Calculating Earthquake Times
Vocabulary
Origin Time – the time that the earthquake began.
Origin Time = Arrival Time - Travel Time
Focus – the location deep within the Earth where the earthquake
actually happened.
Epicenter – the location on the Earth’s surface above where the
earthquake happened. This where the earthquake is felt the
strongest.
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Seismometer
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Seismic Wave – the energy that is released by an earthquake.
This energy travels through the ground in the form of a wave.
There are 3 types of seismic waves:
1. P-Wave – Primary Wave – Compressional Wave:
A wave that makes the ground vibrate in the same
direction that the wave travels.
2. S-Wave – Secondary Wave – Shear Wave:
A wave that makes the ground vibrate at right angles
to the direction that the wave travels.
3. Surface Waves – Both P and S waves that reach and travel
along the Earth’s surface to cause damage.
As time passes, seismic waves actually speed up.
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P-Wave
S-Wave
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Facts about waves
P- waves
1. These waves travel much faster than the other waves.
2. These waves always arrive first after an earthquake.
3. These waves can pass through solids, liquids, and
gases.
S-waves
1. These waves travel the slowest of all of the waves.
2. These waves always arrive second, which is why they are
called secondary waves.
3. These waves can only pass through solids.
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Seismograph
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Locating an Earthquake Epicenter
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A shadow zone is a place on the Earth’s surface where no
seismic waves are received.
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To calculate the distance to an epicenter:
1.
Calculate the DIFFERENCE in arrival times between the P and S waves.
S-P
2. Place a sheet of paper along the Travel Time axis.
3. On this paper, mark a small dot at the time zero and another at the time
corresponding to the difference you calculated in step 1. The distance
between the two dots represents the difference in arrival times of the P
and S waves.
4. Keeping the lower dot on the lower graph line, slide the paper to the right
until the upper dot touches the upper graph line. KEEP THE PAPER
STRAIGHT UP AND DOWN.
5. Read straight down to the x-axis and estimate the epicenter
distance.
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The location of an earthquake epicenter can only be determined if
there is information from three seismic stations. A circle is drawn
around each city with seismic information. The earthquake
epicenter is where the circles intersect.
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