2. Earthquakes An Earthquake is any vibrating, shaking or rapid motion of the Earth’s crust. Most Earthquakes occur at zones of weakness or a break in bedrock known as a fault.

3. When the stress built up in the rock is greater than what the rock can resist, the crust shifts and suddenly releases energy which radiates in all directions as vibrations. The focus is the place underground where the actual break occurs. The epicenter is the surface directly above the focus.

4. After the earthquake occurs the waves move away from the epicenter. When the waves reach a location the earth seems to shake. The further a location is from the epicenter the weaker the waves are when they reach that location.

5. Earthquake vibrations are called seismic waves. Seismic waves travel faster deep within the earth. As the waves change speed they refract (bend).

6. There are two kinds of waves. P (primary waves)- travel through liquids and solids. S (secondary waves)- travel only through solids. The characteristics waves have in different conditions allows geologists to infer properties of the Earth’s interior.

7. Measuring Earthquakes There are two different ways to measure earthquakes. Magnitude scales- based on seismometer readings. Intensity scales- based on reports of damage.

8. Magnitude Scales Seismometer- an instrument designed to detect and measure seismic waves. A pen is attached to a heavy mass on a spring. When the ground shakes, the mass remains stationary and the paper moves under it.

9. Seismograph- a seismometer that makes a recording of seismic waves. Each magnitude is 10 times greater than the previous number. A magnitude 5 earthquake is 10 times stronger than a magnitude 4 earthquake. A magnitude 6 is 100 times stronger than a magnitude 4.

10. Intensity scale- The Mercalli scale is based on the damage to structures and accounts of witnesses. 1-2 - may not be felt 5-6 - some dishes and windows break 7-8 - hard to stand, collapse of some structures. 10-11 - Visible waves on ground, total destruction.

11. Locations of Earthquakes and Volcanoes Most all earthquakes and volcanoes occur in a belts of crustal activity. The largest belt surrounds the Pacific ocean which is an old ocean plate and is subducting all around its borders. It is called the “Ring of Fire”.

12. Damage from Earthquakes Shaking- buildings may be damaged or collapse. Movement of crust- damages roads and railroads. Tsunamis- earthquakes in oceans cause large waves which may swamp coastal areas thousands of miles away.

13. Fires- broken gas pipes often ignite. Broken water mains make it hard to fight fires. Landslides often occur in soft ground. Areas of high risk have stricter building codes to help keep structures from falling apart during quakes. Human Dangers

14. Damage from Volcanoes Ash- may cover large areas, very difficult to deal with. Machines may not work, clogs air filters. Lava- smaller areas covered but nearly unstoppable. Toxic gasses- hot, fast-moving gasses may suffocate people.