Earthquakes:
How to find where they originate…
Terminology:
• Epicenter vs. Focus
• Focus is the point beneath Earth's
surface where rock breaks under
stress (or plates shift) and causes an
earthquake.
• Epicenter is the point on land, directly
above the focus
Graph vs Gram
• Seismograph are instruments that measure
and record motions of the ground,
including seismic waves generated by
earthquakes, nuclear explosions, and other
seismic sources
• Seismogram is a graph output by a
seismograph. It is a record of the ground
motion at a measuring station as a function
of time.
• How Seismograph Works
S waves vs P waves
• P waves
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Primary Waves
Named that because they leave the focus first
Travel faster than S waves
Compression Waves
• S waves
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Secondary Waves
Named that because they leave the focus second
Travel slower than P waves
Transverse Waves
• S wave P wave Animation
• (slinky)
How do we locate earthquakes?
• Seismograph data (seismograms) will tell us
distance from an earthquake epicenter
• We need 3 seismograph readings to find an
epicenter
1400 km from Denver, 2600 km from Houston, 4000 km from Miami
How do we determine distance?
• Lag time between P wave
and S wave tells us distance
• Fast Runners and Slow
Runners...
How to read a Seismogram
Use lag time to figure out distance
traveled
Try the activity:
• http://www.sciencecourseware.com/VirtualEa
rthquake/VQuakeExecute.html