Earthquake Mechanism Lab Exercise
Estimating a Mechanism from First Motions
As explained in the text and in lecture, the P waves radiated from the double-couple
mechanism of an earthquake are compressional in some directions and dilatational in
others. The two compressional quadrants are orthogonal to the dilatational quadrants
(about the intermediate principal strain axis that is along the fault plane).
A P wave propagating out of a compressional quadrant will initially shift the ground
upwards when it reaches a seismic recorder. We will use recorders that have recorded
ground motion in the vertical direction. Seismograms from such instruments will initially
move up or toward positive polarity when they are from a compressional quadrant.
Seismograms from vertical instruments in a dilatational quadrant will show negative
polarity from an initial downward motion.
The polarity of seismogram motions after the first motion can be extremely complicated.
Usually it quickly becomes a series of positive-negative vibrations. Seismometers that are
on the nodal plane between the compressional and dilatational quadrants of an
earthquake do not record a strong first motion. Instead of being impulsive, their first
arrivals are emergent. These are called nodal seismograms.
Problem 1 - First-Motion Polarities
The seismogram plot below shows 13 seismograms recorded from a magnitude 4.7
aftershock of the the M6.2 May 1990 Weber II earthquake on the east side of the North
Island of New Zealand. All the seismograms are from vertical instruments, located as
indicated. Seismogram swings in the up direction have positive polarity. Look for an
initial rise (or fall) out of the pre-arrival noise that looks more like an exponential curve
than a sine wave. Note how the vibrations recorded by more distant stations have lower
frequency. Relative record time increases to the right; these seismograms only plot
arrivals in a 3-second window with the first motion about one second from the left side.
Print the seismograms. Circle the first motion on each one. Identify whether each first
motion is compressional, dilatational, or nodal, and write your identification to the right
of each seismogram.
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