Mysterious earthquakes, unexpected movements

1. Mysterious earthquakes,unexpected movements Episodic Tremor and Slip

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

3. ----- Divergent Boundary ----- Convergent Boundary Washington Juan de Fuca Plate Oregon North American Plate

4. About the Cascadia Region

5. About the Cascadia Region

6. About the Cascadia Region • Animation: The Growth of a Continent(http://www.classzone.com/books/earth_science/terc/content/visualizations/es0808/es0808page01.cfm?chapter_no=visualization) • Focus on the edge of the dark gray continent. A

7. Overview of procedure

8. Earth Exploration Toolbook Chapter Analyzing Plate Motion Using EarthScope GPS Data http://serc.carleton.edu/eet/platemotion/index.html

9. 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 http://www.unavco.org/edu_outreach/data.html

10. 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

11. 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

12. 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

13. Part 2: What’s happening over time? SEAT

14. 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). (… )

15. Part 3-Create a scatter plot • Click the Chart Wizard icon or Insert -Chart . • Choose the XY (Scatter) chart type • Then Next

16. Part 3-Create a scatter plot • Check the data range to ensure that the date and north columns are listed.

17. 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

18. 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.

19. 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.

20. Completed plot

21. 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 North 3.1 mm 4.6 mm

22. Mathematical Method Total Vector Magnitude = ( )

23. Mapping your vectors Legend 10 mm/yr ALBH NEAH P453 SEAT P020

24. What’s happening in the East

25. Velocity Vectors of Western U.S.

26. Cascadia Region GPS Velocity Vectors Neah Bay NEAH Pacific SEAT Beach PABH Othello Kelso KELS Tillamook Wasco La Grande Newport NEWP Corvallis CORV • 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.

27. Further Analyzing the Data

28. Evidence #1: What was that? ALBH (Albert Head)

29. 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

30. 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?

31. 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.

32. 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.

33. Meanwhile … ~200 km There were strange tremors… What do you notice about the seismograms?

34. Meanwhile … Seismologists couldn’t explain the data. Instrument problems? ‘Background noise’: Herd of cattle walking near the instruments Wind Human-made vibrations like cars or trains?

35. 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:

36. 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!

37. 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

38. Putting the Pieces Together Click on movie camera to start animation ETS-GPSmove-seismic-graph-med.mov

39. Does ETS happen Everywhere? So what’s happening???? ALBH Washington No slow slip event occurs beneath PABH (in this time period) ALBH moves southwest during slow slip event PABH Oregon

40. Does ETS happen everywhere?

41. Subducting Plate Animation Click on movie camera to start animation Subduction_3zones-med.mov

42. Stress across the plate boundary ALBH PABH Plates converging at 4 cm / yr

43. Conceptual Model of ETS ALBH PABH Equivalent of Mw = 6.7 Earthquake

44. Blocks demonstration Click on movie camera to start animation BlocksETS-Time_vs_Dist_med.mov

45. ETS slow slip migration Yellow = modeled data Blue = observed data 49oN 48oN 47oN

46. ETS Progression over days http://www.pnsn.org/WEBICORDER/DEEPTREM/fall2006.html Trem_movie_2008eq_time.mov

47. Propagation of Sumatra Earthquake http://www.seismology.harvard.edu/~ishii/sumatra/index.html Ishii 2005

48. Forecasting a Megathrust Earthquake ALBH PABH • 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?

49. 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?

50. 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!