Earthquakes. Vibration of earth produced by rapid release of energy (seismic waves) with radiate in all directions from the source (focus) Elastic Rebound.

EarthquakesEarthquakes

Vibration of Vibration of earth produced earth produced by rapid release by rapid release of energy of energy (seismic waves) (seismic waves) with radiate in with radiate in all directions all directions from the source from the source (focus) Elastic (focus) Elastic Rebound Theory Rebound Theory Explains how Explains how this works.this works.

Like ripples from dropping Like ripples from dropping a stone in a pond, energy a stone in a pond, energy dissipates with distance dissipates with distance

Earthquakes don't occur Earthquakes don't occur randomly. randomly. Occur on faults or Occur on faults or fractures fractures within the earth within the earth

Explained by plate Explained by plate tectonics. tectonics.

Most occur on plate Most occur on plate boundaries. boundaries.

Sometimes in plate Sometimes in plate interiors interiors

if enough stress is built if enough stress is built up up

EarthquakesEarthquakes

Most destructive forces on Earth. But it is Most destructive forces on Earth. But it is buildingsbuildings

and other human structures that cause injury and other human structures that cause injury and and

death, not the earthquake itselfdeath, not the earthquake itself

1988 - Soviet Armenia: magnitude 6.9, 1988 - Soviet Armenia: magnitude 6.9, 25,000 people 25,000 people died died

1985 - Mexico City: magnitude 8.1, 1985 - Mexico City: magnitude 8.1, 9500 people 9500 people 1989 - Loma Prieta, CA: magnitude 7.1, 1989 - Loma Prieta, CA: magnitude 7.1, 40 people died 40 people died 1995 - Kobe, Japan: magnitude 7, 1995 - Kobe, Japan: magnitude 7, ~6000 people died ~6000 people died

Soviet Armenia: 1988Soviet Armenia: 1988

Mexico City - 1985

Kobe, Japan 1995

Alaska 1964

30,000 30,000 earthquakes earthquakes occur occur worldwide worldwide annually that annually that are strong are strong enough to be enough to be felt, but felt, but typically only typically only 75 of them 75 of them are are considered considered to be to be significantsignificant

MechanismMechanismElastic rebound theoryElastic rebound theory Forces bend rock on either side of fault, rock Forces bend rock on either side of fault, rock

strains ever so slowly, then weakest point breaks. strains ever so slowly, then weakest point breaks. Break sends out shock waves, which migrate Break sends out shock waves, which migrate outwards from the original break, causing outwards from the original break, causing shaking. Stress is released shaking. Stress is released

Aftershocks are adjustments to that change in Aftershocks are adjustments to that change in stress. They are always less strong than the main stress. They are always less strong than the main shock, but they may cause more damage to shock, but they may cause more damage to weakened structures.weakened structures.

Duration of Shaking VariesDuration of Shaking Varies 1960 San Francisco: 40 seconds 1960 San Francisco: 40 seconds 1989 Loma Prieta: 15 seconds 1989 Loma Prieta: 15 seconds 1962 Alaska: 4 minutes!!!!1962 Alaska: 4 minutes!!!!

Earth material can either Earth material can either foldfold or or faultfault when put when put

under stressunder stress

If the Earth folds, it is said If the Earth folds, it is said to be to be ductileductile..

If the Earth faults, it is said If the Earth faults, it is said to be to be brittlebrittle..

Normal Reverse

The following animations The following animations are types of faults in are types of faults in

motion: motion: Normal, Reverse/thrust, aNormal, Reverse/thrust, and Strike Slipnd Strike Slip

There are 2 types of There are 2 types of Earthquake Waves: Earthquake Waves: Body Body

Waves and Surface Waves and Surface WavesWaves

Body WavesBody Waves: Because : Because they travel through the they travel through the

body of the earth.body of the earth.

