Geology Reference
In-Depth Information
QUESTIONS (6, PART A1)
Epicenter
1.
In Figure 6.2, use the time scale to determine the lag in
arrival time between the P and S waves at four stations: St.
Louis, Missouri (SLM); Bloomington, Indiana (BLO); Min-
neapolis, Minnesota (MNM); and Bowling Green, Ohio
(BGO). The first major impulse on the left in the seismogram
indicates the arrival of the first P wave at the station, the
second impulse, the arrival of the first S wave. The lag time,
T, is given by the difference between S and P times. Enter the
lag time value for each station below:
FIGURE 6.1
Diagram of directions of vibrations of body (P and S)
and surface (Love and Rayleigh) waves (Hays, 1981).
2.
To determine the distance from the earthquake to each seis-
mograph station we must first determine the time lag between
P and S wave arrivals at a given distance from an earthquake,
say 100 km, knowing the average velocities of the P and S
waves. If the average velocity of the P wave is 6.1 km/sec and
the average velocity of the S wave is 4.1 km/sec, what is the
time required for each wave to travel 100 km? (It may help to
think of this problem like a very fast driving trip: if you want
to go 100 km, and you drive at a rate of 6.1 km/sec, how long,
in seconds, will it take you to get to your destination?)
PART A. EPICENTER, INTENSITY,
AND SEISMIC RISK
Epicenter (6, Part A1)
After an earthquake, seismologists are faced with the
task of finding when and where the shaking began.
They do this by examining the
seismograms
from several
seismograph stations. Because the P and S waves travel
at different rates, the difference in arrival times varies
from station to station depending on the distance from
the source.
The average travel times of P and S waves com-
piled from many earthquake records are used to make
travel-time graphs and tables showing the time
required for waves to travel various distances from a
hypocenter. These records show that P waves travel
more rapidly than S waves. Therefore, travel-time
curves will show P and S waves as separate curves.
Surface waves travel at about 90 percent of the velocity
of S waves because the surface waves are traveling
through lower velocity materials located at the Earth's
surface.
If arrival times are available from several seismo-
graph stations, the distances given by the travel-time
curves may be used to determine the earthquake's
location. The distance provides the radius of a circle
about the seismograph station. The
epicenter
is located
somewhere on that circle. With at least three stations,
the location of the epicenter may be determined as the
point where the three circles intersect.
We can also arrive at the distance to the epicen-
ter by using simple subtraction and a proportional
relationship. Because of their different velocities,
there is a time lag between arrival of the first P and
first S wave at a seismograph station. The time lag
(time of S minus time of P) can be determined from
seismograms. This time lag can be used to compute
the distance to the epicenter, provided the average
velocity of each wave type is known. In the first part
of the exercise, we will use seismograms from four
different stations to locate the epicenter and time of an
earthquake.
P waves (6.1 km/sec) travel 100 km in
seconds.
S waves (4.1 km/sec) travel 100 km in
seconds.
Thus the time lag between the arrival of P and S waves at a
distance 100 km from the hypocenter (
TJOQ
) is
s
econds.
3. Remembering that for longer distances there is a propor-
tionally longer lag time, we can construct a simple equation
to calculate the unknown distance x to each station:
x
100km
T
x
T
10
0
where x = unknown distance in km; Tx = lag time for
distance x;
TJOO
=
lag time at 100 km
Since values for Tx are known from Question 1 and the
value of Tjgo is known from Question 2, the equation can be
solved for x for each station. More than one station is needed
to determine the epicenter since the information from one
station can only give the distance to the earthquake and not
the direction. The minimum number of stations needed to
locate an epicenter is three.
Using the data from Figure 6.2 and the equation above,
determine the distance to the earthquake epicenter from
each station and enter below.
SLM:
km
MNM:
km
km
BLO:
BGO:
km
