Natural Disasters Week 2

Continental plate collisions

  • Subduction consumes ocean basins

  • Ocean closure ends in continental collision

    • Buoyant continental crust will not subduct

    • Subduction ceases and mountains uplift

  • Example: Himalaya (India running into Asia)

Transform plate boundary

  • Shallow earthquakes

  • No volcanoes

Driving Mechanisms

  • Three forces drive plate motions

  • Convection

  • Ridge Pushing

  • Slab Pull

What is earthquake

  • Earth shaking caused by a rapid release of energy

    • Due to tectonic stress that causes rocks to break

    • Energy moves outward as an expanding sphere of waves

    • This waveform energy can be measured around the globe

  • Earthquakes occur along faults (crustal fractures that provide weakness for rock masses to move)

Stress: force on a body over unit area

Strain: Due to external stress

1. Compression Stress

Rocks are squeezed together.

Shortens and thickens crust.

Common at convergent boundaries (subduction zones, continental collisions).

2. Tension Stress

Rocks are pulled apart.

Stretches and thins crust.

Common at divergent boundaries (mid‑ocean ridges, rift valleys).

3. Shear Stress

Rocks are sliding past each other in opposite directions.

Distorts shape without much volume change.

Common at transform boundaries (like the San Andreas Fault).

Fault Initiation

  • Tectonic forces add stress to unbroken rocks

  • The rocks deform slightly

  • Continued stress will cause growth of cracks

Fault Motion

  • Faults move in jumps

  • Once motion starts, it quickly stops due to friction

  • Eventually, strain will build up again, causing failure

  • This behavior is termed slick slip behavior

  • When rocks break, stored elastic strain is released

  • Associated energy radiates outward

Amount of Displacement

  • Displacement scale varies from small to large

  • Large events: 100s km long

  • Displacement diminishes with distance from epicenter

  • Earthquakes relieve stress in one are and often create stress in another

Speed of Body Waves

  • P wave is fastest

  • S wave is 2nd fastest

  • Surface wave is 3rd

  • Mercalli intensity scale: the degree of shaking based on damage (intensity)

Earthquake Size

  • Magnitude: the amount of energy released

  • Magnitude scales are logarithmic

    • Increase of 1 unit = 10x ground motion

    • Increases of 1 unit = 32x the energy

  • Small earthquakes are frequent

    • 100,000 less than 3 magnitude a year

  • Large earthquakes are rare

    • 20 magnitude 7 per year

Intraplate Earthquakes

  • 5% of earthquakes are not near plate boundaries

  • Intraplate earthquakes are not well understood