2.2 Measuring Earthquakes


focus: the area under the Earth's surface where rock that is under stress breaks, causing an earthquake
epicenter: the area on the surface that is above the focus
seismic waves: vibrations that travel through Earth carrying energy released during and earthquake
P waves: earthquake waves that compress and expand the ground
S waves: earthquake waves that vibrate from side to side as well as up and down
surface waves: they move more slowly than P waves and S waves, but theground movements
seismograph: records the ground movement caused by seismic waves as they move through the Earth
magnitude: a measurement of earthquake strength based on seismic waves and movement along faults
Mercalli Scale: rates earthquakes according to their intensity
Richter Scale: a rating of the size of seismic waves as measured by a particular type of mechanical seismograph
moment magnitude scale: a rating system that estimates the total energy released by and earthquake


Seismic Waves

  • sound waves carry force as they move away from their origin
    • during earthquake, seismic waves come out from focus in every way
      • three categories for seismic waves; P waves, S waves, and Surface waves

Primary Waves

  • first waves to come in from core
    • makes buildings become bigger and smaller
    • can travel through solids and liquids

Secondary Waves

  • comes after P waves
  • moves ground back and forth
    • shakes buildings violently
    • can't go through liqueds

Surface Waves

  • when P waves and S waves reach surface, they turn into surface waves
    • moves slower than P waves and S waves
    • has the strongest ground movements

Detecting Seismic Waves

  • to measure seismic waves, scientists use seismographs

Measuring Earthquakes

  • the three types of scales are...
    • the Mercalli scale
    • the Richter scale
    • the Moment Magnitude scale

Mercalli Scale

  • was invented on early 20th century
  • it doesn't give the exact measurement
    • there are 12 steps that describe how earthquakes affect people, buildings, and land surface
    • the same type of earthquake can have different Mercalli ratings because it is in a different location

Richter Scale

  • invented in 1930's
    • used for about 50 years
    • provides correct measurements for small and close earthquakes
      • scale does not work well for large or far earthquakes

Moment Magnitude Scale

  • used to measure earthquakes of all sizes, both close and far away earthquakes

Locating the Epicenter

  • use seismic waves to find the epicenter
    • to find epicenter, measure between arrival times of P waves S waves
      • the farther the epicenter, the farther the amount of time there is between the arrival times of the P waves and S waves
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