EAS 106 LAB WAVES

1
EAS 106 LAB WAVES
1. Locating Earthquakes and Determining their
Magnitude 2. Finding the Speed and Amplitude of
Jet Stream Waves Earthquakes are located by
triangulation. The lag between the arrival time
of P and S waves tells the distance from the
graph of time vs distance but 3 stations are
needed to pinpoint the quake. The combination of
displacement of the S waves on the seismograph
and the distance from the station tell the
magnitude from the nomogram (which includes the
fact that displacements decrease with distance
from the focus or epicenter). Jet Stream waves
generally move from west to east. Speed is found
by finding the difference of positions
(longitudes) of two adjacent troughs or ridges on
two consecutive maps (12 hours apart). Amplitude
is determined by tracing the heights along a line
of constant latitude (here 50) and taking half
the range between troughs and ridge. Over the 12
hour period the wave amplitude increase or
diminish.
2
An Earthquake occurred somewhere in the Western
United States. Your job is to use the map, the
seismographs of the next page, the distance-time
lag graph and the distance-intensity nomogram to
find the quakes epicenter and magnitude.
3
Eureka, CA S-P Interval ______ s
Distance __________km Amplitude _________mm
Elko, NV S-P Interval ______ s Distance
__________km Amplitude _________mm
Las Vegas, NV S-P Interval ______ s Distance
__________km Amplitude _________mm
4
Distance-Time Lag graph for relatively nearby
earthquakes. In the example shown below with red
arrows and yellow ellipse, the time lag is 35
seconds, so the epicenter is 350 km away.
5
Distance-Intensity Nomogram
6
130 120 110 100
90 80 70

7
130 120 110 100
90 80 70

8
130 120 110 100
90 80 70

9
Jet Stream Heights and Wave Amplitudes
140 130 120 110 100
90 80 70 60
10
Jet Stream Wave Worksheet Map source
http//nomads.ncdc.noaa.gov9091/ncep/charts
Jet Stream Table 1
Convert Longitude to Distance At the Equator, a
line of constant latitude that circles the globe
has 360 longitude. Its distance is, d(0)
2prearth rearth 6.37(10)6 m At all other
latitudes d(L) d(0)cos(L). The length of a line
of constant latitude 10 of longitude long is
_______m
Jet Stream Table 2
Jet Stream Table 3