Instructions for Locating an Earthquake Epicenter

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Instructions for Locating an Earthquake Epicenter
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Earthquake Waves

• By determining the time of arrival for both the P
  and S waves at a location we can determine how
  far away the Earthquakes epicenter is from the
  seismograph.
• This does NOT tell us what direction the waves
  are coming from.

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Locating the Epicenter of an EarthquakePart I
Finding the distance to the epicenter

• Step 1
• Determine the difference in arrival time for your
  P-wave, and your S-wave.

S-wave 032715
P-wave 032115
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Calculation difference in arrival time

• 032715
• -032115
• 000600 minutes

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Locating the Epicenter of an EarthquakePart I
Finding the distance to the epicenter

• Step 2
• Take out your ESRT. Open to the Travel Time
  Graph on page 11.
• Step 3
• Use the vertical scale (time) to mark off the
  difference in arrival time on a scrap sheet of
  paper.

000600 minutes
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Locating the Epicenter of an EarthquakePart I
Finding the distance to the epicenter

• Step 4
• Make sure to keep your scrap paper vertical!
  Slide it along the curves until it lines up on
  each of the curves.
• Be very accurate!!!!!

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Locating the Epicenter of an EarthquakePart I
Finding the distance to the epicenter

• Step 5
• Read off the distance from the horizontal axis
  that corresponds to this spot. This is the
  distance between the epicenter and your
  seismograph location.
• This distance is the radius of a circle around
  the seismograph.

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Locating the Epicenter of an EarthquakePart I
Finding the distance to the epicenter

• Step 6
• Repeat steps 1-5 for at least 2 other seismograph
  locations.
• It is necessary to have at least 3 stations, if
  you do not you can not be sure of the exact
  location of the epicenter.

Common Regents Question
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Locating the Epicenter of an EarthquakePart II
Locating the Epicenter

• Step 1
• Find the location of your first seismographic
  station on the map.
• Step 2
• Use a compass or string to create a circle with
  its center at your seismograph location, and a
  radius equal to the distance you found.

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Locating the Epicenter of an EarthquakePart II
Locating the Epicenter

• Step 3
• Repeat this procedure around two other
  seismographic stations.
• Where the three circles all intersect (cross) is
  where your epicenter is located.
• If the circles do not all intersect, but form a
  small triangle, the epicenter is the center of
  the triangle.

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Locating the Epicenter
Epicenter Location
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Figure 2.13
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Locating the Epicenter of an EarthquakePart III
Origin time of the Earthquake

• Step 1
• From the distances determined in Part I,
  determine how long it would take a P-wave to
  travel that distance.

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Origin Time of Earthquake
P-wave Travel Time 000740

• Step 2
• Find the distance on the horizontal axis.
• Go up to the point where you hit the P-wave
  travel line.
• Go over to the vertical axis and read off the
  travel time for the P-wave.

Distance 4,400 km
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Origin Time Story

• We have the P-wave arrival time at the
  seismograph, and the time that it took to get
  there, so we need to find the time the wave left
  the focus of the earthquake (origin time of the
  seismic waves).
• You went to a party, you arrived at 063000, it
  took 001215 to get there, what time did you
  leave your house?

Calculation 063000 arrival time -001215
travel time 061745 time of departure
Check your work 061745 departure 001215
travel time 063000 arrival time
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Origin Time of Earthquake

• Step 3
• Taking the time found in step 1, subtract this
  from the arrival of the P wave and that is the
  original time of the earthquake.
• Calculation
• 032115 - P-wave Arrival Time
• -000740 - Travel Time for P-wave _at_ 4,400km
• 031335 - Origin Time (when the earthquake
  occurred)