Assignment 4 Tutorial

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Assignment 4 Tutorial Locating Earthquake
Epicentres
When an earthquake happens Seismic (shock) waves
travel outward from the centre of the site of
shock wave generation (the focus of the
earthquake).
Primary waves (P-waves) travel fastest through
the Earth and arrive at any point at the Earths
surface before the next fastest waves, the
secondary waves (S-waves).
These shock waves may be detected and measured at
the Earths surface by using a seismograph.
A seismographic chart is a record of such waves
and it records information that includes the
time of arrival of each wave and the amplitude of
the waves.
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In this example, an Earthquake took place
somewhere in the map area shown below.
Seismographs recorded the seismic waves at the
three locations shown.
Using Seismographic charts you will find the
location of the Epicentre of the Earthquake (the
point on the land surface immediately above the
focus).
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The Seismographic Chart, below, shows that at
Location 1 the P-waves began to arrive at the
location of the seismograph at 13 seconds whereas
the S-waves did not arrive until 49 seconds.
The distance to the earthquake epicentre
determines the difference in time of arrival of
the P and S waves.
In this case the difference is S P 49 13
36 seconds.
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The amplitude of the S-wave at the site of the
seismograph depends on the distance to the focus
and the magnitude of the Earthquake (which
increases with the amount of energy released to
produce the shock waves).
In our example, the amplitude of the first S-wave
is 11 mm.
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The scales, below, can be used to interpret the
distance to the Earthquake Epicentre and the
magnitude of the Earthquake.
Give the graph a few seconds, its animated.
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Once again, heres how it works
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The distance to the epicentre can be taken from
the scale on the far left on the basis of the
difference between the P and S wave arrival times.
In this case, to produce a 36 second lag between
the arrival of the P and S waves requires a total
travel distance of 325 kilometres.
Therefore, the Epicentre was 325 kilometres from
Location 1.
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Because we do not know the direction to the
Epicentre from Location 1 we only know that it
must fall on the circle centred on Location 1
with a radius equal to 325 km.
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If we add similar data from Location 2 (taken
from Seismographic charts not included in this
tutorial but collected in the manner used for
Location 1) S P 55 seconds Amplitude 2.1
mm
We find that the distance from Location 2 to the
epicentre was 510 km.
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When we plot the circle with radius 510 km,
centred on Location 2, we find two points of
intersection with the circle for Location 1.
Either is the potential Epicentre.
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Now well add the data for Location 3.
S P 48 seconds Amplitude 4 mm
We find that the epicentre was located 430 km
from Location 3.
Note that the magnitude is the same no matter how
far from the epicentre the seismograph is.
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By drawing the circle with radius 430 km centred
on Location 3 we find a unique intersection of
all three circles this intersection is the
Epicentre.
The procedure followed in this tutorial is
exactly the procedure to use for Assignment 1.