• Are caused by the shaking or trembling caused by the sudden release of energy
  • Usually associated with faulting or breaking of rocks – cracking of the plates also called faults.
  • Continuing adjustment of position results in aftershocks

Unstable Earth

  • The Earth’s crust is subjected to huge forces
  • Very large forces can fracture the rock
  • This can be seen as fault lines in the rock layers
  • The Focus and Epicenter of an Earthquake
  • The point within Earth where faulting  or cracking of the rock begins is the focus
  • The point directly above the focus on the surface is the epicenter

Where Do Earthquakes Occur and How Often?

  • ~80% of all earthquakes occur in the Pacific rim belt
  • most of these result from converging plate activity
  • ~15% occur in the Mediterranean-Asiatic belt
  • remaining 5% occur in the interiors of plates and on spreading ridge centers
  • more than 150,000 quakes strong enough to be felt are recorded each year

Causes of Earthquakes

  • The cause of most major earthquakes is the strain that builds up along faults at or near boundaries between lithospheric plates.
  • A fault is a break in the lithosphere along which movement has occurred.
  • The depth at which an earthquake originates depends upon the type of plate boundary involved
  • Causes of Earthquakes
  • At divergent boundaries such as the M.O.R, earthquakes tend to occur within the first 30 km of the surface
  • This is the case for transform boundaries as well
  • At subduction zones, however, as one plate plunges into the mantle, the focus of the earthquakes can be as far down as 700km below the surface

Body Waves

  • The energy released in an earthquake travels in waves.
  • Waves that travel from the focus of an earthquake through Earth are called body waves
  • Every earthquake produces two different types of body waves – P waves, S waves
  • P waves – compressional waves also known as the Primary wave
  • P waves squeeze and stretch rock material as they pass through Earth – can travel through everything – solid rock, magma, ocean, and even air
  • The second group of waves – the S waves or secondary waves – cause the particles of rock material to move at right angles to the direction in which the waves are travelling.
  • S waves can only travel through solid material but not through liquids or gases
  • The velocity of P and S waves depends on the type and density of the material which they travel through
  • Velocity is greater through materials that are rigid and dense
  • Through all types of solid material, S waves travel at a little more than half of the speed of P waves

Surface Waves

  • These are the waves that travel along the Earth’s surface
  • Surface waves are produced when P and S waves reach the surface
  • The two types of surface waves are Love waves and Rayleigh waves
  • Love waves cause particles to move from side to side, Rayleigh waves travel more slowly and cause particles to move in elliptical patterns.
  • Surface waves cause considerable damage to structures

How do we measure Earthquakes?

  • We measure earthquakes using a seismograph – it measures the seismic waves that are sent out by the earthquake
  • Earthquakes are measured on the Richter Scale
  • 6.1 earthquake on richter scale is 100 times more powerful than a 5.1 event

Locating and Measuring Earthquakes

  • Seismographs – b/c earthquakes produce different types of wave motions, there are different types of seismographs to measure them
  • In a seismograph, a heavy weight is attached to the base anchored in bedrock – this allows the weight to stay still even in an earthquake
  • A record sheet called a seismogram is placed on the drum which records the wavy lines of an earthquake

Interpreting a Seismogram

  • P waves are the first waves to be recorded by a Seismograph, followed by the S waves and then surface waves.
  • The further the Seismograph is from the source of the earthquakes, the longer the lag time between the P waves and S waves
  • A travel – time graph shows the relationship b/w P and S wave arrival times and the distance from an earthquakes epicenter

Locating the Epicenter

  • Since a Seismograph tells scientist the distance to an earthquake, it does not tell them the exact location of the earthquake
  • To determine the exact location of an earthquake, we need to know the distances from at least three different stations in order to plot an epicenters location
  • After getting data from three stations, draw a circle from the station, the point at which all three circles meet is the epicenter

Measuring an Earthquakes Magnitude

  • The magnitude of an earthquake can also be measured using the seismogram.
  • The scale of earthquake magnitude was introduced by Charles F Richter in 1935
  • So each increase in a whole number in the Richter magnitude is 31 times more powerful than the last number
  • ie – magnitude 7 is 31 more powerful than a magnitude 6

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