Locating%20and%20Measuring%20Earthquakes

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Locating and Measuring Earthquakes

• Section 10.2

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Scales

• Modified Mercalli Scale ranks earthquakes from
  intensity 1 (barely felt) to 12 (total
  destruction. It measures the destructions that
  the earthquake caused
• Richter Scale Used to measure the magnitude of
  an earthquake. Each time you go up 1 number, the
  earthquake is 10x stronger
• Magnitude is the measure of an earthquake based
  on the strength of the seismic sound waves
  emitted by the earthquake.

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Seismographs

• Seismographs detects and records waves produced
  by earthquakes
• Scientists use data from seismogrphs to locate
  earthquakes epicenter and to measure their
  magnitudes
• Seismogram A record sheet that is placed on a
  drum attached to the base.
• The attached pen does not shake because it is
  attached to a weight
• Seismograph
• Check this bridge out!

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Interpreting a seismogram

• P waves produced by an earthquake always arrive
  at a seismograph station before the S waves
• The farther a seismograph station is from the
  epicenter, the greater the difference in the
  arrival times of the P and S waves

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Locating the epicenter

• Seismologists use the difference in the speeds of
  P- and S-waves to locate the epicenters of
  earthquakes

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Steps to Finding your distance from an Earthquake
center

1. Subtract the arrival time of the P-waves from the
   arrival time of the S-waves (this can be
   expressed in hours, minutes and seconds, such as
   080640 or just hours and minutes
2. On the clean edge of a sheet of paper, make two
   marks to show the observed interval along the
   travel-time scale on the vertical axis of the
   Earthquake P-wave and S-wave Time Travel graph

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• 3. Keeping the edge of the paper vertical, slide
  the marks on the edge of your paper along the P-
  and S-curves of the travel-time scale until the
  marks match up with both curves
• 4. Carefully follow the marked edge of your paper
  straight down to the horizontal axis to find the
  distance to the epicenter
• 5. If you draw a circle around the recording
  station with a radius equal to that distance, the
  epicenter should be located somewhere within that
  circle.

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• 6. To find the exact location, follow the
  procedure above to find the distance from the
  epicenter to at least three seismic recording
  stations.
• 7. The epicenter is the point at which the
  circles intersect or meet
• 8. If you find that there is a small triangle,
  the epicenter is probably near the center of that
  triangle

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To find origin time

• To find origin time, subtract the travel time
  from the arrival time. To find origin time,
  subtract the travel time from the arrival time.
  It is like if a friend who lives a half hour
  away, arriving at your house at 10am. Based on
  that information, you know your friend started
  out at 930 am. Example if the epicenter is 4000
  km away, the travel time must be 7 minutes. If an
  earthquake was recorded at 225 pm, and the
  P-waves took 7 minutes to get there, the
  earthquake must have actually happened at 218 pm

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Examples

• If an epicenter is located 4000 km away, how
  would the arrival times of S- and P- waves
  compare on seismograms at your location?
• An earth quake occurred at 20000. When would
  the first P-waves arrive at a seismic recording
  station 8000 km away?