# Chapter 9: Earthquakes 9.1: Earthquakes occur along faults

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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 Spring activity - energy travels Focus and epicenter images

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 surface Type of material All Solids _____ Type of movement Push/pull Up/down; side/side Damage 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 ( cm)

Locating an Earthquake
If you know the speed of S and P waves, and you know the time it takes for their arrival, you can determine distance Speed = distance/time While 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. Need seismographs from at least three stations Scientists find the difference between the arrival times of the primary and secondary waves at each of the three stations The time difference is used to determine the distance of the epicenter from each station (the greater time, the father away) 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

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