How Earthquakes work

Investigating Earthquakes--San Francisco

o Earthquakes: Vibrations (seismic waves) within Earth materials are produced by the rapid release of energy

Earth’s crust is in constant motion because of tectonic forces

Earth’s crust can store elastic energy When forces exceed the elastic limits and

structural strength of the rocks, the rocks will break and/or move producing vibrations that travel outward in all directions

What is an Earthquake?

o The actual place underground where the rocks break producing vibrations is called the focus

o The place on the surface directly above the focus is called the epicenter

Earthquakes

Tension Force: stretching or pulling force Makes a normal fault

What types of forces are created?

Compression Force: force pushing something together Makes a reverse fault

What types of forces are created?

Shear Force: a system of forces that

operates against a body from different sides

Makes a strike-slip fault

What types of forces are created?

The two sides of a non-vertical fault are known as the hanging wall and footwall.

•The footwall occurs below the fault

•The hanging wall occurs above the fault plane

Forces in Earth’s CrustAnticlines and Synclines

Compression can cause folds in the crust. Two types of folding are anticlines, which arch up, and synclines, which dip down.

AB

Folded Mountain

Folded Mountain

Fault Block MountainsTension and Normal FaultsAs tension forces pull the crust apart, two normal faults can form a fault-block mountain range.

Fault Block Mountains

Fault Block Mountain

Forces in Earth’s CrustThe Kaibab PlateauLook at the sequence of drawings. In your own words, describe what happens in the last two diagrams.

Plateau

Plateau

Movement along faults: occurs when the energy exceeds the friction holding the sides of the fault together and is suddenly released.

Movement of magma (volcanic)

Volcanic eruptions

What causes Earthquakes?

Originate at the focus and travel outward in all directions

Foreshocks: small earthquakes that come before a major earthquake

Aftershocks: Are adjustments in the crust after in earthquake

o Smaller than main earthquake, but can cause as much or more damage. They can continue for weeks to months. Not every earthquake produces aftershocks

Seismic Waves

Seismic Waves

P waves (primary waves) Compressional wave

Particles move back and forth in the same direction as the wave

Travels the fastest Can pass through solids and liquids

(gases also) Does not cause damage

3 Types of seismic Waves

S wave (secondary wave, shear wave) Particles move at right angles to the

direction of the wave Travels slower than P waves Can pass through solids only Does not cause damage

Types of Waves

L wave (long wave, surface wave, ground wave)

Particles move in elliptical orbit

Originates on the surface after the P and S waves go straight up from the focus and reach the surface

The L wave causes the damage and will be the strongest at the epicenter

Travels the slowest

Types of Waves

How do we Measure Earthquakes?

Earthquake waves are recorded by a seismograph and the recording of waves on paper is called seismogram

How do we Measure Earthquakes?

Intensity – a measure of the effects on an earthquake at a particular location

Magnitude: a measure of the strength or amount of energy released during an earthquake

Richter Scale: Measures the amplitude of earthquake waves on seismograms

Scale from 1-10 Each number is 10 times the amplitude of

the number below

How do we Measure Earthquakes?

Locating the epicenter1. Lag time between the arrival of the P wave and the S

wave to the seismograph station is converted to a distance

2. A circle with a radius that equals the distance is drawn around the station.

3. Two stations can narrow down the location to two places where the two circles intersect

Locating the focus: the lag-time of the L wave will determine the depth of the focus

Measuring Earthquakes

Earthquake Dangers

o Most injuries and deaths are caused by falling objects and most property damage results from fires that start

Tsunami: seismic sea wave sometimes generated when an earthquake originates on the ocean floor

Tsunami—December 2004

Earthquake Dangers

Seiche: rhythmic sloshing of small bodies of water

A seiche is the sloshing of a closed body of

water from earthquake shaking. Swimming

pools often have seiches during earthquakes.

Liquefaction: unconsolidated materials that are water saturated may turn to a fluid causing some underground objects such as storage tanks to float to the surface

Earthquake Dangers

Ground fissures caused by liquefaction near the mouth of the Pajaro River in California during the 1989

Loma Prieta earthquake. When the surface of the ground oscillates, wet, sandy, and muddy soils can

flow like a liquid. This is liquefaction. You can liquefy wet sand at the beach by pumping it up and down with your feet. Photo courtesy of the Loma Prieta

Collection, Earthquake Engineering Research Center, UC Berkeley.

Landslides

Earthquake Dangers

Earthquake Safety

Protect yourself from falling objects (GET UNDER SOMETHING) or stand in a hallway or doorway (watch out for a swinging door)

Do not try to go outside during the earthquake

After the earthquake and before the aftershocks, go outside

Do not return to the building until it has been inspected