Page 426 - Laboratory Manual in Physical Geology

One minute
First L-waves arrive

First P-waves arrive First S-waves arrive

7:12 7:14 7:16 7:18 7:20 7:22 Times
L-wave arrival time is 7:18.3
P-wave arrival time is 7:14.2
S-wave arrival time is 7:17.4

FIGURE 16.4 Seismogram of a New Guinea earthquake. Most of the seismogram, recorded at a location in Australia, shows only

minor background deviations (short zigzags) from a horizontal line, such as the interval recorded between 7:12 and 7:14. Large vertical
deviations indicate motions caused by the arrival of P-waves, S-waves, and L-waves of the earthquake (note arrows with labels). By making
detailed measurements with a ruler, you can determine that the arrival time of the P-waves was 7:14.2 (14.2 minutes past 7:00), the arrival time
of the S-waves was 7:17.4, and the arrival time of the L-waves was 7:18.3.
ACTIVITY Determining Relative Motions along
16.4 San Andreas Fault Analysis the New Madrid Fault Zone
at Wallace Creek The relative motions of blocks of rock on either side
of a fault zone can be determined by mapping the
|
THINK How do geologists use remote sensing, way the pen on a seismograph moved (up or down on
About It geologic maps, seismograms, and first the seismogram) when P-waves first arrived at various
motion studies to analyze fault motions? seismic stations adjacent to the fault. This pen motion
is called first motion and represents the reaction of the
P-wave to dilation (pulling rocks apart) or compression
OBJECTIVE Analyze and evaluate active faults using (squeezing rocks together) as observed on seismograms

remote sensing and geologic maps.
(see FIGURE 16.6 , left).
PROCEDURES If the first movement of the P-wave was up on a
1. Before you begin , this is what you will need : seismogram, then that recording station (where the
seismogram was obtained) experienced compression
___ ruler
___ Activity 16.4 Worksheet (p. 403 ) and pencil during the earthquake. If the first movement of the P-wave
with eraser was down on a seismogram, then that recording station
was dilational during the earthquake. What was the
first motion at all of the seismic stations in FIGURE 16.3 ?
ACTIVITY (Answer: The first movement of the pen was up for
each P-wave, so the first motion at all three sites was
16.5 New Madrid Blind Fault compressional.)
Zone By plotting the first motions observed at recording
stations on both sides of a fault that has experienced
THINK | How do geologists use remote sensing, an earthquake, a picture of the relative motions of the
fault emerges. For example, notice that the first motions
About It geologic maps, seismograms, and first observed at seismic stations on either side of a hypothetical
motion studies to analyze fault motions?
fault are plotted in relation to the fault in FIGURE 16.6

(right side). The half-arrows indicate how motion
OBJECTIVE Interpret seismograms and fi rst motion proceeded away from seismic stations where dilation was

studies to infer fault motion in the blind New Madrid recorded and toward seismic stations where compression
Fault within the North American Plate.
was recorded (for each side of the fault).
PROCEDURES
1. Before you begin , read Determining Relative
Motions Along the New Madrid Fault Zone. Also,
this is what you will need :
___ ruler
___ Activity 16.5 Worksheet (p. 404 ) and pencil

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