reverse, strike-slip, and oblique-slip. What are plate tectonics? The theory of plate tectonics is a interesting story of continents drifting from place to place breaking apart, colliding, and grinding against each other. The plate tectonic theory is supported by a wide range of evidence that considers the earth's crust and upper mantle to be composed of several large, thin, relatively rigid plates that move relative to one another.

See plate tectonic graphic above. The plates are all moving in different directions and at different speeds. Sometimes the plates crash together, pull apart or sideswipe each other. That's about as fast as your hair grows. Earthquakes happen when a plate scrapes, bumps, or drags along another plate. When does this happen?

About a half-million quakes rock the Earth every day. That's millions a year. People don't feel most of them because the quake is too small, too far below the surface, or deep in the sea. Some, however, are so powerful they can be felt thousands of miles away. A powerful earthquake can cause landslides, tsunamis, flooding, and other catastrophic events. Most damage and deaths happen in populated areas. That's because the shaking can cause windows to break, structures to collapse, fire, and other dangers.

The magnitude of an earthquake, and the intensity of shaking, is usually reported on the Richter scale. On the scale, 3 or less is scarcely noticeable, and magnitude 7 or more causes damage over a wide area. An earthquake under the ocean can cause a tsunami. This can cause just as much death and destruction as the earthquake itself. Landslides can happen, too. Earthquakes are part of the Earth's rock cycle.

The ancient Chinese used a device that looked like a jar with dragons on the top surrounded by frogs with their mouths open. When an earthquake occurred, a ball fitted into each dragon's mouth would drop out of the dragon's mouth into the frog's. The position of the frog which received a ball indicated the direction of the earthquake. This was one of the first tools to help figure out where an earthquake originated from.

Earthquakes are caused by tectonic movements in the Earth's crust. The main cause is that when tectonic plates collide, one rides over the other, causing orogeny mountain building , earthquakes and volcanoes. The boundaries between moving plates form the largest fault surfaces on Earth. When they stick, relative motion between the plates leads to increasing stress. This continues until the stress rises and breaks, suddenly allowing sliding over the locked portion of the fault, releasing the stored energy.

There are three main types of geological fault that may cause an earthquake: normal, reverse thrust and strike-slip. Normal faults occur mainly in areas where the crust is being extended. Reverse faults occur in areas where the crust is being shortened.

Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other. Most earthquakes form part of a sequence, related to each other in terms of location and time. Most earthquake clusters consist of small tremors which cause little to no damage, but there is a theory that earthquakes can recur in a regular pattern. A foreshock is an earthquake that occurs before a larger earthquake, called the mainshock.