Chapter 9: Earthquakes 9.1: Earthquakes occur along faults 9.2: Earthquakes release energy 9.3:...
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Transcript of Chapter 9: Earthquakes 9.1: Earthquakes occur along faults 9.2: Earthquakes release energy 9.3:...
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