GEOL: CHAPTER 8 Earthquakes and Earth’s Interior

LO1: Explain Elastic Rebound Theory LO2: Describe seismology LO3: Identify where earthquakes occur, and how often LO4: Identify different seismic waves LO5: Discuss how earthquakes are located Learning Outcomes

LO6: Explain how the strength of an earthquake is measured LO7: Describe the destructive effects of earthquakes LO8: Discuss earthquake prediction methods LO9: Discuss earthquake control methods Learning Outcomes, cont.

Earthquake: shaking or trembling of the ground caused by the sudden release of energy, usually as a result of faulting, which involves the displacement of rocks along fractures Aftershocks: from continued adjustments along a fault; usually smaller than the initial quake Earthquakes

Rocks undergoing deformation bend and store energy When strength of rock is exceeded, they rupture and release energy – the earthquake Rocks rebound to original, undeformed shape Elastic Rebound Theory

Fig. 8-1a, p. 151 Rocks rebound to original undeformed shape Deformation Rupture and release of energy Fence Original position Fault Stepped Art

Seismology: the study of earthquakes Seismic waves: energy from earthquakes Seismographs: detect, record, and measure earthquakes Seismogram: record from a seismograph Earthquakes occur along faults, where movement is stored as energy in rocks Most faults related to plate movements Seismology

Focus: point where energy is first released Epicenter: point on surface above focus Shallow-focus: 0-70 km below surface Intermediate focus: km below surface Deep-focus: >300 km below surface 90% less than 100 km below surface Focus and Epicenter

Divergent and transform boundaries: always shallow-focus Convergent boundaries: –shallow-, intermediate-, and deep-focus –Beniorr-Wadati zones: foci along subducted plate Earthquakes and Plate Boundaries

Plate boundaries: convergent, divergent, and transform 80% in circum-Pacific belt 15% in Mediterranean-Asian belt 5% in plate interiors and ocean spreading ridges Intraplate: from compression of plate along margins Major Earthquake Regions

All waves generated by an earthquake Body waves –P-waves –S-waves –Travel faster through less dense, more elastic rocks Surface waves –R-waves –L-waves Seismic Waves

Primary waves Fastest seismic waves Travel through solids, liquids, and gases Compressional/push-pull: expand and compress material, like sound waves P-Waves

Secondary waves Slower than P-waves Travel only through solids Shear waves: move material perpendicular to direction of wave movement Create shear stresses S-Waves

Fig. 8-7, p. 156 Undisturbed material Focus Surface Primary wave (P-wave) Compression Expansion Undisturbed material Direction of wave movement Stepped Art Secondary wave (S-wave) Wavelength

Travel at or just below the surface Slower than body waves R-waves (Rayleigh waves) –Particles move in elliptical path, like water waves L-waves (Love waves) –Faster than R-waves –Particles move back forth in horizontal plane perpendicular to direction of travel Surface Waves

Fig. 8-8, p. 157 Love wave (L-wave) Undisturbed material Rayleigh wave (R-wave) Rayleigh wave Love wave Stepped Art

P-wave and S-wave average speeds are known Time-distance graphs: difference in arrival time of the 2 waves vs. distance from focus The farther the waves travel, the greater the P-S time interval Epicenter can be determined when the P-S time intervals of at least three seismic stations are known Locating an Earthquake

Subjective measure of earthquake damage and human reaction Modified Mercalli Intensity Scale Maps with intensity zones Earthquake Intensity

Factors that affect earthquake intensity 1.Size of earthquake 2.Distance from epicenter 3.Focal depth 4.Population density 5.Geology of area 6.Building construction 7.Duration of shaking Earthquake Intensity, cont.

Quantitative measure: amount of energy released Richter Magnitude Scale: total amount of energy released at earthquake source Measure amplitude of largest seismic wave Logarithmic: each whole-number increase is a 10-fold increase in amplitude, but a 30-fold increase in energy Earthquake Magnitude

Richter Magnitude Scale underestimates energy of very large quakes –Only measures peak energy, not duration Seismic-moment magnitude scale: –Strength of rocks –Area of fault rupture –Amount of movement of rocks adjacent to fault 1964 Alaska earthquake: –8.6 Richter –9.2 seismic-moment Earthquake Magnitude, cont.

Deaths, injuries, property damage: –Work or school hours –Population density –Magnitude –Duration –Distance from epicenter –Type of structures –Local geological characteristics Earthquake Destruction

Destructive effects: –Ground shaking –Fire –Seismic sea waves (tsunamis) –Landslides –Disruption of services –Panic and psychological shock Earthquake Destruction, cont.

Magnitude and distance Underlying materials –Poorly consolidated and water-saturated materials experience stronger S-waves –Liquefaction: water-saturated sediments behave as a fluid Poor building materials: adobe, mud, brick, poorly built concrete Most common cause of fatalities/injuries Ground Shaking

Common in urban areas 1906 San Francisco earthquake –Severed electrical and gas lines –Fires spread throughout city –Water mains ruptured, so couldn’t put out fires 1923 Tokyo earthquake –71% of houses burned Fire

Indian Ocean: 12/26/2004; 9.0 Seismic sea wave, not tidal wave Caused by: –Submarine earthquakes –Submarine volcanoes –Submarine landslides Can travel across entire oceans Tsunami

Travel at ~600 mph Wave height less than 1 meter Wave length of several hundred miles Shallow water: wave slows and wave height increases 1946 Hilo tsunami: 16.5 m high Tsunami, cont.

Prior: sea withdraws, exposing the seafloor Pacific Tsunami Early Warning System –Seismographs –Instruments that detect seismic sea waves No warning system in the Indian Ocean Tsunami, cont.

Earthquake-triggered landslides Very dangerous in mountain regions Cause many deaths and much damage 1959 Madison Canyon slide 1970: Peru earthquake triggered avalanche that killed 66,000 people Ground Failure

Successful prediction must include: 1.Time frame 2.Location 3.Strength Successful predictions are rare Successful predictions would save lives Seismic hazard maps help U.S., China, Japan, Russia have programs Earthquake Prediction

Plotting locations of earthquakes Locate seismic gaps on fault Slight changes in elevation Tilting of surface Water level fluctuations Changes in Earth’s magnetic field Electrical resistance of ground Earthquake Precursors

Prevention unlikely But may be able to gradually release energy stored in rocks Geologists can potentially inject liquids into locked segments and seismic gaps of faults to release small quakes; but could also cause a big quake Earthquake Control