Description

This is an assignment associated with earthquake. Be sure to annotate seismograms and the graphics as indicated. The values you enter in the table must be what is indicated on the figures. Please understand the instructions carefully.

Geol 1202 Planet Earth

Assignment – Earthquake Hunters

Seismologists study seismic waves not only so as to be able to locate earthquakes but to also better

understand the internal structure of the planet. Seismic waves can be generated, not only from tectonic

activity but also from man-made sources such as explosions. The petroleum industry spends millions of

dollars each years on seismic surveys to help locate potential reservoirs. But we are not looking for oil,

instead we are going to locate the epicentre of a hypothetical earthquake.

Note: Before attempting this assignment you should have read the lecture and understand the concepts

covered.

Recall from the section on locating earthquakes that you can use the difference in arrival time between

P and S waves to determine the distance of the recording station from the epicentre of the quake; and

how, with three distances you can triangulate on the epicenter.

This is just what you are going to do in this assignment. But first an example.

Finding the time difference

Seismic waves are recorded on a seismograph – these

can be as simple as a pendulum and a sheet of paper,

to large digital recorders. The traditional seismograph is

simply a rotating drum with a recording stylus (although

most recordings are digital now).

The time difference is the interval of time between the first arrival of the P and S waves. P waves travel

fastest and so will be the first wave to arrive at the recording station. The slower S waves arrive later and

the difference in the arrival times is a reflection of the distance of the recording station from the epicentre.

In other words- the farther the recording station is from the epicentre the greater the difference between

the P and S wave arrival times.

The figure below is an illustration of a section of paper from a seismograph (called a seismogram)

The P-wave first arrival is at 9:00 and the S-wave is at 9:05. So the time difference is 5 minutes.

paper moves this way

increasing time

recording

stylus (pen)

9:10 UTC

9:05 UTC

S-wave

first arrival

9:00 UTC

P-wave

first arrival

Finding the distance from the epicentre

The easiest way is to mark on the edge of a piece

of paper the distance represented by your time

difference (5 minutes in our example).

This line

represents

5 minutes

on our chart

3000

4000

14

13

12

11

S-wave curve

10

9

Find the

location on the

chart where the

distance between

the 2 curves is

5 minutes apart

8

7

6

5

P-wave curve

4

This is the distance of that particular recording

station from the epicentre of the quake.

2000

16

Move the paper along, keeping it vertical,

until the distance between the 2 curves matches

the distance on your paper.

Then draw a line straight down at this point to find

the distance in miles.

Distance (km)

15

Time (minutes)

Next you look on the Distance Chart and find

the distance at which the time between the P

and S arrivals matches yours.

1000

17

3

2

1

This point

0

represents

500 1000 1500 2000 2500 3000

time zero.

Distance (miles)

The 2 waves

are generated

at the

At 500 miles the S and P waves

epicentre.

are almost 2 minutes apart

Find what

distance

from the

epicentre this

represents

Locating the epicentre

Suppose our example recording station was located in Halifax. Draw a circle centered on the station. The

radius should be equal to the distance from the epicentre (2100 miles in our example). The scale bar at the

bottom of the map figure allows us to set the correct radius.

The circle then represents the locations where the seismic waves could have originated.

radius = 2100 miles

0

Use the scale for the figure to find the

distance of 2100 miles

This will be the radius of your circle.

1000

2000

miles

3000

4000

Two other locations will give us three circles that intersect – and therefore the epicenter of the quake.

The coordinates can be read from the map figure – in our example that would be: Latitude 35 degrees and

Longitude 100 degrees.

35 degrees

radius = 2100 miles

the point where three circles

intersect is the epicentre

of the quake

100 degrees

0

1000

2000

3000

4000

miles

What do I do?

I have provided three seismograms for an fictitious earthquake. Your task is to use this information to

locate the epicentre. To accomplish this you will need a ruler, a drawing compass and a keen alert mind.

(Ok, you don’t need the last but it would be a help)

Print the two sheets with the diagrams and the thrid sheet with the table.

Once you have completed the assignment photograph the annotated figures and the table.

What do I submit?

Table One

the approx. latitude and longitude of the epicentre of the quake. This will be one set of coordinates.

your annotated seismograms and Time/Distance chart

your annotated map figure with the three circles – each centered on the appropriate city

!

!

!

!

Note: We will be using miles rather than kilometres in this exercise to preserve the correct distances.

500

1500

2000

Distance (miles)

1000

2500

3000

P-wave curve

S-wave curve

4000

You must submit an image of this sheet.

Step Two:

Then use this table to determine the distances

of each station from the epicentre.

Record this information in Table One.

Make sure you label your vertical lines on the

chart with the name of the corresponding city.

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

3000

Mexico

Seattle

New York

increasing time

9:00 UTC

9:00 UTC

9:00 UTC

16

2000

Distance (km)

9:05 UTC

9:05 UTC

9:05 UTC

17

Time (minutes)

1000

Student #:

9:10 UTC

9:10 UTC

9:10 UTC

Name:

Step One:

Use these seismograms, recorded at 3 different stations, to determine the arrival times of the P

and S waves. Mark the first arrival times with an arrow and letter (P for the P waves and S for the

S waves. Then calculate the time difference. Record this information in Table One.

0

0

1000

miles

2000

2000

miles

Mexico City

1000

Seattle

3000

3000

4000

4000

New York

Student #:

Step Three:

Finally use the map to locate the epicenter. Do this by drawing a circle, centered on each station, with a radius equal to the

distance from the epicentre that you determined earlier, Your three circles should then intersect at one point – the epicentre.

This is the point you record the coordinates for.

You must also submit an image of this annotated map figure.

Name:

Student #:

Name:

Table One

Location

Arrival time of

first P wave

New York

Seattle

Mexico

Latitude and longitude of epicentre

Arrival time of

first S wave

Time between

P and S waves

Distance from epicenter

(miles)

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