can cause the ground to shake and
crack apart. Earthquakes can be very powerful,
and if they occur in or near areas where people
live, they can make buildings collapse, bridges
sway, and roads buckle.
But not all earthquakes are powerful enough to
cause damage. In fact, earthquakes are happening
all the time, on land and in the ocean. Most are so
small that people don't even feel them.
An earthquake is the sudden movement of the
Earth's crust. Earthquakes occur along fault lines,
cracks in the Earth's crust where tectonic plates
meet. They occur where plates are subducting,
spreading, slipping, or colliding. As the plates grind
together, they get stuck and pressure builds up.
Finally, the pressure between the plates is so great
that they break loose. Depending on how much
pressure has built up, the ground may tremble
slightly or shake forcefully.
Scientists describe the intensity of an earthquake
using the Richter Scale. It measures earthquakes
on a scale of 1 to 10. People barely feel a
magnitude 3 earthquake, and windows might rattle
at magnitude 4. A magnitude 6 earthquake is
considered major, causing houses to move and
chimneys to fall. The largest earthquake on record had a
magnitude of 9.5.