Earthquakes

Chapter 19

Earthquakes

Natural vibrations of ground caused by movement along fractures in Earth’s crust or by volcanic eruptions

Stress and Strain

Stress- forces per unit area exceed the strength of the rocks involvedCompression-Decreases VolumeTension- Pulls material apartShear- Causes Twisting

Strain- Deformation of materials caused by stress

Faults- fracture of Earth’s crust

Reverse- Horizontal compression, Causes shortening, Foot wall goes down

Faults- Con’t

Normal- Horizontal tension, Causes extension, Foot wall goes up

Faults- Con’t

Strike-Slip- Horizontal Shear, causes offset

Waves

Primary- P- Pressure Wave Secondary- S- Side-to-side wave Surface Wave- L-Wave

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Location of Earthquakes

Focus- point where an earthquake originates

Epicenter- Place on Earth's surface where EQ occurred

Seismic Waves

Seismology- Study of Earthquake waves Seismograph- instrument for recording

earthquake waves

Waves

P wave- arrives 1st

S wave- arrives 2nd L wave- last and largest

Clues to Earth’s Interior

Seismic waves change speed and direction when they encounter different materialsP and S waves travel fairly straight through

the mantleP waves strike the core and are bent (11,000

Km from focus) Re-emerge at 16,000 km from focus, after shadow

Clues to Earth’s Interior- Con’t

S-waves cannot travel through liquids so they can’t pass through the core (never beyond shadow)

P waves speed up through solids

Measuring and Locating EQ

Magnitude- amount of energy releasedRichter Scale- size of largest seismic waves

released by quake Each # increase is a 10 fold increase in energy Each increase in amplitude (height of wave) is a 32

time increase in energy

Measuring and Locating EQ- Con’t

Moment Magnitude Scale- uses size of fault rupture, amount of movement and rock’s stiffness

Modified Mercalli Scale- uses damage to buildings at location of worst damage Uses I-XII Roman Numerals- Called Intensity Measures Surface Waves

Depth of Focus- Deeper EQ usually less intense

Measuring and Locating EQ-Con’t

Locating EQ- time and location unknownDistance determined by time between P wave

and S wave--- AT 3 STATIONSTime calculated using same P-S interval

Seismic Belts

Earthquakes not randomRing of Fire- Edge of Pacific Plate- 80% of all

EarthquakesMediterranean- Asian Belt- 15% of EQ

Earthquakes and Society

Earthquake Hazards Structural failure

Ground floor fails → Upper Floors Fail → Pan caking Height- 5-15 stories causes vibrations which are similar to

natural sway; Shorter and taller buildings are different Land/Soil Fracture

Landslides- Common in saturated areas Soft ground conducts waves well → more damage

Fault Scarps- vertical offsets → waterfalls, road problems

Earthquakes and Society- Con’t

Tsunami- ocean waves created by EQTravels fast and are low (less than 4 feet

high) in open ocean, but pile up hundreds of feet tall near shore

Ocean recedes dramatically before tsunami

Earthquake Risk

Here we have a 3% risk over 50 years Hot Spot in US:

CARocky MountainsWA, ORMO