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 .

• When the stress built up in the rock is greater than what the rock can resist, the crust shifts.

• A sudden release of energy 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.

• After the earthquake 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.

San Andreas Fault Video(45MIN) https://www.youtube.com/watch?v=b5f09mh-gIM San Andreas Fault Video (4.5 min) https://www.youtube.com/watch?v=_YLjIvJXhpg

P (primary waves)- travel through liquids and solids.

P waves move faster than S waves.

P waves cause particles to move in the direction of the wave.

S (secondary waves)- travel only through solids.

S waves are slower than P waves.

S waves cause particles to move at right angles to the direction of the wave.

L (Land) Waves are caused when P Waves and S Waves Combine to cause a circular motion of the rock particles.

L waves cause the most destruction during an Earthquake

• The characteristics waves have in different conditions allows geologists to infer properties of the Earth ’ s interior . • http://sunshine.chpc.utah.edu/labs/s eismic/index.htm

Measuring Earthquakes

There are two different ways to measure earthquakes.

• Magnitude scales- based on seismometer readings.

• Intensity scales- based on reports of damage.

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.

• 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.

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.

Since we know how fast P-waves and S-waves travel, we can use this information to calculate the distance to an epicenter

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.

Human Dangers

• 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.

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 ” .

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.