# Geology Assignment

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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