Natural Hazards & Disasters: Earthquakes and Their Causes

Natural Hazards & Disasters: Earthquakes and Their Causes

The Devastation of Earthquakes

  • Port-au-Prince, Haiti Earthquake (January 12, 2010)

    • Occurred at 4:53extpm4:53 ext{ pm}.

    • Magnitude 7.07.0 earthquake.

    • Resulted in 46,000−92,00046,000 - 92,000 fatalities.

    • Characterized by a sudden jolt followed by approximately 3535 seconds of continuous violent shaking.

    • The sky turned gray due to vast clouds of dust from collapsing buildings.

Faults and Their Role in Earthquakes

  • Definition of Faults

    • Faults are fractures within the Earth's crust along which rocks on one side move past rocks on the other side.

  • Measurement of Fault Displacement

    • Displacement is measured by the amount of movement.

    • Example: Over several million years, the west side of the San Andreas Fault (SAF) has moved approximately 450extkm450 ext{ km} to the north.

  • Earthquake Generation

    • Some faults generate earthquakes when they shift, while others produce almost no seismic activity.

  • Location of Faults

    • Most faults are located along plate boundaries.

    • A smaller number of faults are found in the stable interior regions of continents.

  • Fault Detection

    • Faults are sometimes first detected by an earthquake itself.

    • They can also be marked by a geological feature called a fault scarp, which is a small step or offset on the ground surface where one side of a fault has moved vertically with respect to the other.

Types of Faults

  • Normal Faults

    • Associated with crustal extension (pulling apart).

    • Involves rocks above a steeply-inclined fault surface slipping down and over the rocks beneath the fault surface.

  • Reverse Faults

    • Associated with crustal compression (pushing together).

    • Involves rocks above a steeply-inclined fault surface slipping up and over the rocks beneath the fault surface.

  • Thrust Faults

    • Are a type of reverse fault.

    • Distinguished by a more gently-inclined surface (lower dip angle) compared to typical reverse faults.

  • Blind Thrusts

    • Are thrust faults that do not reach the Earth's surface.

  • Strike-Slip Faults

    • Characterized by a vertical fault surface.

    • Rocks on one side of the fault slip laterally (horizontally) past rocks on the other side.

    • Right-lateral strike-slip fault: Rocks on the far side of the fault slip to the right.

    • Left-lateral strike-slip fault: Rocks on the far side of the fault slip to the left.

Orientation of Rock Layers and Faults: Strike and Dip

  • Strike

    • The compass direction of a horizontal line on the surface of a rock layer or fault plane.

  • Dip

    • The inclination angle of the surface, measured perpendicularly to the strike direction.

    • It is measured downwards from the horizontal plane.

Causes of Earthquakes: The Elastic Rebound Theory

  • The 1906 San Francisco Earthquake Investigation

    • A commission, led by Andrew Lawson, was appointed to determine the cause of earthquakes after the great San Francisco earthquake of 19061906.

    • Lawson and his students had previously mapped the San Andreas Fault (SAF) but did not initially recognize its role in generating earthquakes.

    • Following the 19061906 earthquake, it was discovered that the west side of the SAF had shifted northward by as much as 7extm7 ext{ m}.

  • Elastic Rebound Theory

    • Hypothesis: The commission theorized that the two sides of the SAF had been