On January 26, 2017, ICLR conducted a Friday Forum webinar entitled Great Cascadia megathrust earthquakes: Past, present, and future, facilitated by Dr. Kelin Wang of the Geological Survey of Canada. The event marked the 317th anniversary of the Great Cascadia Megathrust Earthquake (January 26, 1700). Megathrust earthquakes and the tsunamis caused by them pose threats to communities along much of the west coast of North America. Without an instrumentally recorded great Cascadia earthquake, our knowledge of these events that guides risk mitigation is based on geological observations, geodetic monitoring, geophysical modeling, and global comparison. In this webinar, Kelin Wang reviewed the state of knowledge of relevant observational and theoretical studies and discussed remaining scientific challenges regarding west coast megathrust earthquakes. Kelin Wang is a senior Research Scientist with the Geological Survey of Canada. He completed his undergraduate studies in geology at Peking University, China, and earned his Ph.D. in geophysics from the University of Western Ontario. Most of his current research is on the geodynamics of subduction zones and related earthquake and tsunami hazards, but he has also worked on a range of other topics regarding the thermal, mechanical, and hydrogeological processes of Earth’s lithosphere. He is an Adjunct Professor at the University of Victoria and an Honorary or Guest Professor for several other scientific institutions. He is Co-Editor-in-Chief for Tectonophysics and was or still is on the Editorial Boards of several other scientific journals. He is a Fellow of the American Geophysical Union and was formerly the Secretary of the Canadian Geophysical Union. In 2015, he was awarded the J. Tuzo Wilson Medal by the Canadian Geophysical Union and named the Birch Lecturer by the American Geophysical Union.

1. Great Cascadia megathrust earthquakes:
Past, present, and future
Kelin Wang
Pacific Geoscience Centre, Geological Survey of Canada,
Natural Resources Canada
January 26, 2017,
On the 317th anniversary
of the AD 1700 great
Cascadia earthquake

2. Kelin Wang
Research Scientist
Geological Survey of Canada
Natural Resources Canada
Megathrust earthquakes and their
tsunamis pose threat to
communities along much of the
west coast of North America.
Without an instrumentally recorded
great Cascadia earthquake, our
knowledge of these events that
guides risk mitigation is based on
geological observations, geodetic
monitoring, geophysical modeling,
and global comparison. In this
webinar, Kelin Wang will review the
state of knowledge of relevant
observational and theoretical
studies and discuss remaining
scientific challenges.

3. Where I live
Rupture zone of M ~ 9
Cascadia earthquake, 1700
M 7.3, 1946
M 6.8, 2001
The ascadia Subduction ZoneM 7.8, 2012

4. 2015
Chile
Mw 8.3
Cascadia
in global context

5. From 1900 – 2004: 71 (Every 18 months)
From 2004 – 2015: 19 (Every 7 months)
Number of great (M  8) earthquakes:
(Lay, 2015 EPSL)
1960
Chile 9.5 1965
Alaska 9.2
2004
Sumatra 9.2
2011
Japan 9.0

6. 2004
Sumatra
9.2
2011
Tohoku
9.0
> 50 m
14 m
Chlieh et al. (2008)
Shao et al., 2012

7. 2004
Sumatra
9.2
14 m

8. 2004
Sumatra
9.2
14 m
2011
Tohoku
9.0
> 50 m
200 km

9. 2004
Sumatra
9.2
2011
Tohoku
9.0
> 50 m
14 m
Different types of giant earthquakes
1. Tohoku type (compact):
small rupture, short duration, huge slip;
locally strong patch
2. Sumatra type (spread-out):
long rupture, long duration, smaller slip;
no locally strong patch
Lay and Kanamori (2011)

10. 2004
Sumatra
9.2
2011
Tohoku
9.0
> 50 m
14 m
1964
Alasaka
9.2
Lay and Kanamori (2011)
200 km
Freymueller et al.
2009

11. 2011
Tohoku
9.0
1964
Alasaka
9.2
2004
Sumatra
9.2
200 km
> 50 m
14 m
1960
Chile
9.5
40 m
20 m
Moreno et al., 2009
What about
Cascadia?
Wang et al., 2013
1700
Cascadia
~ 9.0

12. Cascadia:
The day before yesterday

13. Between
Earthquakes
Earthquake
Great Earthquakes Recorded
in Coastal Geology
From Brian Atwater
Coos Bay
From Brian Atwater
Sand sheet 1680-1720

