Earthquakes

Main Topics for Chapter 19

• Forces within the Earth

• Seismic Waves and Earth’s Interior

• Measuring and Locating Earthquakes

• Earthquake Hazards

What is an earthquake?

• Natural vibrations of the ground caused by movement along gigantic fractures in the Earth’s crust or by volcanic eruptions.

Forces within the Earth

• Earthquakes occur because of a release of energy when rocks break inside the Earth.

• Rocks break when stress (force on an area) is greater than the strength of the rock.

• There are 3 types of stress that act on rock in the Earth.

Stress1. Compression – pushing material together.

2. Tension – pulling material apart.

3. Shear – twisting a material.

Strain• Strain is the deformation of a material in

response to stress.• Strain is the result of stress.• 3 types of strain:1. Elastic strain – low stress causes a

material to bend and stretch temporarily. (rubber band)

2. Ductile Deformation – higher stress causes a material undergo permanent change.

3. Failure – high stress breaks the material.

Stress-Strain Curve

Strain

Str

ess Elastic limit

Failure

Elastic deformation

Ductile deformation

How do rocks behave under stress?

• At lower temps. rocks are brittle (break easily).

• Rocks become more ductile as temp. increases.

• If stress is too great rocks can fail causing a fracture called a fault.

• There are 3 main types of faults.

Faults

• Reverse Faults – result from horizontal compression.

• Normal Faults – result from horizontal tension.

• Stike-slip Faults – result of horizontal shear.

Reverse Fault

• Horizontal compression!

Normal Fault

• Horizontal tension!

Stike-Slip Fault

• Horizontal shear!

Earthquake Waves

• Most earthquakes are caused by movements along faults.

• Earthquakes travel in waves that vibrate the ground, these waves are called seismic waves.

• Earthquakes cause 3 main types of waves.

Earthquake Waves1. Primary waves or P-waves – squeeze and

push rock particles in the same direction. They are the first to be felt.

2. Secondary waves or S-waves – causes rock particles to move up and down in relation to the direction of the wave. They are second to be felt in an earthquake.

3. Surface waves – causes rock to move up and down and side to side as the wave passes.

Earthquake Origin

• Focus – point of rock failure inside the Earth where the energy is first release and the earthquake starts.

• Epicenter – location on the surface of the Earth directly above the focus.

• Body waves – the P and S waves that travel through the interior of the Earth.

Earthquake Origin

Locating an Earthquake

• Seismometer – a sensitive instrument that can record the vibrations from earthquakes.

• Seismogram – the record produced by the seismometer, usually a long sheet of paper that shows the movement of a pen due to vibrations in the earth over time.

• Travel-Time Curves – standard curves used to predict the time vs. distance from epicenter. (ESRT page 11)

Locating an Epicenter 1. Determine P-wave arrival time.2. Determine S-wave arrival time.3. Subtract P-wave time from S-wave time, this will

be your S-P time difference.4. Using ESRT p. 11 and the Travel Time (y-axis),

mark off a scrap piece of paper with your S-P time difference.

5. Slide your scrap paper along the S and P lines on the graph until both marks touch each line. (Hint: scrap paper must be vertical at all times)

6. Draw a line vertically down and read the Epicent Distance of the x-axis.

7. PRACTICE….this skill takes lots of practice!!

Earth’s Interior

• Seismic waves can tell us a lot about the earth’s interior.

• Remember that body waves (P and S) move in all directions.

• P-waves move through solids and liquids.

• S-waves only move through solids only!

• S-waves will not pass through the outer core.

Clues to Earth’s Interior

• Most of the information we know about the interior of the Earth comes from seismic waves.

• P and S waves that travel in the mantle take direct paths.

• P waves are bent as they enter the liquid outer core, causing a shadow zone on the opposite side of the Earth.

• S waves cannot pass through the liquid outer core at all, and will not reach the opposite side of the Earth.

Layers of the Earth

Mantle

Outer core (liquid)

Inner core (solid)

Crust

Measuring an Earthquake

Magnitude – the amount of energy released during an earthquake.

What tools can we use to measure an earthquake?

1. Richter Scale

2. Moment Magnitude Scale

3. Modified Mercalli Scale

Richter Scale• A numerical scaled used to measure the

magnitude of an earthquake.

• This scale was created by Charles Richter in 1935.

• He used a mathematical model to compare the height of the seismic wave on a seismograph to the energy released in an earthquake.

• Each number on the scale represents at 32 times increase in energy released!

Richter Scale

• Example: A magnitude-7 earthquake releases 32x more energy than a magnitude-6 earthquake!

How much more energy does a magnitude-7 earthquake release than a magnitude-5 earthquake?

1024x more energy!!

Moment Magnitude Scale

• This scale takes into account the size of the actual failure in the rock at a fault.

• Scientists use this scale to calculate the specific amount of energy released the moment the earthquake happened at the focus.

Modified Mercalli Scale

• This scale does not actually measure seismic waves during an earthquake.

• Damage after the earthquake is assessed to place an earthquake on this scale.

• Roman numerals I to XII are used to indicate intensity. (see table 19-1)

Modified Mercalli ScaleI. Not felt except under unusual conditions.II. Felt only by a few people, suspended objects may swing.III. Noticeable in a building.IV. Felt in a building, but few feel it outside.V. Felt by nearly everyone. Some dishes and windows break.VI. Felt by all, Furniture moves, chimneys are damaged.VII. Some chimneys break, slight damage in well build structures,

considerable damage in poorly built structures.VIII. Chimneys, smokestacks, and walls fall, heavy furniture is

overturned, partial collapse of ordinary buildings.IX. Great damage occurs, buildings shift off foundations, ground

cracks, underground pipes break.X. Most ordinary structures are destroyed, rails are bent,

landslides are common.XI. Few structures remain standing, bridges are destroyed, broad

fissures form in the ground.XII. Damage is total, objects are thrown up into the air.

Earthquake Hazards

• Structural failure

• Land and soil failure

• Tsunami

http://earthquake.usgs.gov/