Understanding Earthquakes and Faults

What Causes Earthquakes?

• Earthquakes are caused by the rapid release of energy, often from tectonic plate movement.
• Earthquakes occur daily, with over a million detectable events annually.
• Other causes of earthquakes include:
  • Sudden slip along a new or existing fault.
  • Movement of magma in a volcano.
  • Giant landslides.
  • Nuclear detonations.

Components of Earthquakes

• The hypocenter (focus) is where fault slip occurs, typically on a fault surface.
• Earthquake waves expand outward from the hypocenter.
• The epicenter is the point on the Earth's surface directly above the hypocenter.

Faults in the Crust

• Faults are breaks or fractures in the Earth's crust.
• Active faults are those where ongoing stress produces motion.
• Inactive faults experienced motion in the geological past.
• A fault trace represents the intersection of a fault with the ground surface.
• Displacement at the surface creates a fault scarp.
• Some faults, known as blind faults, do not extend to the surface.

Generating Earthquake Energy

• Tectonic forces apply stress (push, pull, or shear) to rocks.
• Rocks bend slightly without breaking due to their elastic properties.
• Continued stress leads to crack development and growth within rocks.
• Eventually, cracking leads to failure (faults = brittle deformation), releasing stored elastic energy at once.
• Stick-slip behavior describes the cyclical process of fault movement:
  • Stick phase: friction prevents motion.
  • Slip phase: Friction is briefly overcome by motion.

Seismic Waves

• Seismic waves are energy waves emanating from the focus of an earthquake.
• Body waves travel through Earth's interior:
  • P-waves (primary or compressional waves):
    • Travel by compressing and expanding material.
    • Material moves back and forth parallel to wave direction.
    • Fastest seismic waves, traveling through solids, liquids, and gases.
  • S-waves (secondary or shear waves):
    • Travel by moving material back and forth.
    • Material moves perpendicular to wave travel direction.
    • Slower than P-waves, traveling only through solids.
• Surface waves travel along Earth's exterior:
  • L-waves (Love waves):
    • S-waves intersecting the land surface.
    • Move the ground back and forth like a snake.
    • Slowest, but most destructive, surface waves.
  • R-waves (Rayleigh waves):
    • P-waves intersecting the land surface.
    • Cause the ground to ripple up and down like water.

How Do We Measure Earthquakes?

• Seismometer: instrument that records ground motion.
• A weighted pen on a spring traces the movement of the frame.
  • Vertical motion: Records up-and-down movement.
  • Horizontal motion: Records back-and-forth movement.
• Seismogram: Data recording earthquake wave behavior.
  • Earthquake waves arrive at a station in a specific order:
    • P-waves first.
    • S-waves second.
    • Surface waves last.
  • Arrival times determine the distance to the epicenter.

Finding the Epicenter

• P-waves consistently arrive before S-waves.
• The time separation between P-wave and S-wave arrivals increases with distance from the epicenter.
• Triangulation: Data from three or more seismic stations pinpoints the epicenter by intersecting circles.

Earthquake Size

• Earthquake size is described using two measurements:
• Intensity (Mercalli Scale):
  • Measures the severity of damage.
  • Damage is assigned Roman numerals (I = low, XII = high).
  • Damage occurs in zones and diminishes with distance.
• Magnitude (Moment Magnitude Scale):
  • Most accurate measure of earthquake size.
  • Measures the maximum amplitude of ground motion.
  • Magnitude scales are logarithmic:
    • M6.0 is 10 times larger than M5.0.
    • M6.0 is 100 times larger than M4.0.
    • M8.0 is 10,000 times larger than M4.0.

Earthquake Occurrence

• Earthquakes are concentrated along plate tectonic boundaries.
• Divergent boundaries:
  • Shallow earthquakes (mid-ocean ridges).
• Transform boundaries:
  • Shallow earthquakes.
• Convergent boundaries:
  • Shallow, intermediate, and deep earthquakes.

Earthquakes at Plate Boundaries

• Divergent plate boundaries (mid-ocean ridges):
  • Two types of faulting occur.

This quiz explores the causes and components of earthquakes, including the role of faults in the Earth's crust. Test your knowledge on how tectonic movements, volcanic activity, and other factors contribute to seismic events, as well as key terminology like hypocenter and epicenter.