A earthquake is the movement and trembling that results from the movement of the rock beneath the Earth's surface. This movement of Earth's plates creates forces that squeeze or pull the rock in the crust. These forces are examples of stress, a force that acts on the rock to change it's shape or size.
*Volume is the amount of space an object takes up.*
Because stress is a force, it adds energy to the rock. The energy is stored in the rock until the rock either breaks or changes later on in a bit of a while.
There are three different kinds of stress that occurs in the crust:
a. Shearing
b. Tension
c. Compression
There are three different types of faults:
a. Strick-slip faults are created by shearing
b. Normal faults are caused by tension
c. Reverse faults are produced from compression
'Friction is the force that opposes the motion of the surface as it moves across another surface.'*
Seismic Waves -
Primary waves (p waves) are the first to arrive.
Secondary waves (s waves) vibrate side to side as well as up and down.
Surface waves occur when p waves and s waves reach the surface.
A seismograph records the ground movement. Magnitude is a measure of earthquakes strength. Mercati scale rates earthquakes according to their affect on the people, buildings, and surrounding land. Richter Scale measures an earthquake by a particular type of mechanical seismograph. Moment magnitude scale can be used to rate earthquakes of all sizes, near or far.
This is from an earthquake in California.
To observe the changes, geologists put in place instruments (like the one above) that measure stress and deformation in the crust.