14. (Atwater et al., 2005)
Great Cascadia earthquake of Jan. 26, 1700, recorded in Japan

15. (Satake, Wang, Atwater. 2003 JGR)
Great Cascadia earthquake of Jan. 26, 1700, recorded in Japan

16. 13
Great Earthquakes Recorded in Seafloor Sediment Deposits
(Goldfinger et al., 2005)
(Adams, 1990)

17. M 8.8-9.2
~ 20 events
Simplified from Goldfinger et al. (2016 Marine Geology)
Great Cascadia earthquakes recorded in turbidite deposits
over the past 10,000 years
Many additional
events in the south?

18. Cascadia: Today

19. Continuous GPS
Stations in 1994:
Western Canada
Deformation Array
began operation

20. Continuous GPS
Stations in 1994:
Western Canada
Deformation Array
began operation
Continuous GPS
Stations today:
U.S. Plate Boundary
Observatory (PBO)
and Western Canada
Deformation Array
Plus numerous
temporary GPS sites
and numerous seismic
stations

21. Two points 100 km
apart get closer to
each other by 5 cm
every decade
Today
AD 1700
GPS data:
McCaffrey et al. (2013),
after correction for long-
term crustal rotation

22. A few years Four decades Three centuries
Wang, Hu, He (Nature, 2012)
Deformation cycles of giant earthquakes in a viscoelastic Earth
Sumatra (and Japan) S. Chile (and Alaska) Cascadia

23. Cascadia since the 1700 earthquake
England and France
began to fight in
eastern North
America (Queen
Anne’s War).
Captain Chirikov
(Russia) landed on
northwest coast of
North America
(Prince of Wales
Island).
Dr. Wang presents
ICLR webinar on
317th anniversary
of the earthquake.
1702 1741 20171778
Captain Cook sailed
along west coast of
North America and
stayed at Nootka
Sound.
Nootka
Sound
Model by Yan Hu

24. Cascadia:
The day after tomorrow

25. E-Wgroundvelocity(m/s)
0.5
0.3
-0.1
-0.3
-0.5
0.1
0 1 2 3 minutes
Seattle
Portland
Eugene
Kelin Wang’s
home
Medford
Cascadia M9 shaking scenario
From A. Frankel and W. Stephenson,
USGS (first presented at 2015 SSA
annual meeting)
Shaking
Landslides
Liquefaction

27. Tsunami simulation
by Eric Geist, USGS

28. 10 minutes 20 minutes 80 minutes
Seattle
Portland
Eugene
Medford
Victoria
Seattle
Portland
Eugene
Medford
Victoria
Seattle
Portland
Eugene
Medford
Victoria
Tsunami simulation by Randall LeVeque, Universityu of Washington

30. 1 km
1 mile
Inundation
scenario
SM1
M1
L1
XL1
XXL1
(Bandon)
Witter et al., 2013 Geosphere

31. Cascadia:
Scientific challenges

32. Is the seismogenic zone fully locked?

33. With thermal constraints Geodetic inversion only
Used in Canadian and U.S.
seismic zoning

34. Why do we assume “shallow and narrow”? –– Very warm megathrust
Gao and Wang
(2014)
Because of young slab and thick sediment cover,
megathrust temperature > 400C at 20 km depth, too warm
for seismic rupture if fault-zone material is quartz-rich.

35. GPS data:
McCaffrey et al. (2013)
Not interplate
Not interplate
Interplate
Why do we assume full locking? –– Eerily quiet megathrust
SeaJade OBS monitoring:
Three months in 2010 and nine months in 2012
detected almost no earthquakes on megathrust.
Obana et al., 2015 BSSA

36. Partial
Landward velocity

37. Proposed
seafloor geodesy
(Wilcock et al. 2012)

38. Interseismic stress relaxation

39. 1973 (Bott and Dean)
Today
1984 (Thatcher and Rundle)
The Earth’s mantle exhibits a viscoelastic behaviour
Wang, Hu, He (Nature, 2012)

40. Cascadia since the 1700 earthquake
England and France
began to fight in
eastern North
America (Queen
Anne’s War).
Captain Chirikov
(Russia) landed on
northwest coast of
North America
(Prince of Wales
Island).
Dr. Wang presents
ICLR webinar on
317th anniversary
of the earthquake.
1702 1741 20171778
Captain Cook sailed
along west coast of
North America and
stayed at Nootka
Sound.
Nootka
Sound
Model by Yan Hu

