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Mysterious earthquakes, unexpected movements. Episodic Tremor and Slip. About the Cascadia Region. Satellite view of the US. The northwestern portion of US is the Cascadia Subduction Zone Let’s zoom in. ----- Divergent Boundary ----- Convergent Boundary. Washington. Juan de Fuca

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Mysterious earthquakes, unexpected movements

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about the cascadia region
About the Cascadia Region
  • Satellite view of the US.
  • The northwestern portion of US is the Cascadia Subduction Zone
  • Let’s zoom in. . .

----- Divergent Boundary

----- Convergent Boundary


Juan de Fuca



North American


about the cascadia region3
About the Cascadia Region
  • Animation: The Growth of a Continent(
  • Focus on the edge of the dark gray continent.


earth exploration toolbook chapter
Earth Exploration Toolbook Chapter

Analyzing Plate Motion Using EarthScope GPS Data

step 1 download the data
Step 1 – Download the Data
  • Zoom in on Washington State
  • Click on the balloon for the first station assigned to your row.
  • Click on the Download Time Series Data In Excel Compatible Format link

Start at Data for Educators

part i creating time series plots from gps data
Part I: Creating time series plots from GPS data
  • See how GPS data can show the movement of a tectonic plate in the Cascadia subduction zone in the Pacific Northwest.
  • Calculate velocity vectors
  • Row 1
    • ALBH
    • PABH
  • Row 2
    • NEAH
    • CHZZ
  • Row 3
    • SEAT
    • NEWP
  • Row 4
    • TWHL
    • CORV
part 2 view the data
Part 2: View the data
  • Station’s 4-character identity
  • Date range & release date for the data
  • Reference position
  • North, East, and Vertical (mm) change in position from reference point
more information about the data
More information about the data
  • In the year 2004… processed GPS data became available
  • North, East, and Vertical Standard Deviation
  • Qualifier about the data quality
part 3 create a scatter plot
Part 3-Create a scatter plot
  • Select the cell that contains the column heading Date.
  • Press Shift then Ctrl (or Apple’s Command key) thenthe down arrow
  • Hold the Ctrl key and click “North (mm)"
  • Continue to hold the Ctrl key and press Shift + the down arrow key
  • The date column and the North mm should be highlighted (selected). (… )
part 3 create a scatter plot1
Part 3-Create a scatter plot
  • Click the Chart Wizard icon or Insert -Chart .
  • Choose the XY (Scatter) chart type
  • Then Next
part 3 create a scatter plot2
Part 3-Create a scatter plot
  • Check the data range to ensure that the date and north columns are listed.
part 3 create a scatter plot3
Part 3-Create a scatter plot
  • Make a title to the graph [such as SEAT GPS Station: North (mm)] and label the X [Date] and Y [North (mm)] axes.
  • De-select the “show legend” option
  • Place the chart in a new sheet and call it SEAT North Plot
now let s make the graphic readable
Now, let’s make the graphic readable
  • Double click on the x axis
  • Click on the Scale tab
    • Minimum = 37987
    • Maximum = 39500 (Feb 22, 2008)
    • Major unit = 182.5
    • Minor unit = 30.35
  • Double-click on the y axis
  • Click on the Scale tab.
    • Value (x) axis crosses at:
    • Use the minimum y value , -10.
part 4 calculating station motion
Part 4: Calculating Station Motion
  • Select Add Trendline... from the Chart menu (chart > Add Trendline).
  • Choose Linear as the Trend/Regression type.
  • Click Options. Check the box next to Display equation on chart.
adding vectors together
Adding Vectors Together
  • SEAT GPS Station has moved
    • 3.1 mm to the North in one year
    • 4.6 mm to the East in one year


3.1 mm

4.6 mm

mathematical method
Mathematical Method

Total Vector Magnitude =

( )

mapping your vectors
Mapping your vectors


10 mm/yr







Cascadia Region GPS Velocity Vectors













La Grande





  • Coastal: Fastest movement to NE.
  • Corridor: some NE movement
  • Eastern stations… are not moving due to deformation (relative motion is zero)

Note: Data from the UNAVCO Plate Boundary Observatory uses a reference frame where the interior of the North America Plate is not moving.

going back in time
Going Back in Time

Geologic Survey of Canada scientists saw something strange at ALBH.

