Download Episodic Tremor and Slip Lab (doc)

Mysterious motions along the California
coast - Student Worksheet
On April 25, 1992 an area of Northern California, called
Cape Mendocino, moved. This movement was detected
and recorded by Global Positioning System (GPS) stations
installed throughout the area by scientists. Cape
Mendocino (shown to the right) is located at a triple
junction or a place where three tectonic boundaries all
meet at a point. In this case, San Andreas Fault,
Cascadia Subduction Zone and Mendocino Fracture (a
transform plate boundary between the tiny Gorda Plate
and the Pacific Plate) also join up.
Below is a map that shows the location of several of the
GPS stations that recorded the ground movement, and the
amount and direction of the movement, which occurred
over a very short time.
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Use the map on the previous page to fill in the table below. Use a ruler and the information in
the Map Legend to measure the distance the GPS station was displaced on April 25, 1992.
Station Name
Bear Ridge 2
Amount Displaced (cm)
King Peak 2
Pierce Ecc
Rhonerville
Average for these 4 stations
What was the greatest overall displacement that a station experienced?
What do you think might have happened within the Earth to cause the ground to move in this
region?
How would you confirm your answer to the previous question? What other data would you
need to support your hypothesis?
If you suspected an earthquake occurred, use the IRIS Earthquake Browser at
www.iris.edu/ieb/ to see if you are correct. Input the date 1992/4/25 for both the latest and
earliest time to see where an earthquake occurred that day.
Was your hypothesis confirmed, or do you need to formulate a new hypothesis for why the
ground moved that day?
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Part 2
Now let’s take a look at some more recent GPS data recorded near the Cascadia Subduction
Zone. This data was recorded by station ALBH located on Victoria Island, British Columbia,
which is north of Cape Mendocino, California.
Here is a picture of the GPS station, along with a map of its location:
What plate boundary is located near this GPS station?
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The data for this station is displayed a little
differently. Instead of vectors, as was shown
previously, the position of the station is recorded
and plotted for each day as a GPS Time Series
Plot. While the position of the station is recorded in
three dimensions (that is, the North-South, EastWest, and Vertical displacements are measured),
we are only going to look at the data for the EastWest motion of the station.
In this graph, if the station is displaced eastward, a
dot is drawn above the previous measurement
(e.g. sample data top). If the dot is drawn below
the previous measurement the station has a
‘negative’ eastward motion, or is moving to the
west (e.g. sample data middle). If the station were
not moving East or West, we would expect the dot
to be placed beside the previous dot (e.g. sample
data bottom).
What patterns do you see in the GPS Time Series Data for Station ALBH? Based on this
graph, how far did the station move eastward between 2005 and 2010?
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On average, how many mm a year does the Station ALBH move eastward?
The overall eastward motion of the station is related to the general motion of the North
American plate. If we take out this overall eastward motion and re-graph the data, the ‘zig-zag’
pattern is more pronounced:
Describe what you think may be physically happening to the station (and the ground beneath
it) to produce the ‘zig-zags’ pattern recorded by the GPS?
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What is the average East-West displacement in mm during one of the episodes of abrupt
change in direction? Over what sort of time period dies this appear to occur (e.g. years,
months, weeks, etc)?
What do you think is happening inside the Earth to produce these episodes of westward
displacement of the Earth’s surface in this region?
If you hypothesized that an earthquake may have caused these episodes of westward
displacement, test your hypothesis by using the IRIS earthquake browser (www.iris.edu/ieb) to
check to see if there might have been an earthquake during one of these episodes of
westward displacement. What does this earthquake data tell you? Did you find any
earthquakes the size of the one that occurred on April 25, 1992?
If you did not find any earthquakes, then you need to reformulate your hypothesis for what is
causing these periodic changes in the displacement of the GPS station.
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In 2003 scientists were confused by the seeming conflict between the GPS and: They noticed
that GPS stations in this region of Canada and Washington State were experiencing episodes
of westward motion despite the fact that there were no earthquakes occurring to produce this
motion on the surface of the Earth.
To investigate this phenomena further, they sifted through lots and lots of seismic data, but
instead of finding large earthquakes (sudden releases of stored seismic energy), they found
that the there were episodes of gradual energy release (gradual, but equivalent to the total
amount of energy released by a Magnitude 7 earthquake!) that corresponded to the westward
motion. These low-level seismic signals, called tremors, are weak and slowly undulating.
By studying these tremors, geologists realized that a portion of the major fault in this region of
the US and Canada, the Cascadia Megathrust, was slipping to the westward displacement at
the surface of the Earth.
Examine this figure:
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The figure above shows the GPS and seismic signals that are produced by different types of
fault behavior along with the where they occur along the megathrust fault.
Large, destructive earthquakes tend to occur in the ‘locked’ zone of the fault where the energy
from the plate motions is stored elastically and then is suddenly released as an earthquake. In
contrast, the deepest segment of the fault simply “creeps” without storing energy because this
material behaves plastically rather than elastically. Between the “locked” and “creeping”
segments of the fault is a zone where the fault stuck, but is weaker than the locked zone above
it. In this zone, it is believed that Episodic Tremor and Slip or gradual release of seismic
energy (the phenomena you just learned about) occurs because fluid pressure reduces the
load of the overriding plate enough to overcome the locking friction but not enough to allow the
rupture to occur suddenly.
Episodic Tremor and Slip was not known to scientists before 2003, but since then there has
been a lot of research to understand why it happens. It is important to understand how and
why it happens, because a number of scientists think it might ‘load’ the locked zone, leading to
a destructive earthquake in the future.
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