Earthquakes and Faults

• Earthquake: Sudden release of stored energy (stress) caused by movement along a fault.
• Fault: Fracture in the Earth where two sides move relative to each other.

Fault Components

• Hanging wall: Above the fault.
• Footwall: Below the fault.

Types of Faults

• Dip-slip faults: Dominated by vertical movement.
  • Normal fault: Hanging wall moves down due to extensional forces.
  • Reverse fault: Hanging wall moves up due to compressional stress.
• Strike-Slip Faults: Move side by side.
  • Right lateral: Moves to the right when looking across the fault.
  • Left lateral: Moves to the left when looking across the fault.
• Hypocenter (or focus): Location within the earth where the earthquake occurs.
• Epicenter: Point directly above the hypocenter on the earth's surface.
• Seismograph: Instrument that records passing energy waves (seismic waves).
• Seismogram: Actual recording of the passing waves.
• Wavelength and amplitude describe seismic waves.

Seismic Waves

• Body waves: Pass through the entire Earth.
  • P (primary) waves: Fastest waves, first to reach a recording station.
  • S (secondary) waves: Reach a recording station after primary waves.
• Surface waves: Move near the surface only.
  • Love waves: Side-to-side movement, cause most destruction.
  • Rayleigh waves: Elliptical motion.
• Earthquake location requires distance from three stations.

Earthquake Magnitude

• Measurement of earthquake strength based on the largest amplitude wave and distance from epicenter.
• Richter scale: Log scale; a Richter magnitude increase of one number increases energy by about 32 times and amplitude by 10.
• Large earthquakes are part of a series over years.
  • Mainshock: Largest event in the series.
  • Foreshocks: Smaller events preceding the mainshock.
  • Aftershocks: Smaller events following the mainshock.
• Seismic wave velocity depends on the material it moves through.
• Shaking is amplified when tall buildings shake at the same period as the seismic waves.
• Mercalli intensity scale: Quantifies what people feel during an earthquake and the amount of damage; scale ranges from I to XII.

Factors Affecting Damage

• Earthquake magnitude: Larger earthquakes increase death and damage.
• Distance from hypocenter: Closer earthquakes usually cause more damage.
• Type of rock or sediment: Soft sediments amplify shaking.
• Building style: Rigid short buildings amplify shaking.
• Duration of shaking: Longer shaking causes more damage; larger earthquakes have longer durations.

Earthquakes and Plate Tectonics

• Most earthquakes are explained by plate tectonics.
• Divergent plate boundaries: Earthquakes are typically small and non-threatening.
• Transform plate boundaries: Generate large earthquakes.
• Convergent plate boundaries: Generate the largest earthquakes.

Convergent Boundaries

• Seismic-Gap Method: Unmoved fault segments are expected to move next to fill the gaps if other segments have moved recently.
• Mexico City, 1985: Long-Distance Destruction
• 350 km from Mexico City with intense damage.
• The Michoacan seismic gap was expected to produce an earthquake.
• 9,000+ fatalities due to building collapses.
• Resonance occurred between seismic waves, soft lake-sediment foundations, and improperly designed buildings.

Earthquakes in the United States

• Western US earthquakes result from plate interaction between the North America and Pacific plates.
• San Andreas fault: Transform boundary causing earthquakes.
  • The San Andreas fault is not straight, which can lead to compressional stress and thrust faults.
  • Big Bend near Los Angeles (Northridge 1994).
• Damaging earthquakes can result from normal faulting (tensional stress).
• Long-term predictions are fairly good based on paleoseismology.
• Short-term predictions are not possible due to the unpredictable behavior of faults.

Human-Triggered Earthquakes

• Pumping fluids underground and disposal wells of drilling can lead to small earthquakes.

Seismicity of the United States

• Western North America: Plate Boundary-Zone Earthquakes
  • Due to subduction of small plates and continued effects of the overridden Farallon plate.
  • Includes California into Utah (Western Great Basin, Basin and Range), eastern edge around Salt Lake City (Wasatch Front Mountains), and the Rio Grande Rift in New Mexico.
• Intraplate Earthquakes: “Stable” Central United States
  • Clusters of earthquakes at few locations in the central U.S.
  • Away from active plate edges; represents ancient rifts similar to the Rio Grande Rift.
  • Fewer earthquakes, but can be large.
• New Madrid, Missouri, 1811 to 1812: Future earthquakes will affect St. Louis, Memphis, and Nashville.
  • Buildings not designed for earthquake shaking.
  • Soft sediments will amplify ground shaking.
  • Large area will be subject to shaking.
• Intraplate Earthquakes: Eastern North America
  • Similar to central quakes, mostly occur at sites of ancient rift valleys.
  • Charleston, South Carolina, 1886: Largest earthquake on the east coast.

Volcanic Hazards and Plate Tectonics

• Most volcanism is associated with plate boundaries.
  • 73% at spreading centers.
  • 15% at subduction zones.
  • 12% above hot spots.
• Variations in magma's chemical composition, ability to flow, gas content, and volume determine eruption style.
• The 3 V’s: Viscosity, Volatiles, Volume.

Viscosity

• Viscosity: Resistance to flow, high silica content magma has high viscosity.
  • Lower viscosity means more fluid behavior.
  • Higher viscosity means thicker magma.
  • Basaltic magma has the lowest viscosity.
  • Rhyolitic magma has the highest viscosity.

Volatiles

• Volatiles: Dissolved gases, more gases mean more explosive eruptions.
  • As pressure decreases, gases become bubbles that expand rapidly, leading to violent eruptions.
  • Basaltic magma has low dissolved gases, causing peaceful eruptions.
  • Rhyolitic magma has the highest dissolved gases, causing violent eruptions.

Volume

• Volume: Large volumes create more hazard.
  • Spreading center volcanoes have relatively peaceful eruptions.
  • Subduction zone volcanoes explode violently.

Magma Types

• Nonexplosive: Icelandic and Hawaiian eruptions.
• Somewhat explosive: Strombolian eruptions.
• Explosive: Vulcanian and Plinian eruptions.
• Low-water content, low-viscosity magma results in nonexplosive eruptions.
• High-water content, high-viscosity magma results in explosive eruptions.

Volcanic Landforms

• Shield Volcano: Low viscosity, low volatiles, large volume = peaceful.
• Flood basalts: Low viscosity, low volatiles, very large volume, connected to mass extinctions.
• Scoria (Cinder) Cone: Low/medium viscosity, low/median volatiles, small volume.
• Stratovolcano: Medium/high viscosity, medium/high volatiles, large volume = explosive.
• Caldera: High viscosity, high volatiles, very large volume = explosive.

Volcanism at Subduction Zones

• Most of the world’s famous volcanoes are subduction zone volcanoes.
• Many regions around subduction zone volcanoes are heavily populated.
• Erupt directly into the atmosphere, impacting worldwide climate.
• Cascade Range, Pacific Coast of the United States and Canada.

Volcanic Processes and Killer Events

• Pyroclastic Flows: 29%, superhot, high-speed turbulent cloud of ash, gas, and air; deadly.
• Tsunami: 21% of fatalities.
• Lahar: 15%, volcanic mudflows; Mount Rainier is on alert.
• Indirect (famine): 23%
• Gas: 1%
• Lava flow