Instructions for Locating an
Earthquake Epicenter
Earthquake Waves
• By determining the time of arrival for both the
P and S waves at a location we can
determine how far away the Earthquakes
epicenter is from the seismograph.
• This does NOT tell us what direction the
waves are coming from.
Locating the Epicenter of an Earthquake
Part I: Finding the distance to the epicenter
• Step 1:
– Determine the difference in arrival time for
your P-wave, and your S-wave.
P-wave
03:21:15
S-wave
03:27:15
Calculation – difference in arrival time
03:27:15
-03:21:15
00:06:00 minutes
Locating the Epicenter of an Earthquake
Part I: Finding the distance to the epicenter
• Step 2:
– Take out your ESRT.
Open to the Travel
Time Graph on page
11.
00:06:00 minutes
• Step 3:
– Use the vertical scale
(time) to mark off the
difference in arrival
time on a scrap sheet
of paper.
Locating the Epicenter of an Earthquake
Part I: Finding the distance to the epicenter
• Step 4:
– Make sure to keep
your scrap paper
vertical! Slide it along
the curves until it lines
up on each of the
curves.
– Be very accurate!!!!!
Locating the Epicenter of an Earthquake
Part I: Finding the distance to the epicenter
• Step 5:
– Read off the distance
from the horizontal axis
that corresponds to this
spot. This is the distance
between the epicenter
and your seismograph
location.
– This distance is the radius of a
circle around the seismograph.
Locating the Epicenter of an Earthquake
Part I: Finding the distance to the epicenter
• Step 6:
– Repeat steps 1-5 for at least 2 other
seismograph locations.
– It is necessary to have at least 3 stations, if
you do not you can not be sure of the exact
location of the epicenter.
Common Regents
Question
Locating the Epicenter of an Earthquake
Part II Locating the Epicenter
• Step 1:
– Find the location of your first seismographic
station on the map.
• Step 2:
– Use a compass or string to create a circle with
its center at your seismograph location, and a
radius equal to the distance you found.
Locating the Epicenter of an Earthquake
Part II Locating the Epicenter
Step 3:
• Repeat this procedure around two other
seismographic stations.
• Where the three circles all intersect
(cross) is where your epicenter is
located.
• If the circles do not all intersect, but
form a small triangle, the epicenter is
the center of the triangle.
Locating the Epicenter
Epicenter Location
Figure 2.13
Locating the Epicenter of an Earthquake
Part III: Origin time of the Earthquake
Step 1:
• From the distances determined in Part I,
determine how long it would take a Pwave to travel that distance.
Origin Time of Earthquake
P-wave Travel Time = 00:07:40
• Step 2:
• Find the distance on the
horizontal axis.
• Go up to the point where
you hit the P-wave travel
line.
• Go over to the vertical axis
and read off the travel time
for the P-wave.
Distance = 4,400 km
Origin Time Story
• We have the P-wave arrival time at the
seismograph, and the time that it took to get
there, so we need to find the time the wave left
the focus of the earthquake (origin time of the
seismic waves).
• You went to a party, you arrived at 06:30:00, it
took 00:12:15 to get there, what time did you
leave your house?
Calculation:
Check your work:
06:30:00 – arrival time
06:17:45 – departure
-00:12:15 – travel time
+00:12:15 – travel time
06:17:45 – time of departure
06:30:00 – arrival time
Origin Time of Earthquake
• Step 3:
– Taking the time found in step 1, subtract this
from the arrival of the P wave and that is the
original time of the earthquake.
– Calculation
03:21:15 - P-wave Arrival Time
-00:07:40 - Travel Time for P-wave @ 4,400km
03:13:35 - Origin Time (when the earthquake occurred)