Chapter 11: EarthquakesChapter 11: Earthquakes

IntroductionIntroduction Earthquake:

Vibration of the Earth produced by rapid release of energy

Most often caused by slippage along a fault Energy is in the form of waves

Focus: AKA Hypocenter Source of earthquake from which energy is

released in all directions Epicenter:

Surface location directly above the focus Seismometer:

Instrument sensitive to vibrations Located worldwide & used to measure

earthquakes

Earthquake: Vibration of the Earth produced by rapid

release of energy Most often caused by slippage along a fault Energy is in the form of waves

Focus: AKA Hypocenter Source of earthquake from which energy is

released in all directions Epicenter:

Surface location directly above the focus Seismometer:

Instrument sensitive to vibrations Located worldwide & used to measure

earthquakes

Focus & EpicenterFocus & Epicenter

Elastic ReboundElastic Rebound Stress applied

along pre-existent fault

Stress builds up, straining the rock

Eventually, frictional resistance is overcome

Slippage occurs, releasing the built up strain

Vibrations (earthquake) occur as the rock elastically returns to original shape

Stress applied along pre-existent fault

Stress builds up, straining the rock

Eventually, frictional resistance is overcome

Slippage occurs, releasing the built up strain

Vibrations (earthquake) occur as the rock elastically returns to original shape

Foreshocks & AftershocksForeshocks & Aftershocks

Small earthquakes, called foreshocks, often precede a major earthquake by days or, in some cases, by as much as several years

Adjustments that follow a major earthquake often generate smaller earthquakes called aftershocks

Small earthquakes, called foreshocks, often precede a major earthquake by days or, in some cases, by as much as several years

Adjustments that follow a major earthquake often generate smaller earthquakes called aftershocks

San Andreas FaultSan Andreas Fault Best studied fault system Fault creep:

Slow, gradual displacement

Some segments slip at ~regular intervals Results in small to

moderate quakes Stick-slip motion:

Storing elastic energy for hundreds of years before rupturing in great earthquakes

Quakes every 50 to 200 years

Best studied fault system Fault creep:

Slow, gradual displacement

Some segments slip at ~regular intervals Results in small to

moderate quakes Stick-slip motion:

Storing elastic energy for hundreds of years before rupturing in great earthquakes

Quakes every 50 to 200 years

SeismologySeismology Study of earthquake waves Dates back 2000 years to the

Chinese Seismographs:

Instruments which record earth’s movement relative to stationary mass

Stationary due to high inertia

Study of earthquake waves Dates back 2000 years to the

Chinese Seismographs:

Instruments which record earth’s movement relative to stationary mass

Stationary due to high inertia

Seismographs

Seismographs

Vertical motion

Horizontal motion

Parts of WavesParts of Waves

Frequency = 1/T

Types of seismic waves: P-waves S-waves Surface waves

Frequency = 1/T

Types of seismic waves: P-waves S-waves Surface waves

T

P-wavesP-waves Primary waves Compressional waves ( AKA Push-pull

waves) Travel through liquids & solids

Primary waves Compressional waves ( AKA Push-pull

waves) Travel through liquids & solids

S-wavesS-waves Secondary waves Shake waves Travel only through solids

Secondary waves Shake waves Travel only through solids

Surface WavesSurface Waves Travel along Earth’s surface Greatest amplitude, slowest velocity Cause greatest destruction AKA Long waves or L-waves

Travel along Earth’s surface Greatest amplitude, slowest velocity Cause greatest destruction AKA Long waves or L-waves

Locating the SourceLocating the Source Epicenter located from difference in arrival

time of P-wave and S-wave Requires seismograms from three locations

AKA Triangulation

Epicenter located from difference in arrival time of P-wave and S-wave

Requires seismograms from three locations AKA Triangulation

TriangulationTriangulation

Practice TriangulationPractice TriangulationLocation

Time Interval

Distance (km)

Distance (cm)

Cape Town

13 min 30 sec

Rio de Janeiro

10 min10 sec

Luanda, Angola

7 min45 sec

0

60

120

180

240

300

360

420

480

540

600

660

720

780

840

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Distance to Epicenter (km)

P-S Interval (seconds)

Practice TriangulationPractice TriangulationLocation

Time Interval

Distance (km)

Distance (cm)

Cape Town

13 min 30 sec

4800

Rio de Janeiro

10 min10 sec

Luanda, Angola

7 min45 sec

0

60

120

180

240

300

360

420

480

540

600

660

720

780

840

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Distance to Epicenter (km)

P-S Interval (seconds)

Practice TriangulationPractice TriangulationLocation

Time Interval

Distance (km)

Distance (cm)

Cape Town

13 min 30 sec

4800

Rio de Janeiro

10 min10 sec

3700

Luanda, Angola

7 min45 sec

0

60

120

180

240

300

360

420

480

540

600

660

720

780

840

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Distance to Epicenter (km)

P-S Interval (seconds)

Practice TriangulationPractice TriangulationLocation

Time Interval

Distance (km)

Distance (cm)

Cape Town

13 min 30 sec

4800

Rio de Janeiro

10 min10 sec

3700

Luanda, Angola

7 min45 sec

2900

0

60

120

180

240

300

360

420

480

540

600

660

720

780

840

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

Distance to Epicenter (km)

P-S Interval (seconds)

Practice TriangulationPractice Triangulation

Location

Time Interval

Distance (km)

Distance (cm)

Cape Town

13 min 30 sec

4800

Rio de Janeiro

10 min10 sec

3700

Luanda, Angola

7 min45 sec

2900

1.2 cm = 1050 km

1.2 cm = x

1050 km distance

Luanda

Practice TriangulationPractice Triangulation

Location

Time Interval

Distance (km)

Radius (cm)

Cape Town

13 min 30 sec

4800 5.5

Rio de Janeiro

10 min10 sec

3700

Luanda, Angola

7 min45 sec

2900

1.2 cm = 1050 km

1.2 cm = x

1050 km distance

Luanda

Practice TriangulationPractice Triangulation

Location

Time Interval

Distance (km)

Radius (cm)

Cape Town

13 min 30 sec

4800 5.5

Rio de Janeiro

10 min10 sec

3700 4.2

Luanda, Angola

7 min45 sec

2900

1.2 cm = 1050 km

1.2 cm = x

1050 km distance

Luanda

Practice TriangulationPractice Triangulation

Location

Time Interval

Distance (km)

Radius (cm)

Cape Town

13 min 30 sec

4800 5.5

Rio de Janeiro

10 min10 sec

3700 4.2

Luanda, Angola

7 min45 sec

2900 3.3

1.2 cm = 1050 km

1.2 cm = x

1050 km distance

Luanda

Measuring Earthquake SizesMeasuring Earthquake Sizes Intensity Scale

Mercalli Intensity Scale Based on destruction of buildings

Magnitude Scales Richter Magnitude (logarithmic) Based on the Amplitude of the largest

seismic wave recorded on seismogram

Intensity Scale Mercalli Intensity Scale Based on destruction of buildings

Magnitude Scales Richter Magnitude (logarithmic) Based on the Amplitude of the largest

seismic wave recorded on seismogram

Mercalli Intensity Scale

Mercalli Intensity Scale

Richter Magnitude Scale

Richter Magnitude Scale

Earthquake DestructionEarthquake Destruction

Seismic Vibrations Tsunami Landslides & Ground Subsidence Fire

Seismic Vibrations Tsunami Landslides & Ground Subsidence Fire

Formation of TsunamisFormation of Tsunamis

Global Distribution of Earthquakes

Global Distribution of Earthquakes

Evidence for Plate TectonicsEvidence for Plate Tectonics