Chapter 11, Section 1 - How and Where Earthquakes Happen
Earthquake - A movement of the ground caused by a sudden release of energy when the rocks along a fault move.
Fault - A break in a body of rock where one block moves relative to the other.
Elastic Rebound - The sudden return of elastically deformed rock to its undeformed state. Click here for a good animation of elastic rebound from the USGS.
a. Friction stops the blocks from sliding past each other.
b. Pressure builds up, and rocks deform.
c. The rocks suddenly slip past each other, releasing the built-up energy.
d. The rocks return to their original shape, but are displaced.
Focus - The place inside the crust where the first motion of an earthquake occurs.
Epicenter - The place on the surface that is directly above the focus.
- 90% of continental earthquakes have shallow (<70km) foci.
- The deeper the focus, the more the waves dissipate before reaching the surface.
Seismic (Earthquake) Waves
- Body Wave - A seismic wave that travels through the body of a medium.
- Primary (P) Waves
- These are the fastest type of seismic wave.
- P-waves cause rock to move parallel to the wave (compression waves).
- Can travel through solids, liquids and gasses.
- Secondary (S) Waves
- S-waves cause rock to move up and down, perpendicular to the wave.
- Can travel through solids only.
- Surface Wave - A seismic wave that travels along the surface of a medium, and that has a stronger affect near the surface than it has in the interior.
- Surface waves are the slowest, but most destructive.
- Love Waves
- Cause the surface of the ground to move side to side, perpendicular to the wave.
- Rayleigh Waves
- Cause the ground to move with an elliptical, rolling motion.
Shadow Zones - locations on the Earth's surface where no body waves from a particular earthquake can be detected.
- Waves bend as they pass through layers of different compositions/rigidity.
- S-Waves do not travel through liquids, but P-waves do.
- This allows us to use seismic waves to "view" the internal layers of the Earth. [Video]
Locations of Earthquakes
- Most earthquakes occur at plate boundaries...
- Convergent Oceanic Boundaries (subduction)
- Divergent Oceanic Boundaries (mid-ocean ridges)
- Continental Boundaries (convergent, divergent, transform)
- Fault Zone
- A region of numerous, closely-spaced faults.
- Often form along plate boundaries.
- Away from plate boundaries...
- ex. Madrid, MO earthquakes of 1811-1812
- Ancient fault zone deep underground.
- Formed 600 m.y.a.; buried by sediment.
Chapter 12, Section 2 - Studying Earthquakes
Seismology - The study of earthquakes and seismic waves.
Seismograph - An instrument that records vibrations in the ground.
- Three sensing devices to record ground movement...
- vertical motion
- horizontal motion - north-south
- horizontal motion - east-west
Seismogram - A tracing of earthquake motion that is recorded on a seismograph.
- P-waves are recorded first, then S-waves, then surface waves.
- The difference in time between P- and S-waves is used to determine distance to the epicenter.
- Triangulate the source by using seismographs at different locations.
Magnitude - A measure of the strength of an earthquake.
- Richter Scale - measures ground motion from an earthquake.
- Moment Magnitude Scale
- size of area of fault that moves
- avg distance the fault blocks move
- rigidity of rocks in the fault zone
Number scale - higher numbers for larger earthquakes.
- Largest recorded: 9.5 (Chile)
- Less than 2.5, not felt by most people.
Intensity - The amount of damage caused by an earthquake.
- Depends on...
- Distance to epicenter
- Local Geology
- Duration of Quake
- Human infrastructure
- Modified Mercalli Intensity Scale
- scale of I (felt by very few, under ideal conditions) to XII (total destruction)
Chapter 12, Section 3 - Earthquakes and Society
Dangers from earthquakes include...
- collapse of building/structures
- falling objects
- flying glass
- explosions caused by broken electric and gas lines
- floodwater from collapsed dams
Tsunami - A giant ocean wave that forms after a volcanic eruption, submarine earthquake or landslide.
- A sudden rise/drop in the ocean floor causes a large mass of water to also rise/drop, creating the tsunami waves.
- Buildings with weak walls may collapse.
- Very tall buildings may tip over from swaying violently.
- Buildings constructed on loose sand and rock can experience exaggerated motion.
- Before an Earthquake
- Have a supply of canned food, bottled water, flashlights, batteries, portable radio.
- Have a plan of action for if you are caught in an earthquake at home, at school, in the car.
- Discuss the plan with your family.
- Know how to turn off the gas, electricity and water at your home.
- During an Earthquake
- Stay calm.
- Between tremors, move to a safer position.
- Stand in doorway OR hide under a desk or table (protection from falling debris).
- Stay away from windows (broken glass) heavy furniture or other objects that could tip over on you.
- In a car...
- Stop in a place away from tall buildings, tunnels, power lines, bridges.
- Remain in the car until the tremors stop.
- After an Earthquake
- Stay alert.
- Check for fire, other hazards.
- Wear shoes in areas where there could be broken glass.
- Avoid downed power lines (and objects touching them).
Earthquake Warnings and Forecasts
- There is currently NO reliable way to predict when or where earthquakes will hit.
- Predictions may be off by several years.
- Seismic Gap - An area along a fault where relatively few earthquakes have occurred recently, but where strong earthquakes are known to have occurred in the past.
- May be likely locations for future earthquakes.
- Foreshocks - Little earthquakes that sometimes precede a larger quake.
- May precede the earthquake by a few seconds or a few weeks.
- Changes in Rocks
- Scientists use sensors to measure tilting of the ground, changes in stress and strain, changes in magnetic and electrical properties of rock, and seepage of natural gas.
- Predictions are unreliable, but zones of relative earthquake hazard have been identified. (See map.)