Earthquake Epicenters
An earthquake is a shaking motion of a portion of the earth's crust. This motion results
from the passage of waves traveling through the earth away from a place of rupture or
faulting. The location of the rupture or faulting, which may be within or below the earth's
crust, is called the focus. The point on the earth's surface directly above the focus is
called the epicenter. In this exercise you will use the records of an earthquake from
several different locations to find the epicenter of the earthquake.
A seismograph is used to record several types of waves radiating through the earth away
from the focus. Two of these waves can be used to locate earthquake epicenters. These
are:
1) "P"- waves or longitudinal wave (also called primary waves because they are
the first to appear on a seismogram).
2) "S"- waves or transverse waves (also known as secondary waves because they
are the second major wave to appear on the seismogram).
The P waves travel through the crust at a velocity of about 5 miles per
second and the S waves at a velocity of about 3 miles per second. Because
of this velocity difference, the seismogram will record the arrival of the Pwave first. At a later period of time the seismogram will record the S-wave
arrival. The difference in time between the S-wave arrival time and the Pwave arrival time will be a measure of the distance of the seismograph
from the epicenter.
Travel time curves are graphs that show how long it takes for a particular
wave to travel a certain distance. If the difference in the arrival times for
the P and S at some particular station can be determined, then by using the
travel time curves, the distance from that station to the epicenter can be
obtained.
For example, in Figure 6-2 ( a sample seismogram recorded in Dallas) notice that the P
wave arrives ~ 2.1 minutes and the S wave arrives ~ 5.4 minutes. The difference in
arrival times (5.4-2.1 = 3.3) is 3.3 minutes.
To get the distance, you must use the travel time curves given in Figure 6-1:
1)
Lay a sheet of blank tracing paper along the S-P (time) vertical axis in
Figure 6-1, and draw a vertical line on the tracing paper
2)
Using the vertical scale on Figure 6-1, mark two dots on the vertical line
on the tracing paper so there is a gap of 3.3 minutes (a 3.3 minute time
interval) between the dots.
3)
Keeping the vertical line of the tracing paper parallel to the vertical lines
on Figure 6-1, slide the tracing paper across Figure 6-1 until the two dots
on the tracing paper lie exactly on the S and P curves.
4)
Draw a vertical line through the S and P curves at these points until it
intersects the horizontal axis and gives you a distance in miles.
This gives the distance between the station and the epicenter. For the sample given in
Figure 6-2, the epicenter is 1200 miles from Dallas.
Exercises
Using the sections of seismograms from three stations that are given in Figure 6-3:
1
Estimate to the nearest tenth of a minute the arrival times of the P and S waves at
each station.
Site Name
Time of P arrival
Time of S arrival
Difference (S-P)
Sitka
______________
______________
______________
Columbia
______________
______________
______________
Honolulu
______________
______________
______________
2.
Using the S minus P times, determine the distances corresponding these values
Site Name
Distance to epicenter
Sitka
___________ miles.
Columbia
___________ miles.
Honolulu
_____________ miles.
3) Next you will find the location of the earthquake epicenter distances just obtained.
a) Locate the three sites on the world map (Figure 6-4). The latitude and
longitude of each site is as follows:
Site Name
Lat.
Long.
Sitka
57 N
135 W
Columbia
35 N
81 W
Honolulu
21 N
158 W
b) Draw a circle about each site with a radius equivalent to the distance to
the epicenter determined for that site. Use a drawing compass and the
scale given on the map for this purpose. The three circles should intersect
at one point. The location of the epicenter is at the following:
Lat._________
Long.________.
Source: http://mimp.mems.cmu.edu/~ordofmag/earthqua/earqua.htm