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How to Use the Earthquake Travel Time Graph (Page 11 of the Earth Science Reference Tables). The graph on page 11 can be used to find several different variables. Let’s start with:. TRAVEL TIME or DISTANCE from EPICENTER (Given one, find the other).
of the Earth Science Reference Tables).
The graph on page 11 can be used to find several different variables. Let’s
TRAVEL TIME or DISTANCE from EPICENTER (Given one, find the other)
Sample Problem: A seismic station is 3000 kilometers from the epicenter of an
earthquake. How long will it take P waves from this earthquake to reach the
You must always ask yourself two questions:
1) What information do I have?
3000 kilometers from the epicenter
2) What information am I looking for? If you don’t know what you’re looking for
it’s very unlikely that you’ll find it!
How long (time)
We’re trying to find the time it will
take P waves to travel this distance.
Find 3 (really 3000km) on the bottom
axis and, using a straight edge
(a ruler) draw a line up to meet the
P wave line.
Now, using the ruler, draw a line over
to the left axis and read the travel
time: 5 minutes and 40 seconds.
Notice that each small box is worth
Now try the same problem in reverse:
It takes P waves 5 minutes and 40
seconds to travel from an epicenter
to a seismic station. How far is the
seismic station from the epicenter?
5 minutes and 40 seconds.
We are trying to find the distance
between the epicenter and the
Using a ruler, draw a line from the
time (5:40) to the P wave line.
Now draw a line straight down to
the bottom axis and read the
All of these problems are the same.
Given the time, find the distance.
Given the distance, find the time.
And it doesn’t matter whether you
are given P or S wave travel time
as long as you are careful to use
the correct line and, most important:
TAKE THE TIME TO BE CAREFUL AND ACCURATE!
Let’s take this one step further. A seismic station is 3000 km from the epicenter of an earthquake. If P waves from that quake arrive at the station at 4:25:40 PM (4 hours, 25 minutes and 40 seconds), at what time did the earthquake occur?
We do exactly the same thing we
did before, we find the travel time which is 5:40 (5 minutes, 40 sec.)
Now some math. If the waves arrived at 4:25:40 and they’ve been traveling for 5:40, when did they start out?
The quake occurred at 4:20:00 PM
OK, let’s try another type of problem involving both P and S waves. Here’s
It takes P waves 7:20 (7 minutes and 20 seconds) to travel from an earthquake
epicenter to a seismic station. How long will it take S waves from the same
earthquake to reach the seismic station?
What do we know?
We know P wave travel time is 7:20
What are we trying to find?
We want to find S wave travel time.
Do we have enough information to do the problem?
We have to divide the problem into 2 parts. First, we do just what we did in
the previous problem. We use the P wave travel time to find the distance
to the seismic station and then.................................
We use the distance to the seismic station to find how long it took S waves
to travel that same distance.
Let’s see how it’s done....................................
so, using a ruler, draw a line from
7:20 over to the P wave line.
Now draw a line straight down to find
the distance from the epicenter.........
The distance is 4200 km. Notice that
each small box on the bottom axis is
Now we can get on with the second
half of the problem. If the seismic
station is 4200 km from the quake
epicenter, how long (time) did it take
the S waves to travel that same
Draw a line from 4200 km straight up
to the S wave line.
Now draw a line over to the vertical
axis and read the time..........
13 minutes exactly (13:00)
And now for something completely different.......................
“difference in travel time between P
waves and S waves”.
Whenever you are given the difference
in arrival times OR you are given both
times so that you can subtract and find
the difference, you should immediately
realize that you will be dealing with the
shaded area (yellow) between the two
Here’s a sample problem:
P waves arrive at a seismic station
4:30 (4 minutes, 30 seconds) before
the arrival of S waves from the same
earthquake. How far from the epicenter
is the seismic station?
arrive at a seismic station 4 minutes and
30 seconds (4:30) apart. You are asked
to find the distance to the epicenter.
1) Take a sheet of paper and line up
the left edge with the vertical axis
(time). Be sure that most of the paper
is hanging down below the graph. This
2) Make a small, thin, and accurate mark
on the paper at 0 time. Make another
small, thin, and accurate, mark at 4:30
(4 minutes, 30 seconds).
