How to Use the Earthquake Travel Time Graph (Page 11 of the Earth Science Reference Tables).

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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).

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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

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

seismic station?

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 know the distance (3000km)

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

20 seconds.

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?

We know the P wave travel time:

5 minutes and 40 seconds.

We are trying to find the distance

between the epicenter and the

seismic station.

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

distance.............

3000 kilometers!

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?

Subtract: 4:25:40

5:40

------------

4:20:00

The quake occurred at 4:20:00 PM

OK, let’s try another type of problem involving both P and S waves. Here’s

an example...........................

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.

NO!

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....................................

We know P wave travel time is 7:20

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

200 km.

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

distance?

Draw a line from 4200 km straight up

to the S wave line.

Now draw a line over to the vertical

13 minutes exactly (13:00)

And now for something completely different.......................

Another type of question involves the

“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

lines.

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?

Here’s what you know: P & S waves

arrive at a seismic station 4 minutes and

30 seconds (4:30) apart. You are asked

to find the distance to the epicenter.

Here’s how:

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

is important.

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!

Now slide your paper to the right

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

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!

A seismic station is 3000 km from

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

the P line and the S line.

Then..............................

Slide your paper to the left so that your

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.

12:36:30

- 12:32:00

----------------

00:04:30 0 hours, 4 minutes, and

30 seconds.

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

arrival times.

Now count the number of

minutes between the

arrival of P and S waves at

the station.

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?

Using the seismogram (on the previous

page) you determined that P & S waves

arrived at a seismic station 6 minutes

apart.

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.

Now slide your paper until one

mark is on the S wave line and the

other is on the P wave line. Be

is straight (vertical).

Read the distance at the point

where the edge of the paper

crosses the bottom (distance)

axis.

Remember that each small box is

4400 km!

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!

One last type of problem: Find the

‘time of origin of the earthquake’.

In other words, use the information

given to find out when the quake

occurred.

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

- 7:00

---------------

Origin time: 2:41:00 PM

Let’s try another one: P waves arrive

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

arrival times:

4:28:50

- 4:22:10

--------------

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

Slide the paper until one mark

is on the P wave line and the

other is on the S wave line.

the epicenter on the bottom:

4000 km.

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

they started.

4:22:10

- 7:00

--------------

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.