PSC 150

1
PSC 150 EXERCISE "EARTHQUAKES"
2
Definitions
1.) Focus- This is the point, usually deep
underground, where the initial dislocation and
energy release occurs.
2.) Epicenter- This is the point on the Earth's
surface directly above the earthquake's focus. It
is the point where the waves will have their
greatest amplitude.
3.) Primary(P) Waves- These are the longitudinal
sound waves generated by the earthquake. They are
the fastest moving of all earthquake waves and
are capable of propagating through both solids
and liquids.
4.) Secondary(S) Waves- These are transverse
shear waves. They travel more slowly than "P"
waves and can only propagate through solids. The
inability of "S" waves to travel through the
Earth's outer core has lead seismologists(geologis
ts who specialize in the study of earthquakes) to
conclude that this region is liquid.
Together the "P" and "S" waves are called body
waves since they travel through the Earth.
3
Both P-Waves and S-Waves Recorded
Only P-Waves Recorded
4
5.) Long(L) Waves- These are the waves that
travel along the surface of the Earth and are
responsible for most of the damage associated
with an earthquake. They are the slowest
traveling of the three earthquake waves. There
are two types Love and Rayleigh.
5
6.) Magnitude- The magnitude of an earthquake is
a measure of the amount of energy released by the
earthquake.
It is determined by measuring the largest
amplitude of the waves arriving at a seismic
recording station, adjusting for the distance
between the station and the earthquake's
epicenter and then plotting on a Richter Scale.
Using the Richter Scale an earthquake is assigned
a magnitude of 1 or greater.
The Richter Scale is logarithmic so that an
increase of one unit on the scale corresponds to
a ten-fold increase in the amplitude of the waves
detected. An increase of one unit on the scale
also corresponds to a 30-fold increase in energy
released.
6
For example, an earthquake with a magnitude of
"7" released 30 times as much energy as an
earthquake with a magnitude of "6" and 900 (30 x
30) times as much energy as one with a magnitude
of "5".
NOTE An earthquake with a magnitude of 6.5
releases as much energy as the atomic bomb
(20-kilotons) dropped on Hiroshima, Japan at the
end of World War II.
7
(No Transcript)
8
Part One Determining the Distance to the
Earthquakes Epicenter
9
Because the three types of earthquake
waves(P,S,L) travel at different speeds they will
require different times to travel to a particular
recording station. The "P" waves will arrive
first, followed by the "S" waves and then the "L"
waves. This difference in arrival times can be
used to determine the stations distance from the
epicenter of the earthquake.
Consider the following analogy Two cars, A and B
are to travel down a long straight track. They
will start at the same place and time, but car A
will travel at 100 mph while car B will travel at
only 50 mph. At certain points down the track
observers are stationed and equipped with timers
and communications equipment. Observer 1 is
placed 100 miles from the starting line, observer
2 is 200 miles from the starting line and
observer 3 is at an unknown distance.
10
(No Transcript)
11
(No Transcript)
12
100 miles
x miles
200 miles
13
Car A arrives at 100 miles
100 miles
x miles
200 miles
14
Car A arrived at 100 miles
Car A arrives at 200 miles
Car B arrives at 100 miles
Difference in arrival times _at_ 100 miles 1 hour
100 miles
x miles
200 miles
15
Car A arrived at 200 miles
Car B arrives at 200 miles
Difference in arrival times _at_ 200 miles 2 hour
100 miles
x miles
200 miles
16
Car A arrives at x miles
100 miles
x miles
200 miles
17
Car A arrived at x miles
Car B arrived at x miles
Difference in arrival times _at_ x miles 4 hour
x 400 miles
100 miles
x miles
200 miles
18
The distance from a seismic recording station to
the earthquakes epicenter can be determined by
measuring the difference in arrival times of the
"P" and "S" waves.
Based on the speeds of "P" and "S" waves the
following table has been calculated
19
EXAMPLE Suppose that a seismograph in Orangeburg
detects the arrival of P waves at 110509 am and
the arrival of S waves at 110527 am.
The difference in arrival times is 18 seconds.
According to the table above the earthquakes
epicenter was 150 km from Orangeburg.
20
Listed below are the arrival times of the P and S
waves at three other seismic recording stations
Problem 1. Determine the distance from each
station to the earthquake's epicenter
21
Part Two Determining the Location of the
Earthquakes Epicenter
22
The distances from the three stations to the
epicenter can be used to determine the location
of the epicenter. On a map where the positions of
the stations are marked, draw circles centered on
each station. The radius of each circle
represents that station's distance from the
epicenter. The region where the three circles
intersect is the location of the epicenter. The
circles are of course drawn to the same scale as
used on the map, in this case 1 cm 25 km.
Problem 2 Using the map provided determine the
location of the epicenter. Draw the smallest
possible circle enclosing the region where the
epicenter is located.
23
(No Transcript)
24
Part Three Determining the Earthquakes Magnitude
25
The amplitudes of the waves reaching a certain
seismic recording station will depend on the
magnitude of the earthquake and that station's
distance from the epicenter. The Richter scale
takes these effects into account and provides a
means of assigning a magnitude to an earthquake
which is independent of the station doing the
measurements.
Shown below are the seismograph tracings recorded
at stations 1,2 and 3.
26
Orangeburg Station Recording 150 km from Epicenter
Amplitude 50mm
27
The Richter Scale
Magnitude 7.4
Orangeburg Station
Amplitude 50mm
distance 150km