Your 2 marks are now 4:30 apart!
until one of your marks is exactly
on the S wave line and the other
is exactly on the P wave line. It is
very important to be sure your
paper is straight (vertical).
Now look to see where the bottom
of your paper crosses the lower
(epicenter distance) axis. In this
case it crosses at exactly 3000 km
which is the answer.
When P & S waves arrive 4 minutes
and 30 seconds apart it means that
the seismic station is exactly 3000
km from the epicenter of the quake.
Of course we can do the same
problem in reverse!
the epicenter of an earthquake.
How long after P waves arrive will
S waves from the same earthquake
arrive at the station?
This time place your paper on the
3000 km mark taking care to keep
the paper straight and to cover
both the P and S wave lines.
Make small, thin, and accurate
marks where your paper crosses
the P line and the S line.
lower mark is on time 0.
Now just read the time indicated by your
top most mark: 4:30 (4 minutes and
30 seconds) which is the answer.
This kind of question may also be
phrased this way:
“P waves arrive at a seismic station at
12:32:00. S waves from the same
earthquake arrive at 12:36:30. How far
from the epicenter is the seismic station?
All you need to do is subtract in order
to get the difference in arrival times.
00:04:30 0 hours, 4 minutes, and
Just remember, if you’re given 2 times, just subtract
to get the difference.
One last kind of problem to become familiar with. Sometimes, instead of giving you
the difference in arrival times, you will be given a seismogram (a record made by a
Using this seismogram
find when the P waves
and the S waves arrive.
Do this by carefully
making a mark at the
P wave and S wave
Now count the number of
minutes between the
arrival of P and S waves at
From this point on it’s just like the previous problem: If the difference in arrival times
between P and S waves is 6 minutes, how far is the seismic station from the
epicenter of the earthquake?
page) you determined that P & S waves
arrived at a seismic station 6 minutes
Place your paper along the vertical axis
(time) and make small and accurate
marks at time 0 and time 6. Remember
to keep most of the paper hanging down
below the graph.
mark is on the S wave line and the
other is on the P wave line. Be
absolutely sure that your paper
is straight (vertical).
Read the distance at the point
where the edge of the paper
crosses the bottom (distance)
Remember that each small box is
200 km. The answer is............
That all there is to it. But doing
this once or twice won’t help. The
ONLY way to get good at anything
is to practice, practice and then
practice some more!
‘time of origin of the earthquake’.
In other words, use the information
given to find out when the quake
Here’s a sample problem: A seismic
station is 4000km from the epicenter
of an earthquake. P waves arrive
at the station at 2:48:00 PM. At what
time did the earthquake occur?
First, use the distance to find the P wave travel time.
OK, the P waves took 7 minutes to
travel the 4000km distance.
If they arrived at 2:48:00 and the
trip took 7 minutes, they must have
started out 7 minutes before 2:48 PM
So subtract. 2:48:00
Origin time: 2:41:00 PM
at a seismic station at 4:22:10 AM.
S waves from the same earthquake
arrive at 4:28:50 AM. What is the time
of origin of the earthquake?
First subtract to get the difference in
difference = 6:40 (6 min, 40 sec)
Now, as we did before, get a piece
of paper and carefully and
accurately make marks at time 0
and time 6:40
is on the P wave line and the
other is on the S wave line.
Now read the distance from
the epicenter on the bottom:
Now that we know the distance
to the epicenter we can easily
tell how long it took P waves
to travel that distance.
P waves took 7 minutes to
travel the 4000 km and since
they arrived at 4:22:10 we
can subtract to find out when
Origin time: 4:15:10
Many students read a question and immediately realize that they do
not know what to do. They feel they are completely lost. In that case
there’s one important thing to remember:
DO SOMETHING! Just don’t sit there! Do something with the information
you have. For instance, if the question mentions P waves and gives a
distance, use that to find a travel time.
If the question gives a travel time for S waves use that to find a
distance to the epicenter.
Now you have a little more information than you had before. Re-read
the question. Be sure you know what you’re looking for and see if
you now have enough information to get the answer.
Never give up. Never let yourself sit there doing nothing!