Earthquakes and Earth’s Interior GEOL: CHAPTER 8 Earthquakes and Earth’s Interior

This shop in Olema, California, is rather whimsically called the Epicenter, alluding to the fact that it is in the San Andreas fault zone.

Learning Outcomes 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, cont. 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. LO10: Describe Earth's interior LO11: Examine Earth's core LO12: Examine Earth's mantle LO13: Describe Earth's internal heat LO14: Examine earth's crust

Earthquakes 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

Elastic Rebound Theory 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

Figure 8.1 The Elastic Rebound Theory

Figure 8.1 The Elastic Rebound Theory

Figure 8.1 The Elastic Rebound Theory

Seismology 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

Figure 8.2 Seismographs

Figure 8.2 Seismographs

Figure 8.2 Seismographs

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

Figure 8.3 The Focus and Epicenter of an Earthquake

Figure 8.3 The Focus and Epicenter of an Earthquake

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

Earthquake Epicenters and Plate Boundaries This map of earthquake epicenters shows that most earthquakes occur within seismic zones that correspond closely to plate boundaries. Approximately 80% of earthquakes occur within the circum-Pacific belt, 15% within the Mediterranean–Asiatic belt, and the remaining 5% within plate interiors and along oceanic spreading ridges. The dots represent earthquake epicenters and are divided into shallow-, intermediate-, and deep-focus earthquakes. Along with many shallow-focus earthquakes, nearly all intermediate- and deep-focus earthquakes occur along convergent plate boundaries. Figure 8.4 Earthquake Epicenters and Plate Boundaries This map of earthquake epicenters shows that most earthquakes occur within seismic zones that correspond closely to plate boundaries. Approximately 80% of earthquakes occur within the circum-Pacific belt, 15% within the Mediterranean–Asiatic belt, and the remaining 5% within plate interiors and along oceanic spreading ridges. The dots represent earthquake epicenters and are divided into shallow-, intermediate-, and deep-focus earthquakes. Along with many shallow-focus earthquakes, nearly all intermediate- and deep-focus earthquakes occur along convergent plate boundaries.

Benioff Zones Focal depth increases in a well-defined zone that dips approximately 45 degrees beneath the Tonga volcanic arc in the South Pacific. Dipping seismic zones are called Benioff or Benioff–Wadati zones. Figure 8.5 Benioff Zones Focal depth increases in a well-defined zone that dips approximately 45 degrees beneath the Tonga volcanic arc in the South Pacific. Dipping seismic zones are called Benioff or Benioff–Wadati zones.

Major Earthquake Regions 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

Figure 8.6 Earthquake Damage in the Circum-Pacific Belt Damage in Oakland, California, resulting from the October 1989 Loma Prieta earthquake. The columns supporting the upper deck of Interstate 880 failed, causing the upper deck to collapse onto the lower one.

Seismic Waves 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

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

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

Direction of wave movement Undisturbed material Primary wave (P-wave) Compression Expansion Undisturbed material Direction of wave movement Focus Surface Figure 8.7 Primary and Secondary Seismic Body Waves Body waves travel through Earth. Secondary wave (S-wave) Wavelength Stepped Art Fig. 8-7, p. 156

Surface Waves 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

Rayleigh wave (R-wave) Undisturbed material Rayleigh wave (R-wave) Rayleigh wave Love wave Love wave (L-wave) Figure 8.8 Rayleigh and Love Seismic Surface Waves Surface waves travel along Earth’s surface or just below it. Stepped Art Fig. 8-8, p. 157

Locating an Earthquake 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

Figure 8.9 Determining the Distance from an Earthquake

Figure 8.10 Determining the Epicenter of an Earthquake Three seismograph stations are needed to locate the epicenter of an earthquake. The P–S time interval is plotted on a time–distance graph for each seismograph station to determine the distance that station is from the epicenter. A circle with that radius is drawn from each station, and the intersection of the three circles is the epicenter of the earthquake.

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

Earthquake Intensity, cont. Factors that affect earthquake intensity Size of earthquake Distance from epicenter Focal depth Population density Geology of area Building construction Duration of shaking

Earthquake Magnitude 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

Figure 8.11 Richter Magnitude Scale The Richter Magnitude Scale measures the total amount of energy released by an earthquake at its source. The magnitude is determined by measuring the maximum amplitude of the largest seismic wave and marking it on the right-hand scale. The difference between the arrival times of the P- and S-waves (recorded in seconds) is marked on the left-hand scale. When a line is drawn between the two points, the magnitude of the earthquake is the point at which the line crosses the center scale.

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

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

Ground Shaking 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

Figure 8.12 Relationship between Seismic Wave Amplitude and Underlying Geology The amplitude and duration of seismic waves generally increase as the waves pass from bedrock to poorly consolidated or water-saturated material. Thus, structures built on weaker material typically suffer greater damage than similar structures built on bedrock, because the shaking lasts longer.

Figure 8.13 Liquefaction The effects of ground shaking on water-saturated soil are dramatically illustrated by the collapse of these buildings in Niigata, Japan, during a 1964 earthquake. The buildings, which were designed to be earthquake resistant, fell over on their sides intact when the ground below them underwent liquefaction.