41. ETS and the seismogenic zone

42. Episodic Tremor
and Slip (ETS)
?
Seismogenic zone

43. Wang and Trehu (2016 J. Geodyn.)

44. Tsunamigeneic seafloor deformation

45. Wang and Trehu (2016 J. Geodyn)
Currently considered
in Cascadia tsunami
source modeling
1 km
1 km
1 km
Kodaira et al. (2012 Nature Geoscience)
Before earthquake
After earthquake
50 m displacement

46. Wang and Trehu (2016 J. Geodyn.)
British Columbia
Washington
Oregon

47. Along-strike heterogeneity and
earthquake recurrence behaviour

48. Model by Wang et al. (2013 JGR)
Microfossil data: A. Hawkes, S. Engelhart, B. Horton, etc.
Great Cascadia earthquake of AD 1700

49. M 8.8-9.2
~ 20 events
Simplified from Goldfinger et al. (2016 Marine Geology)
Great Cascadia earthquakes recorded in turbidite deposits
over the past 10,000 years
Many additional
events in the south?

50. • Cascadia likely has a Sumatra-type megathrust featuring spread-out ruptures
(large strike length, moderate slip).
• No doubt, the Cascadia megathrust is presently locked to some degree.
• Narrow and shallow locking is assumed because of high temperature.
• Full locking is assumed because of paucity of interface seismicity.
• Seafloor geodesy and seismometry are needed to define the true locking
state.
• Multidisciplinary research is needed to understand the relation between ETS
and the seismogenic zone.
• Geophysical imaging and geological studies are needed to investigate
whether Tohoku-like trench-breaching rupture occurs at Cascadia.
• Structural and paleoseismic research needs to examine possibly persistent,
geologically controlled along-strike heterogeneity in seismogenic behaviour.
• Local-scale research is needed to study site conditions pertaining to shaking
amplification, tsunami inundation, and secondary hazards.
Some points to emphasize

51. • Cascadia likely has a Sumatra-type megathrust featuring spread-out ruptures
(large strike length, moderate slip).
• No doubt, the Cascadia megathrust is presently locked to some degree.
• Narrow and shallow locking is assumed because of high temperature.
• Full locking is assumed because of paucity of interface seismicity.
• Seafloor geodesy and seismometry are needed to define the true locking
state.
• Multidisciplinary research is needed to understand the relation between ETS
and the seismogenic zone.
• Geophysical imaging and geological studies are needed to investigate
whether Tohoku-like trench-breaching rupture occurs at Cascadia.
• Structural and paleoseismic research needs to examine possibly persistent,
geologically controlled along-strike heterogeneity in seismogenic behaviour.
• Local-scale research is needed to study site conditions pertaining to shaking
amplification, tsunami inundation, and secondary hazards.

52. • Cascadia likely has a Sumatra-type megathrust featuring spread-out ruptures
(large strike length, moderate slip).
• No doubt, the Cascadia megathrust is presently locked to some degree.
• Narrow and shallow locking is assumed because of high temperature.
• Full locking is assumed because of paucity of interface seismicity.
• Seafloor geodesy and seismometry are needed to define the true locking
state.
• Multidisciplinary research is needed to understand the relation between ETS
and the seismogenic zone.
• Geophysical imaging and geological studies are needed to investigate
whether Tohoku-like trench-breaching rupture occurs at Cascadia.
• Structural and paleoseismic research needs to examine possibly persistent,
geologically controlled along-strike heterogeneity in seismogenic behaviour.
• Local-scale research is needed to study site conditions pertaining to shaking
amplification, tsunami inundation, and secondary hazards.
Questions welcome
www.earthquakescanada.nrcan.gc.ca

54. GPS Observations
(1991-2004)
Station velocities
with respect to
stable North America
(Compiled by
McCaffrey et al., 2007)
Red –
continuous
stations
Blue –
campaign
stations
Velocity scale
10 mm/year

55. Secular motion of
coastal region
(Model of Wells et al.,
1998, based on
Earth’s magnetic field
recorded in rocks and
other geological data).

56. Wang and Trehu (2016 J. Geodyn.)

57. Frontal thrusts off Vancouver Island identified from seismic survey profiles
D. Gao, 2016 MSc thesis