Exploration Guide: Earthquake - Determination of Epicenter
[NOTE TO TEACHERS AND STUDENTS: You will need your data table and the
graph made while doing our last Gizmo: Earthquake—Recording Center. The
table and graph should contain data, T (elapsed time between the P and s
waves) and distance, from the earthquakes you experimented with in that Gizmo.
If you do not have those from the last Gizmo, or you didn’t do the Gizmo before,
you may use my sample page. NOTE: Minor variations in lines-of-best-fit may
cause major differences in distance calculation.]
When an earthquake occurs, people nearby can feel (and sometimes hear) a
rumbling. In some of the strongest earthquakes, buildings and other structures
can be damaged. But, in most cases, an earthquake just feels like a short vibration
or rumble and then it's over.
Soon after an earthquake, the news reports where the epicenter of the
earthquake was. (The epicenter is the point on Earth's surface closest to the
focus, or origin, of the earthquake.) When all you feel is a short rumble, how can
anyone figure out exactly where the earthquake originated?
Earthquakes release energy in the form of vibrations called seismic waves.
Seismologists (scientists who study earthquakes) detect and record seismic
waves with an instrument called a seismograph. A seismograph records the
vibrations on a printout known as a seismogram.
Finding the Epicenter of an Earthquake
In this activity, you will explore the type of data collected by seismologists when
an earthquake occurs.
1. Notice the three small graphs at the top left corner of the Gizmotm
(labeled A, B, and C). These are simulated seismographs. Real
seismographs run constantly at many locations around the world. Click
Play (
) and watch the seismograms as an earthquake occurs
somewhere in the world. When the vibrations have mostly subsided,
click Pause.
a.
Describe in general what the three seismograms look like. How
are they similar? How are they different?
b.
When an earthquake occurs, it sends out fast-moving P waves
and slower-moving S waves. P waves cause a fairly small
vibration. S waves are much stronger and cause taller "spikes" in
the seismogram. The amount of time between the arrival of the
first P wave and the arrival of the first S wave is called T. (Note:
Each recording station will have a different value for T.) To find
T, click on Show time probe. Drag one of the vertical probes and
place it on the first seismogram where the first P wave arrived.
Then place the second probe where the first S wave arrived. What
is T for Recording station A? What is T for Recording Station
B? For Recording station C?
c.
The larger the value of T, the further that recording station was
from the epicenter. Of the three recording stations in the Gizmo,
which one was closest to the epicenter? Which was furthest?
2. Next you will use your data table and graph (from our last Gizmo),
along with the values for T that you just calculated, to determine how
far each recording station was from the epicenter.
a.
What was the value of T that you found for Recording station
A? Based on that, use your data table and graph to determine how
far Recording station A was from the epicenter. (Make your best
estimate, based on how this value for T fits in with your previous
data.) What was the distance?
b.
In the Gizmo, select Show station A. Set the Radius slider to
the distance from the epicenter. You will see a circle around
Recording station A. A circle shows all the points that are the
same distance from the center. In this case, the circle shows all the
points that are the correct distance from Recording station A.
According to your calculations, the epicenter lies somewhere on
that circle.
c.
Repeat the process for Recording station B and Recording
station C. What distances did you find for each?
d.
Once you have all three radii set, the three circles shown in the
Gizmo should intersect at one point. That point is the epicenter of
the earthquake. If you have found the correct epicenter you will
see a red symbol like the one shown below. (A small red dot will
appear if the three circles intersect at a point that is not the
epicenter.) [NOTE: Due to variations in your graphs and the lines
of best fit, you may NOT end up finding the epicenter expected by
the GIZMO. If your data results in the three circles intersecting,
EVEN if you get the red dot and not the bulls-eye symbol, you have
read your graph correctly.]
e.
You should have found that Recording station A was about 430
km from the epicenter, Recording station B was about 800 km
from the epicenter, and Recording station C was about 400 km
from the epicenter. If you have not already seen this in the Gizmo,
adjust the Radius slider settings to see the epicenter. NOTE: If
you pressed reset since the start of the GIZMO, these three
answers may be very different for you! That is OK!
3. Click Reset. Then click Play and watch as a new earthquake happens
somewhere else in the world.
Repeat the same process to find the three values for T, calculate the
three distances from the epicenter, and then locate the epicenter.
(You should do at least three earthquakes, finding the epicenter for
each of the three earthquakes.) When you feel you have practiced
enough, you may go on to answer the five questions at the bottom of
the Gizmo.