L2.1 Earthquakes & Faults

• What is the focus of an earthquake?


  The place inside Earth's crust where an earthquake originates
• What is the epicenter of an earthquake?


  The part of the earth's surface directly above the focus of an earthquake
• How is magnitude measured in earthquakes?

  By using the Richter Scale
• What does intensity describe in the context of earthquakes?


  The severity of an earthquake in terms of its effects on the earth's surface and on humans
• Which scales are used to measure earthquake intensity?

  Modified Mercalli, Rossi Forel scales, and PEIS
• What does PHIVOLCS stand for?


  The Philippine Institute of Volcanology and Seismology
• What is an earthquake?


  The shaking of a surface of the earth caused by a sudden slip on a fault
• What causes an earthquake?


  A sudden slip on a fault
• What are the vibrations of the earth caused by?

  The rapid release of energy
• What are the types of earthquakes?

  • Tectonic Earthquake: caused by geological forces on rocks.
  • Volcanic Earthquake: occurs with volcanic activity.
  • Collapse Earthquake: small earthquakes in underground caverns.
  • Explosion Earthquake: results from nuclear or chemical detonations.
• What form does the energy released during an earthquake take?

  Seismic waves
• What is seismology?


  The study of earthquakes
• Who is a seismologist?


  A person who studies earthquakes
• What does a seismograph do?


  Measures and records seismic waves through vibrations
• What are the zigzag lines seen in a seismograph called?


  Seismogram
• How many detectable earthquakes are estimated to occur each year?

  500,000
• What are faults?


  Rock fractures where a block of rock moves with respect to another
• What causes faults?


  Forces brought about by moving plates
• What are the types of faults?

  • Normal faults: blocks pulled apart, hanging wall moves down.
  • Reverse faults: hanging wall moves up, caused by compression.
  • Strike-slip faults: horizontal movements of blocks.
  • Right lateral: displacement to the right.
  • Left lateral: displacement to the left.
• What are the types of stresses in the Earth’s crust?

  • Compression Stress: rocks push against each other.
  • Tension Stress: rocks pull apart.
  • Shear Stress: plates rub against each other.
  • Confining Stress: applies to all sides, causing sinkholes.
• What does compression stress do to rocks?


  Causes the rocks to push or squeeze against one another
• What does tension stress do to rocks?


  Causes the rocks to pull apart
• What usually causes shear stress?


  When two plates rub against each other as they move in opposite directions
• What is confining stress?


  Stress that applies to all sides of the crust, causing a sinkhole
• What causes an earthquake?

  Geologic forces inside the Earth
• How do geologic forces lead to earthquakes?

  They build up until rocks break
• What happens when underground rocks break?

  Tremendous energy is released
• What are seismic waves?

  Waves of energy traveling through the Earth
• How do seismic waves behave?

  They behave differently depending on their encounters
• What are the two main types of earthquake waves?

  Body waves and surface waves
• What are body waves?

  Seismic waves that travel through Earth's interior
• How do body waves compare to surface waves?

  Body waves have higher frequency than surface waves
• What are the two types of body waves?

  Primary waves and secondary waves
• What are primary waves also known as?

  1. waves or pressure waves
• Why are P-waves the first to arrive at a seismic station?

  They are the fastest kind of seismic waves
• What can P-waves move through?

  Solid rocks and fluids
• How do P-waves move rocks?

  They push and pull rocks as they move
• What is the velocity range of P-waves?

  4 to 6 km/s
• What are secondary waves also known as?

  1. waves or shear waves
• Why do S-waves arrive after P-waves?

  They are slower than P-waves