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Experiential Science 30—Freshwater Systems
DID YOU KNOW?
Around 132 AD, Chinese scientist Zhang
Heng invented the first seismoscope,
an instrument that could register the
occurrence of an earthquake. Zhang’s
invention was called a “dragon jar.”
The dragon jar had eight dragon
heads around its brim; each dragon had a
ball in its mouth. Around the foot of the
jar were eight frogs, each directly under
a dragon head. When an earthquake
occurred, a ball fell out of a dragon’s
mouth into a frog’s mouth, making a
noise that was supposedly loud enough
to wake the Emperor’s household. The
Emperor’s servants would check which
ball had dropped, and that would tell
them which direction the
earthquake was coming
from.
In subsequent
years, the instrument
correctly detected every
earthquake that occurred
in Luoyang, the Emperor’s
capital.
Zhang Heng’s invention,
called a “dragon jar,” was
the first seismoscope.
Locating the Epicentre
To the untrained eye, a seismogram looks like a jumble
of wavy lines, but scientists who are trained to interpret
them can tell the strength of an earthquake and the location of its epicentre.
The basis of reading a seismogram is understanding
the rates at which the three types of waves travel before
they reach the seismograph. You will recall that P waves
are faster than S waves, and surface waves are the slowest
of all. This basic knowledge can be used to identify the
beginning of each type of wave, as shown in Figure 2.5.
Surface waves
P Wave
The seismogram shows the intensity or amplitude of
the ground motion, although in most cases the height of
the wave is greatly magnified by the equipment so that it
is visible. By examining the differences in amplitude, you
can identify the arrival times of each different wave type.
In the travel-time simulation you did in activity 7,
your travel times were, of course, much slower than those
of real seismic waves. P waves travel through the Earth’s
crust and upper mantle at about 6,000–8,000 metres per
second (m/s) and S waves at about 3,500–4,500 m/s.
Also, as you probably noted in activity 7, the further
Figure 2.5 Seismogram
showing the arrival sequence
of P, S, and surface waves and
their relative magnitudes.
S Wave
22:20:00
:30:00:40:00:50:00
Time (hr:min:sec)
Chapter 2 Seismology91
Figure 2.6 Using a travel-time graph to find the distance to the
epicentre. This example shows a travel-time lag of 7 minutes,
40 seconds between the arrival times of P and S waves.
The distance the station is from the earthquake epicentre
is determined using the method you will use in activity 8. In
this example, the distance is 6,000 km.
22
21
20
19
18
Stn
18
16
15
14
Travel time (min)
12
av
e
Sw
13
7 min, 40 s
11
10
9
8
7
e
av
Pw
6
5
4
3
2
1
0
0 1,000 2,0003,0004,0005,000 6,0007,0008,0009,00010,000
Epicentre distance (km)
away the epicentre is from the recording station, the longer it takes the waves to reach the seismograph. The further away from the epicentre the recording station is, the
further behind the S wave will be. But the seismograph
shows only the time when the waves reached the station,
not where they began. So how are seismologists able to
tell how far a wave has travelled? The answer
lies in the relationship between the timing of the P and S waves.
Seismologists use a travel-time
Seismograph 1
graph to locate the epicentre of an
earthquake. Over the course of
more than a hundred years of recording seismic waves, scientists
d1
have developed a travel-time curve
that enables them to calculate the expected speeds that P and S waves travel.
Seismograph 2
By calculating the difference or lag in
travel time, they can use the graph to
read the distance from the epicentre.
Figure 2.6 shows a travel-time graph.
It shows a time lag of 7 minutes, 40 seconds.
There is only one position on the graph that can have
that measure, and by reading the epicentre distance on
the x-axis, you see that the distance from the epicentre is
about 6,000 km.
Determining the distance that a seismograph station
is from the epicentre does not tell you where it is located.
It could be anywhere on a circle having a radius of the
distance from the station. To locate the actual epicentre,
seismologists use a technique called triangulation. This
requires measuring the distance from at least three different stations. Seismologists draw circles around each
station, using as the radius the distance from the epicentre calculated with the travel-time graph. The point
where all three circles intersect is the approximate location of the epicentre. Figure 2.7 shows an example of
how an epicentre is located.
Figure 2.7 From the seismographs at three stations, a circle
can be drawn with a radius equal to the distance to the
epicentre. The point where the three circles intersect is the
location of the epicentre.
Epicentre of
earthquake
d3
Seismograph 3
d2
94
Experiential Science 30—Freshwater Systems
Activity 8
field activity
lab activity
library activity
4 classroom activity
chapter project
research team activity
Locating the Epicentre
Purpose
Hypothetical Earthquakes
To use triangulation to plot the epicentre of an earthquake.
Materials and Equipment
• geometry compass
• outline map of Yukon and Northwest Territories
• 3 different-coloured pens or pencils
Procedure
Distance to epicentre (km)
Station
Event A
Event B
Event C
YKW
601
693
1,205
WHY
518
284
812
INK
830
594
148
Reflections and Conclusions
Nahanni Earthquake, December 1985
Location
Distance to epicentre (km)
YKW (Yellowknife)
507
WHY (Whitehorse)
722
INK (Inuvik)
808
1. Use the map of Northern Canada. Locate the three stations
shown in the above table and label them with the station
codes.
2. Using a compass, draw a circle around each station with
the radius equal to the distance in kilometres to the
epicentre. Use the map scale to establish the radius.
3. Locate the epicentre of the December 1985 Nahanni
earthquake. If the circles do not intersect at a precise
point, identify a spot in the centre of the triangle created
by the circles. Label the epicentre on your map.
4. The Hypothetical Earthquakes table shows data for some
imaginary earthquakes in Northern Canada. Use the same
method as you used above to find their epicentres. If you
are using the same map as above, use different-coloured
pens or pencils to draw the circles.
1. In step 4, how accurately were you able to plot the
epicentre? Did the circles intersect at one point, or did you
need to estimate?
2. In step 4, if these earthquake data were real, what
community would be closest to the epicentre of each
quake?
3. Why are three stations needed to pinpoint the epicentre?
How might the epicentre be located more accurately?
4. What errors could occur when someone is determining the
epicentre of an earthquake?