Note that the “0” on the x-axis is Jan. 1, 1994.

The green line shows the general trend of the data.

The break in the line (arrow) indicates a sharp change in the trend of the line.

How much slip occurred?

Jan 1994

what was that
What was that??
  • Oct 1 – Oct 15, 1994
    • SW movement measures
    • ~2 mm South
    • ~6 mm West.

Could this be a problem with the equipment?

a pattern seems to emerge
A Pattern Seems to Emerge…

The technology of GPS measurement increased dramatically.

The pattern was seen again… and again…

In 1999, Herb Dragert from the Geological Survey of Canada confirmed this strange reversing movement patterns in the continuous data from more GPS stations.

a pattern seems to emerge1
A Pattern Seems to Emerge . . .

How often is the reverse motion happening?

  • And the answer is:
  • These ‘slow slip’ events occur about once every 14 months.
Meanwhile …

~200 km

There were strange tremors…

What do you notice about the seismograms?

Meanwhile …

Seismologists couldn’t explain the data.

Instrument problems?

‘Background noise’:

Herd of cattle walking near the instruments


Human-made vibrations like cars or trains?

putting the pieces together
Putting the Pieces Together

Herb Dragert and other geophysicists looked a lot more carefully at data from the past.

They compared the seismographs from the 9 seismometers and this is what they saw:

putting the pieces together1
Putting the Pieces Together

The black lines along the bottom represent the time when the slow slip ‘tremors’ were happening, 6-14 days every 14 months

  • Plotted together
  • Tremors
  • Timing of the southwest motion of the North American plate!
more recent data
More Recent Data

Updated from Rogers & Dragert, Science, 2003

  • Blue dots show GPS east component at Victoria
  • Bottom graph shows total # of hrs with tremor activity in a 10-day window
  • The shaded bars show predicted time windows for ETS

Tremors are called: “slow earthquakes” or “slow-slip events”

The events are:

Episodic Tremor and Slip

putting the pieces together2
Putting the Pieces Together

Click on image to start animation

does ets happen everywhere
Does ETS happen Everywhere?

So what’s happening????



No slow slip event occurs beneath PABH (in this time period)

ALBH moves southwest during slow slip event



subducting plate animation
Subducting Plate Animation

Click on image to start animation

stress across the plate boundary
Stress across the plate boundary



Plates converging at 4 cm / yr

conceptual model of ets
Conceptual Model of ETS



Equivalent of Mw = 6.7 Earthquake

blocks demonstration
Blocks demonstration

Click on image to start animation

ets progression over days
ETS Progression over days

propagation of sumatra earthquake
Propagation of Sumatra Earthquake Ishii 2005

forecasting a megathrust earthquake
Forecasting a Megathrust Earthquake



  • Stress builds in Locked zone during each ETS
  • More likely to cross critical threshold of stress during ETS events than at other times
  • Sudden release of stress is an earthquake – which could rupture the locked zone… but how big an earthquake?
let s add some numbers
Let’s Add Some Numbers
  • During ETS events, the transition zone of the deep plate slides ~ 3cm / yr in a southwest direction.
  • Stress accumulates across the locked zone.
  • The locked zone prevents relative plate motion; a ‘slid-deficit ‘ builds up for the locked zone
  • In 500 years, how much ‘slip deficit’ would build up on the locked portion of the plate interface?
consequences of ets earthquake why you need to know this
Consequences of ETS: EarthquakeWhy You Need to Know This!

What did you get?

1500 centimeters slip deficit

If the locked zone breaks, the ground will snap to the southwest …

… 1500 centimeters, or 15 meters! That’s almost 45 feet!

That’s a HUGE earthquake!

modeling a tsunami
Modeling a Tsunami

Eric L. Geist

tsunami of 1700
Tsunami of 1700

January 26, 1700 megathrust earthquake: mathematical model showing the tsunami passing Hawaii.

consequences of ets tsunami why you need to know this
Consequences of ETS: TsunamiWhy You Need to Know This!

If this huge earthquake happens right off the west coast of North America…

…a tsunami will be generated along the coast for hundreds of miles…

Inundating coast regions for hundreds of meters inland

Impacting many, many lives

  • Ishii 2005
earthquake history of canada
Earthquake history of Canada