Body WavesBody WavesP wavesP waves: : PPressure or compressional waves. Vibrate ressure or compressional waves. Vibrate

pparallel to direction of wave travel like a arallel to direction of wave travel like a slinky.slinky.

Fast travel: 4-7 km/sec (15,000 mph) Fast travel: 4-7 km/sec (15,000 mph) PP is is pprimary, or first wave to arrive at rimary, or first wave to arrive at

recording station recording station

Body WavesBody WavesS wavesS waves: : SShear waves. Vibrates perpendicular to hear waves. Vibrates perpendicular to

direction of wave travel. Like direction of wave travel. Like ssnapping a rope napping a rope

SSlower than P wave: 2-5 km/sec (11,000 mph)lower than P wave: 2-5 km/sec (11,000 mph)

So So SS is is ssecondary, or second wave to arrive at econdary, or second wave to arrive at recording station recording station

Body WavesBody Waves

S wavesS waves: : SShear waves. Vibrates hear waves. Vibrates

perpendicular to perpendicular to direction of wave direction of wave travel. Like travel. Like ssnapping a napping a rope rope

SSlower than P wave: 2-lower than P wave: 2-5 km/sec (11,000 5 km/sec (11,000 mph)mph)

So So SS is is ssecondary, or econdary, or second wave to arrive second wave to arrive at recording station at recording station

Earthquake waves in reviewEarthquake waves in review

Examples of Body Waves Examples of Body Waves on a Seismogramon a Seismogram

Surface WavesSurface Waves: Because : Because they travel along the they travel along the surface of the Earth. surface of the Earth. These waves travel These waves travel

slower than Body Waves slower than Body Waves and are very destructive.and are very destructive.

Love Waves: Love Waves: cause cause particles to move from particles to move from

side to side, perpendicular side to side, perpendicular to direction of travel, to direction of travel,

similar to S wave.similar to S wave.

Rayleigh Waves: Rayleigh Waves: travel travel more slowly than Love more slowly than Love

Waves and cause Waves and cause particles to move in particles to move in

elliptical patternselliptical patterns

Types of WavesTypes of Waves

Earthquake waves = seismic waves. Earthquake waves = seismic waves. Recorded on seismometers on Recorded on seismometers on seismographs.seismographs.

Types of WavesTypes of Waves Surface wavesSurface waves - travel on Earth's - travel on Earth's

surface, away from epicenter. surface, away from epicenter. • Very slow waves. Cause a lot of damage, Very slow waves. Cause a lot of damage,

rolling feeling at end of earthquake rolling feeling at end of earthquake Body WavesBody Waves - travel through Earth's - travel through Earth's

interior, spread outward from focusinterior, spread outward from focus

Earthquake waves in reviewEarthquake waves in review

Seismogram Tracings

Locating the Epicenter of an Locating the Epicenter of an EarthquakeEarthquake

P, S and surface waves all start out at same P, S and surface waves all start out at same time. time.

The further you are away from the quake, the The further you are away from the quake, the longer the time span between arrival of P and longer the time span between arrival of P and S wave. S wave.

The distance of the seismometer to the The distance of the seismometer to the earthquake can be determined by the time earthquake can be determined by the time between the arrival of P wave and arrival of S between the arrival of P wave and arrival of S waves. waves.

Can tell the distance, Can tell the distance, but not the directionbut not the direction. . Therefore, at least 3 sites must be used to Therefore, at least 3 sites must be used to

find epicenter.find epicenter.

Studying Earth’s InteriorStudying Earth’s Interior

Travel of P and S Waves Travel of P and S Waves Through the Body of the Through the Body of the

EarthEarth

How did geophysicists How did geophysicists discover what was inside of discover what was inside of our earth?our earth?

Travel of P and S Waves Travel of P and S Waves Through the Body of the Through the Body of the

EarthEarth

They They discovereddiscovered that P that P waves will travel through waves will travel through solid and liquid, whereas S solid and liquid, whereas S Waves will only travel Waves will only travel through a solid.through a solid.