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What is an Epicenter?• A point on Earth’s surface where EQ
originates.
Focus: A point of an EQ within the Earth.
• Primary wave- travels phastest so it arrives at seismic stations phirst.
• Compressional wave• Pass through solids and liquids
(magma).
• Secondary wave- arrives at a seismic station second.
• Vibrates side-to-side.• Solids wave- only travels through
solids.• Shear wave
Between each minute, we notice that there are 3 segments.
If there are 60 seconds in a minute, each one of these segments must be ________seconds.
Notice the horizontal axis
This represents DISTANCE (in thousands
of Kilometers!)
Let’s enlarge a small section
This chart can be used for answering many different questions!
How long does it take an S-Wave to travel 5,000km?
The recording station tells us it took 6 minutes and 20 seconds for the P-Wave to reach them. How far away from the epicenter of the earthquake must they be?
Use the data from the recording stations:
• Station A: San Francisco, California
P-Wave arrival 3:02:20 S-Wave arrival 3:06:30
What is the time difference between P and S wave arrivals?
Use the data from the recording stations:
• Station B: Denver, Colorado
P-Wave arrival 3:01:40 S-Wave arrival 3:04:10
What is the time difference between P and S wave arrivals?
Use the data from the recording stations:
• Station C: Missoula, Montana
P-Wave arrival 3:01:20 S-Wave arrival 3:03:00
What is the time difference between P and S wave arrivals?
Recording BoardDifference in arrival times:
San Francisco: 4:102,800km
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Slide your compass to the EXACT distance on the scale.
Need a closer view to be sure? Go to http://earthquake.usgs.gov/regional/states.php and retrieve the seismicity map (like the one on the left) for the state containing your possible solution.
USGS
Based on this map, earthquakes have previously occurred in the region where our epicenter fell. Thus our answer seems likely
What if our epicenter had fallen here instead? Does this mean our solution is wrong or less likely? What could help us find out?