Fire 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

Tsunami 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, cont. 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

Ground Failure 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

Figure 8.14 Ground Failure On August 17, 1959, an earthquake with a Richter magnitude of 7.3 shook southwestern Montana and a large area in adjacent states.

Figure 8.14 Ground Failure On August 17, 1959, an earthquake with a Richter magnitude of 7.3 shook southwestern Montana and a large area in adjacent states.

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

Figure 8.15 Global Seismic Hazard Assessment Map The Global Seismic Hazard Assessment Program published this seismic hazard map showing peak ground accelerations. The values are based on a 90% probability that the indicated horizontal ground acceleration during an earthquake is not likely to be exceeded in 50 years. The higher the number, the greater the hazard. As expected, the greatest seismic risks are in the circum-Pacific belt and the Mediterranean–Asiatic belt.

Earthquake Precursors 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

Figure 8.16 Earthquake Precursors Seismic gaps are one type of earthquake precursor that can indicate a potential earthquake in the future. Seismic gaps are regions along a fault that are locked; that is, they are not moving and releasing energy. Three seismic gaps are evident in this cross section along the San Andreas fault from north of San Francisco to south of Parkfield. The first is between San Francisco and Portola Valley, the second near Loma Prieta Mountain, and the third is southeast of Parkfield. The top section shows the epicenters of earthquakes between January 1969 and July 1989. The bottom section shows the southern Santa Cruz Mountains gap after it was filled by the October 17, 1989, Loma Prieta earthquake (open circle) and its aftershocks.

Earthquake Control 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

Figure 8.17 Population Density of San Francisco Downtown San Francisco sits atop the active plate boundary between the Pacific plate and the North American plate. The high density of population poses a risk to residents in the event of an earthquake. Although pumping seismic fluids may relieve the pressure of a fault and prevent major earthquakes from occurring, people in populated areas are reluctant to risk earthquake control, lest a major earthquake be initiated by the process.

Earth’s Interior Crust Mantle Outer core Inner core

Figure 8.18 Earth’s Internal Structure The inset shows Earth’s outer part in more detail. The asthenosphere is solid but behaves plastically and flows.

Seismic Waves and Earth’s Interior P-wave and S-wave velocity determined by density and elasticity of material S-waves don’t travel through liquids Seismic waves change velocity and direction when enter material with different density or elasticity (refraction)

Seismic Waves and Earth’s Interior, cont. Some waves are reflected Calculate depths of boundaries Discontinuity: significant change in materials or their properties

Figure 8.19 Refraction and Reflection of Seismic Waves Refraction and reflection of P-waves as they encounter boundaries separating materials of different density or elasticity. Notice that the only wave ray not refracted is the one perpendicular to boundaries.

Figure 8.20 Seismic Wave Velocities Profiles showing seismic wave velocities versus depth. Several discontinuities are shown, across which seismic wave velocities change rapidly.

Figure 8.20 Seismic Wave Velocities Profiles showing seismic wave velocities versus depth. Several discontinuities are shown, across which seismic wave velocities change rapidly.

Figure 8.20 Seismic Wave Velocities Profiles showing seismic wave velocities versus depth. Several discontinuities are shown, across which seismic wave velocities change rapidly.

The Core P-wave velocity decreases at a depth of 2,900 km: core-mantle discontinuity P-wave shadow zone Weak P-wave energy does penetrate the shadow zone: from solid inner core S-wave shadow zone: shows the outer core is liquid, because S-waves can’t travel through liquids

Figure 8.21 P-Wave and S-Wave Shadow Zones

Figure 8.21 P-Wave and S-Wave Shadow Zones

Core Density and Composition 16.4% Earth volume ~33% of mass Outer core: 9.9 to 12.2 g/cm3 Earth center: pressure 3.5 million times of surface Outer core: iron, sulfur, silicon, oxygen, nickel, potassium Inner core: iron and nickel

Earth’s Mantle Moho: discontinuity about 30 km deep Asthenosphere: P- and S-waves slow down Plastic Magma generation Lithospheric plates ride across it 3.3 to 5.7 g/cm3; probably periodotite

Seismic Discontinuity Andrija Mohorovičić studied seismic waves and detected a seismic discontinuity at a depth of about 30 km. The deeper, faster seismic waves arrive at seismic stations first, even though they travel farther. This discontinuity, now known as the Moho, is between the crust and mantle. Figure 8.22 Seismic Discontinuity Andrija Mohorovičić studied seismic waves and detected a seismic discontinuity at a depth of about 30 km. The deeper, faster seismic waves arrive at seismic stations first, even though they travel farther. This discontinuity, now known as the Moho, is between the crust and mantle.

Earth’s Internal Heat Geothermal gradient: 25ºC/km Greater in active volcanic regions Most heat generated by radioactive decay Regions of equilibrium temperature Base of crust: 800ºC to 1200ºC Core-mantle boundary: 2,500ºC -5,000ºC

Continental Crust Granitic composition 2.5 to 3.0 g/cm3; average = 2.7 g/cm3 20 to 90 km thick; average = 35 km thick Thickest under large mountain ranges

Oceanic Crust Gabbro overlain by basalt Average density = 3.0 g/cm3 5-10 km thick Thinnest at spreading ridges