Earthquakes and Earth’s interior

• Earthquakes occur when energy stored in rocks is suddenly released. Most occur at plate boundaries.
• Earthquakes releases mechanical energy transmitted through rocks as seismic waves, which are recorded at seismic stations.
• What causes earthquakes?

  Movement along the faults, volcanic eruptions, magma movement, landslides, and explosions.
• Elastic Rebound Theory is how energy is released during an earthquake.
• Elastic Rebound Theory

  1. Rocks deform or bend
  2. Rocks rupture when pressure accumulates in rocks on either side of a fault and build to a level which exceed the rocks’ strength.
  3. Rocks rebound and return to their original shape when the accumulated pressure is released.
• Seismology is the study of earthquakes
• Seismograph is an instrument that measures earthquakes. Record: seismogram.
• Where do earthquakes occur?

  Hypocenter/focus: below surface along the fault where the earthquake is generated.

  Epicenter: on the surface directly above the hypocenter

  Along the plate boundaries
• Seismic waves causes most of the damage and shaking people feel during an earthquake.
• Body waves (P- and S-waves) travel through earth and Suface waves travel along or just below the surface.
• P (primary) Waves

  Compresses material

  Fastest wave, and it goes through solid and liquids.
• Secondary (S-waves)

  Shears materials

  slower than P-wave, and it only goes through solid.
• Surface Waves

  Horizontal waves shear materials

  Vertical waves compresses materials
• Two types of surface waves

  Rayleigh wave and Love Wave
• STRENGTH OF AN EARTHQUAKE: INTENSITY

  • A qualitative measure of the kind of damage that occurs during the earthquake
  • Expressed with the Modified Mercalli Intensity Scale (I to XII)
• Intensity of an earthquake depends on:

  • Distance from epicenter
  • Focal depth
  • Population density
  • geology of the area
  • type of building construction
  • duration of ground shaking
• STRENGTH OF AN EARTHQUAKE: MAGNITUDE

  • A quantitive measure of the amount of energy released during an earthquake
  • Ritcher Magnitude
• Destructive effects of earthquakes

  • Ground shaking
  • Liquefaction
  • Fire
  • Tsunami
  • Ground failure
  • humans
• GROUND SHAKING is the most destructive effect of an earthquake. It lasts longer and more vigorous in loose and wet sediment than solid rocks.
• LIQUEFACTION is when water saturated clays become liquid during ground shaking.
• FIRE occurs when natural gas and water lines break. In some cases, it is more deadly than the earthquake itself.
• TSUNAMI: an earthquake on the seafloor can generate deadly waves. An example is the Sumatra (Indian Ocean) and in Japan (Pacific Ocean).
• GROUND FAILURE: earthquakes trigger landslides and rockslides that are responsible for many deaths and damage. Can result in building and road collapse.
• HUMANS

  Poorer, more rural dominated countries have more loss of life and damage from earthquakes. Wealthier countries have eq resistant buildings and warning systems.
• Seismic risk maps helps geologists in determining the likelihood and potential severity of future earthquake based on the intensity of the past eq.
• Earthquake precursors is the short-term and long-term changes within the Earth prior to an earthquake that assist in prediction.
• EARTHQUAKE PRECURSORS

  1. Seismic gaps-locked portions of the fault where pressure is building—fewer eq
  2. Surface evaluation changed and tilting of land surface
  3. Ground water table fluctuations
  4. Local changes in earth’s magnetic field.
• Earthquake prediction: programs

  Research programs (US, Russian, China, Japan) have lab and field studies of rock behaviors before, during, and after a large earthquake. They monitor major active faults.
• Earth’s interior

  Refraction (bending) and reflection of P- and S-waves are studied by geologists to help understand Earth’s interior.
• P- and S-waves indicate the locations of boundaries between layers of different densities called discontinuities. Changes in speed and refract and reflect when waves cross them.
• EARTH’S INTERIOR: THE CORE

  P- and S-waves both refract and reflect as they cross discontinuities.

  Sudden decrease in P-waves velocity occurs at the core-mantle boundary.
• EARTH’S INTERIOR: THE CORE

  P-wave shadow zone: formed from strong diffraction of p-waves
• S-wave shadow zone: S-waves can’t pass through liquids, meaning it cannot go through the outer core because it blocks s-waves, creating a shadow zone.
• Seismic tomography is a technique for developing better models of the earth’s interior.
• Tomography uses seismic waves to map out changes in velocity within the mantle which allows geologists to track the subducting plates.
• Geothermal gradient measures the increase in temperature with depth in the earth.
• Continental crust is mostly granite and has low density.

  Oceanic crust is mostly gabbro overlain by basalt.