Chapter 9: Earthquakes

9.1: Earthquakes occur along faults

9.2: Earthquakes release energy

9.3: Earthquake damage can be reduced

9.2 Earthquakes release energy Terms:

faults, plate boundaries stress earthquake

All earthquakes occur along ______ The force exerted when an object pushes, pulls, or

presses against another object is called _______ Most faults are located along _______

Earthquakes release energy Energy from earthquakes travel through Earth

Ripple of rock in pond, but in all directions Energy travels as seismic waves: vibrations caused

by earthquakes Earthquakes start beneath Earth’s surface

Focus: point underground where rocks first begin to move Seismic waves travel outward from the focus

Epicenter: point on Earth’s surface directly above the focus

If equal strength earthquakes occur, the more shallow the focus results in greater damage

Depth is related to the direction in which the plate move Pulling apart: shallow: new crust that forms is thin Subduction zones: wide range of depths, anywhere

along sinking plate

CA video

Waves and Energy

All waves (sound, seismic, light) carry energy from place to place As a wave moves through a material, particles of

the material move out of position temporarily, causing the particles next to them to move: Energy moves through the material, matter does not

(*light is different) October 17th earthquake in San Francisco: shook

the stadium around for 15 seconds 20 minutes later the seismic waves reached the other

side of the Earth: detect only by sensitive scientific instruments

Three types of waves

Each type moves through material differently Can reflect, or bounce, off boundaries

between different layers Can bend as pass from one layer to another Scientists learn about Earth’s layers by

studying the paths and speeds of seismic waves traveling through Earth

waves video

Three types of waves

Primary (P waves) the fastest seismic waves First to reach any

particular location after an earthquake Travel through Earth’s crust at an average speed

of 5 km/s (3 mi/s) Can travel through solids, liquids, gases As they travel through a material the particles are

slightly pushed together and pulled apart Buildings experience this push and pull as p waves pass

through the ground

Three types of waves

Secondary waves (S waves) the second waves to reach a location after an earthquake Originate at the same time as P waves, but travel at half the

speed As they pass through a material, the material’s particles are

shaken up and down or side to side This rocks buildings back and forth as they pass

can travel through rock but not liquids or gases Primary waves alter the material density and volume slightly

Particles are pushed and pulled in the direction the waves travel Secondary waves alter the material’s shape

Liquids and gases have no definite shape Particles move at a right angel to the direction the waves travel

When scientist learned S waves cannot pass through the earth’s outer core they realized it was not solid!

Three types of waves

Surface waves Move along Earth’s surface, not through the

interior Make the ground roll up and down or shake from

side to side Cause the largest ground movement and most

damage Slowest type of seismic wave As depth increases, motion of the particles

decreases

Types of waves

Name Primary Secondary Surface

Speed Fastest Medium Slowest

Location Earth’s interior

Earth’s interior

Earth’s surface

Type of material

All Solids _____

Type of movement

Push/pull Up/down; side/side

Up/down; side/side

Damage Some Some Most

Seismic waves can be measured

Seismograph: an instrument that constantly records ground movements Separate ones are needed to record

side-to-side movements and up-and-down movements

Side-to-side: uses a heavy weight attached to a wire, which remains still as the ground moves beneath it

Up-and-down: uses a heavy weight hanging from a spring, which remains almost still as the spring absorbs the movement by getting longer or shorter

Can detect movements as little as one hundred-millionth of a centimeter (0.000001cm)

Locating an Earthquake

Need seismographs from at least three stations

1. Scientists find the difference between the arrival times of the primary and secondary waves at each of the three stations

2. The time difference is used to determine the distance of the epicenter from each station (the greater time, the father away)

3. A circle is drawn around each station, with a radius corresponding to the epicenter’s distance from that station. Where the three circles meet is the epicenter

If you know the speed of S and P waves, and you know the time it takes for their arrival, you can determine distanceSpeed = distance/timeWhile the speeds vary, the ratio of the speeds do not! Just multiply the S-minus-P (S-P) time, in seconds, by the factor 8 km/s to get the approximate distance in kilometers.

Locating an Earthquake

Seismographs can also be used to locate the focus of an earthquake Can study waves that have reflected off boundaries inside

Earth Can help determine the earthquake’s depth

Record the time when the first primary wave arrives, by a direct path and also the first reflected primary wave arriving, which first

reflects off the surface before reaching the station The difference in arrival time indicates the depth of the focus

Also used to determine earthquakes’ magnitude (strengths) More energy an earthquake releases, the greater the

ground movement recorded