Download ď - Google Sites

Inquiry 12.3 – Locating the Epicenter of an Earthquake
Since Body waves travel through the earth, scientists use the
information gathered by seismographs to learn about the
interior structure of the planet. Any earthquake of
magnitude 5.0 or higher will be detected by any and all
seismographs on earth. Because waves lose energy as they
travel, the farther a seismograph is from the epicenter, the
less energy it will register for the quake.
Seismic waves change speed and direction as they travel
through substances of different densities. Since they know
how fast P and S-waves travel, scientists can get a profile of
what lies beneath the crust. P-waves are deflected as they
move through the earth, and S-waves are blocked by the
molten outer core because they cannot travel through liquid.
Seismologists also use the information from seismographs to
figure out the epicenter of the quake. The epicenter is the
spot on the earth’s surface where the shaking will be the most
violent, because it’s closest to the focus (the point within the
crust where the energy is first released). It’s important to
identify the epicenter quickly so that emergency services can
be sent right away. The graph below, a Time Distance Graph,
shows the known rates that P and S-waves travel.
Notice that both waves start at the same moment in time and
at the same place on the graph – the origin or focus. As the
waves travel, the amount of time between them begins to
increase. This is because P-waves travel faster than S-waves.
The farther the waves travel, the greater the difference in
time between them. What this means is that seismographs
close to the epicenter of a quake will show very little
difference between P and S-wave arrival times; seismographs
far away from the epicenter will show a much bigger
difference in the arrival of P and S-waves.
The Seismograms above all registered the energy from the
same earthquake. Comparing them tells us that the epicenter
of the quake was closest to Tokyo because…
The P-wave reached Tokyo before any of the others.
The amplitude of the waves (the squiggly lines) was highest in
Tokyo, so the seismic waves had the most energy there.
The arrival times of P and S-waves were the closest in Tokyo,
meaning they were detected very quickly after the energy was
first released at the focus.
How do seismologists calculate the epicenter of a
quake???
The information on a seismogram can tell scientists how far
that seismograph station is from the epicenter of a quake.
As seismic waves travel, the amount of time difference
between the arrival of P and S-waves increases. A
seismograph near the epicenter of a quake will show very
little difference in time between P and S-waves; a station
very far away will show a big difference between the arrival
of P and S-waves.
Let’s say that at 8:07 am, a seismograph station in Sitka,
Alaska registers a P-Wave. Four minutes later, the S-waves
are picked up. The seismogram for that station will look
something like this:
The seismologist at the station then goes to the time distance
graph and looks to see where there is a 4-minute difference in
the arrival of P and S-waves…
The chart shows that his seismograph station must be
2,500 km away from the epicenter of the earthquake.
This allows him to identify possible locations. Since
measuring out from a single point in many directions will
basically give him the circumference of a circle around the
station, he uses a compass and the scale on the map to
create that circle.
Clearly, this information alone is not enough to identify a
single point along the perimeter of the circle as the actual
epicenter (by the way, the distance around a circle is called
the circumference of the circle)
Information is also gathered from the same quake at a station
in Charlotte, North Carolina. The seismogram is shown below:
Using the Time-Distance graph, the seismologist figures out
that for there to be a 5 minute difference between P and Swaves, his station must be 3,500 km away from the epicenter.
A third Seismograph station in Honolulu, Hawaii picks up the P-wave
from the quake at 8:09, and it takes 6 minutes for the S-waves to
arrive:
The Time-Distance Graph shows that his station must be 4,500 km
from the epicenter…
When all three maps are put together, the single possibility
for the location of the epicenter that satisfies all three
seismograph stations is
identified…
This method is called Triangulation, and is what is
programmed into supercomputers that calculate the
information almost instantly.