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o EarthquakesEarthquakes: 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 focusfocus
o The place on the surface directly above the focus is called the epicenterepicenter
Earthquakes
Tension ForceTension Force: stretching or pulling force Makes a normal normal fault
What types of What types of forces forces are created?
Compression ForceCompression Force: force pushingpushing something togethertogether Makes a reversereverse fault
What types of What types of forcesforces are created?
Shear ForceShear Force: a system of forces that
operates against a body from different sides
Makes a strike-slipstrike-slip fault
What types of What types of forcesforces are created?
Movement along faults: occurs when the energy exceeds the friction holding the sides of the fault together and is suddenly releasedreleased.
Movement of magma (volcanic)
Volcanic eruptions
What causes Earthquakes?
Originate at the focus and travel outward in all directions
ForeshocksForeshocks: small earthquakes that come before a major earthquake
AftershocksAftershocks: 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 weeksweeks to monthsmonths. Not every earthquake produces aftershocks
Seismic Waves
P wavesP 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 waveS 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 waveL 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?
IntensityIntensity – a measure of the effects on an earthquake at a particular location
MagnitudeMagnitude: a measure of the strength or amount of energy released during an earthquake
Richter ScaleRichter 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 epicenterLocating the epicenter1. Lag time between the arrival of the P wave P wave
and the S waveand 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 focusLocating 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.
TsunamiTsunami: seismic sea wave sometimes generated when an earthquake originates on the ocean floor.
Earthquake Dangers
SeicheSeiche: 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.
LiquefactionLiquefaction: 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.
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