OCE 661
Plate Motions and Earthquake Mechanisms
Relative plate Motions
Calculate the velocity and azimuth of the motion of the Pacific plate with respect to the North
American plate at 40N, 124W using the UNAVCO plate motion calculator: http://sps.unavco.org/crustal_motion/dxdt/nnrcalc/
Use all the available models. What is the biggest variation in the models (the pair with the largest difference)? Why might this be, based on the nature of the models.
Problem 2.
Calculate the velocity and azimuth of Plate B relative to plate C using the flat earth vector method. The fault strikes and half spreading rate at the ridge are given.
What does this configuration imply about the motion of the triple junction?
Estimating a Mechanism from First Motions
As explained in the lecture and notes, 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. This is because the body waves from a distant source (as opposed to surface waves) that arrive at distant recorders started with a downward trajectory, and have refracted back upward to the station (Fig. below)
For this exercise however, we will use a set of local recorders and a nearby source, so that the travel path is more less a straight line through the crust.
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 plot below (click on the image for a printable Adobe Acrobat version) 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.
Problem 2 - Plotting Polarities on the Focal Sphere
The data that you can use to interpret a focal mechanism are your first-motion
polarities plotted on a stereonet representing the focal sphere.
Inferring the compressional and dilatational quadrants of the focal sphere
allow you to suggest the strikes and dips of the fault plane and the auxiliary
plane.
| Longitude |
Latitude |
Distance, km |
|---|
| 176.35° | -40.250° | 3.403 |
| 176.17° | -40.292° | 14.414 |
| 176.37° | -40.061° | 19.072 |
| 176.28° | -40.408° | 20.154 |
| 176.06° | -40.106° | 25.886 |
| 176.47° | -40.453° | 27.993 |
| 176.63° | -40.339° | 29.118 |
| 176.09° | -40.429° | 29.880 |
| 176.27° | -40.618° | 43.544 |
| 176.81° | -39.989° | 48.965 |
| 176.35° | -39.699° | 59.008 |
| 176.88° | -39.665° | 78.454 |
| 176.82° | -39.541° | 87.502 |
To plot a seismogram's first-motion polarity on the stereonet, you need to
estimate the take-off angle of the seismic ray at the focal sphere. Two
angles are needed: the ray's azimuth from North and its inclination above
horizontal. Assume that all the rays here propagate more or less in a
straight line from the source at depth to the station at the surface.
Since none of the rays are refracting off a deeper interface, we use the
stereonet to represent the upper hemisphere of the focal sphere. This only works for a NEARBY EVENT!
To get the ray's inclination, use the (horizontal) distance in the table below
from the earthquake's epicenter to the recording station, and the earthquake's
16.7 km depth. The depth divided by the distance is the tangent of the inclination.
The order of the stations in the table is the same as in the seismogram plot.
The easiest way to get the azimuth is to use a protractor on a printout of
the map below (click on the map image for a printable PDF file).
Measure from the earthquake (filled, hatched circle) to each
station (crosses). You are also welcome to calculate them. The epicenter is Lon=176.3284° Lat=-40.2295°.
(I hate to say it, but don't bother to estimate or propagate errors in any of these
calculations.)
Turn in a stereonet with all 13 polarities plotted on it. Use a filled circle for
a compressional first motion, an open circle for a dilatational first motion,
and a plus sign for a nodal seismogram.
Problem 3 - Interpreting the Focal Mechanism
This is the most difficult part of the lab. You will notice that your first
motions plotted on the stereonet do not define unique focal planes.
You have to make an interpretation of the most likely type of fault,
and its likely strike and dip. Then you have to draw the focal plane of the
fault on the stereonet (remember, we are using it for the upper hemisphere).
You also have to draw the auxialiary plane, at exactly a right angle to the
fault plane (this is a cinch with a good stereonet).
If you have made the correct interpretation, all the compressional first
motions will fall within the compressional quadrants, all the dilatational
motions in the dilatational quadrants, and all the nodal rays within
10° of the fault or auxiliary plane. This rarely happens; you may
have to look at the seismograms again and re-interpret a polarity or two.
You could use the information that the Weber II
earthquake sequence is above a shallow zone of westward subduction, that
the subduction zone strikes northeast, and that the Weber II aftershocks
trend along a northwest-dipping plane to help the interpretation. If you do, please describe the caveats to this. Should such information be used to define the planes? Or used afterward to decite which plane is the more likely fault plane?
Turn in your stereonet copy with the first motions plotted on it. Also plot in
the fault and auxiliary planes, with your interpreted fault plane a solid line
and your auxiliary plane a dashed line. Hatch in the compressional quadrants
(try not to obscure your first motion symbols).
Describe the fault you have interpreted. Describe the possible range of fault strike
and dip that could still fit the polarities. List any polarities that you can't
fit. You may have one or two.
Include the answer to the question above about use of geologic information.
