Snap Your Fingers and observe what is happening. When you snap your fingers, imagine that each finger is a big chunk of rock deep inside the earth's surface. Like your fingers, one rock mass is forced against another.

Seismic Waves Deep inside the earth, rocks are constantly being pressured to move until the strain is so great the rocks can no longer bear the tension. Suddenly, there is a movement that releases all the energy called an earthquake --some of which has been building up for years. This released energy travels through rocks in the form of vibrations called seismic waves. Blue primary waves followed by red secondary waves move outward in concentric circles from the epicenter of an earthquake off British Columbia and Washington State.

Hypocenter/Epicenter The waves travel outward from the spot where rocks of the earth's crust snapped under the strain. This spot is called the Focus. The spot on the earth's surface right above the hypocenter is called the epicenter. Earthquakes are felt the strongest at the epicenter.

Three Seismic Waves Energy released at the hypocenters of earthquakes travel as three different types of waves. You can think of these waves by picturing the ripples created when a stone is thrown into a puddle or lake. The first set of waves are called P-waves or primary waves. The second ones are called S-waves or secondary waves. The third waves are called L-waves or Love waves, named after the scientist who first discovered it.

Three Seismic Waves The energy of all three types of waves is passed from one rock particle to another in the same way one domino hits another and then another in a line of falling dominoes. These seismic waves cause back-and-forth, side-to-side, and up-and-down motions in the earth. These motions are what people sense during a earthquake. These motions also cause the movement of objects we often see in video images of areas hit by an quake.

Primary Waves (P-waves) Primary waves are compression waves. They push and pull the rocks of the earth's surface in a forward-backward motion.

Primary Waves (P-waves) Primary waves travel the fastest. They can move through solids (like the rocks of the crust) and liquids (like water on the surface or in cracks in the crust or the liquid outer core of the earth) and gases (like the atmosphere or like air in the cracks of the crust). P-waves are the first waves to reach the Earth's surface after an earthquake.

Primary Waves (P-waves) Primary waves push and pull on the rocks through which they are traveling. This creates a back and forth movement on the Earth's surface. This is just like sound waves pushing and pulling the air. Have you ever heard a big clap of thunder and heard the windows rattle at the same time? The windows rattle because the sound waves were pushing and pulling on the window glass much like P-waves push and pull on rock.

Secondary Waves (S-Waves) Secondary waves move through solid rock only. These shear waves move the earth's surface side to side.

Secondary Waves (S-Waves) Secondary waves travel slower than primary waves. Also called shear waves, S-waves create a side to side motion in the rocks through which they are traveling.

Secondary Waves (S-Waves) Unlike the other earthquake waves, S-waves cannot travel through liquids or gases, ONLY SOLIDS. S-waves will rock buildings side-to-side.

Surface Waves (L-waves also called Land waves) Land waves move along the surface of the earth and cause it to move up and down like something bobbing on the surface of the ocean.

L-Waves - Surface Waves Land waves only move along the surface of the earth, close to the epicenter. Their speeds vary depending on the material, rock or soil, forming the surface. These waves cause the surface to move up and down. These are the waves that cause the to buildings and other structures during an earthquake.

Seismograph This is an image of a seismograph, an instrument used to record the energy released by an earthquake. When the needle is moved by the motion of the earth, it leaves a wavy line.

When you look at a seismogram, there will be wiggly lines all across it. These are all the seismic waves that the seismograph has recorded. Seismogram

The P wave will be the first wiggle that is bigger than the rest of the little ones (the microseisms). Because P waves are the fastest seismic waves, they will be the first ones that are recorded. The next set of seismic waves will be the S waves. These are usually bigger than the P waves. So which wiggles are the earthquake?

The Surface waves are the other, larger waves marked on this seismogram. Surface waves travel slower than S waves (which are slower than P waves) so they arrive last at the seismograph after the S waves. Surface Waves Arrive Last

The End Bonus : Where do earthquakes happen?