Chapter 19 Notes

Earthquakes

Stress and Strain

• Earthquakes occur when stress builds up and causes movement in the Earth’s crust

• Compression: squeezing forces• Tension: pulling forces• Shear: opposing parallel forces/twisting• Elastic deformation: initial response to stress• plastic deformation occurs under high stress

just before breaking

Faults

• Normal fault: produced by tension forces

• Reverse fault: produced by compression forces

• Strike-slip fault: produced by shear forces

Earthquake Waves

• Body Waves– Primary (P-waves): compression waves; move

parallel to the direction of wave motion– Secondary (S-waves): transverse waves; move at

right angles to the direction of wave motion

• Surface waves: move sideways and up and down

Earthquake Waves

• P-waves move fastest

• Surface waves are slowest, last the longest, and are the most destructive

Earthquake location

• Focus: the origin of the EQ waves

• Epicenter: “above” the center

Earthquake Measurement

• Seismometer: an instrument that records EQ waves

• Seismogram: the record of EQ waves

Time travel curves

• The difference in arrival time of the P-waves and S-waves can show how far the waves have travelled.

Clues to Earth’s Interior

• EQ waves are reflected and refracted in different ways by the different materials inside the Earth

Measuring Earthquakes

• Richter Scale: – measures magnitude – energy released– numeric scale (no true upper limit)

• Modified Mercalli Scale: – measures intensity– based on the amount of damage– Roman numerals I to XII

Richter Scale

Mercalli Isoseismic Map

Locating an Earthquake

• The P-wave and S-wave arrival times determine the distance a station is to an EQ

• Three stations are required to locate the epicenter of an EQ

Seismic Belts

• Circum-Pacific and Mediterranean-Asian belts

Earthquake Hazards

• “Earthquakes don’t kill people, buildings do.”• Structural failures– Brittle building materials cause more damage– Building height oscillations

• Land and soil failure– Soil liquefaction

• Tsunami – Ocean wave generated by sea floor movement

U.S. Earthquake Risk