How Do We Know Where an Earthquake
Originated?
Modified From:
http://www.iris.edu/hq/files/programs/education_and_outreach/lessons_and_reso
urces/docs/Epicenters_Teachersguide.pdf
Objectives:
1. Identify P and S waves on seismograms
2. Determine the distance of an epicenter from a seismic station using travel
time curves
3. Locate the epicenter of an earthquake by triangulation
Materials needed:
1. 1 world map with latitude and longitude
2. 1 station map
3. 1 travel time curve graph
4. Note cards for marking/measuring distances
5. Ruler
6. Pencil
7. Compass for drawing circles
8. Calculator
Part 1
Identify P and S waves on your seismograms.
What are the P arrival times for each station?
What are the S arrival times for each station?
What is the amount of time between arrival of the P and S wave (S-P time) for
each station?
Part 2
Determine the distance the epicenter is away from each station using the travel time
curve graph.
Do this by creating a marker on a note card/scrap of paper on the y-axis of the graph
that is equal to the S-P time you found in part 1. The image below shows the correct
process for a S-P time of 8 minutes.
Keeping the note card edge parallel to the y-axis, slide the note card to the right and
upward. Find the spot in the graph where the marker you created fits between the S
and P curves on the travel time graph. The S curve is marked by the bright blue line
and the P curve is marked by the bright red line.
Read down from the edge of the marker to the x-axis to determine the distance from
the station to the epicenter. This process is shown in the figure below.
Once you read the epicenter distance in degrees from the travel time graph you can
convert this degree distance into kilometers by multiplying the degrees * 111.
What is the epicenter distance from each station in degrees and kilometers?
Part 3
Use the each station’s latitude and longitude location and the latitude and longitude
world map to assign the proper station name to each marker on the station map.
Label the correct station names on the station map.
Part 4
Use a compass and the scale provided on the station map to draw a circle around
each station. The radius of each circle should be equal to the epicenter distance
found for each station in part 2.
Part 5
In an ideal scenario, this triangulation method will result in one point on the map
where all three circles intersect.
Did you find one point on the map where all 3 circles intersect? If so, what is
the latitude and longitude of this point?
If you did not find a point on the map where all three circles intersect, why do
you think that this happened?
Even if the circles do not intersect at one exact point, using this method, they
should come close in one geographic region. Using this information, on which
continent do you think the epicenter is located? In which country do you think
the epicenter is located?
This was a magnitude 8.8 earthquake occurring in 2010. Given your estimate
of the earthquake location, what tectonic setting/events are occurring in this
location that would cause such a large earthquake?