Magnitude 9! How We Learned That the Largest Earthquakes on Earth Happen in the Pacific Northwest
John J. Clague SFU Centre for Natural Hazard Research
What is an earthquake?
Prince 1946 Prince 2001 Rupert Vancouver Island Rupert Nisqually, Washington earthquake Shake maps earthquake magnitude 7.3 (Mercali intensity) magnitude 6.9 Campbell River: 75% of chimneys General alarm. collapse VII Difficult to stand. Damage to buildings. >$2 billion Lions Gate dollars in Bridge sways Courtenay: Felt by everyone. damage Post Office VI Difficult to walk. wall collapses Objects fall. Vancouver Vancouver Felt outdoors, Port Alberni: Victoria V Victoria Hundreds of Buildings sway. houses lose Seattle Seattle chimneys. Water Felt by most mains break. buildings IV people indoors. Power out for sway 10 days. Portland Portland
0 km 300 0 km 300
fig27.cdr How did we learn that giant earthquakes occur in the Pacific Northwest?
A 50-year odyssey marked by
spreading transform ridge fault earthquake subduction ("teeth" on zone overriding volcanoes zone plate)
Eurasia Plate North America Plate Japan g o f F i n i r R e
Pacific Plate
South America Plate Nazca Plate Indo-Australia Plate
New Zealand
spreading Andes Pacific ridge volcanoes Ocean
Nazca Plate South America Pacific Plate Plate mantle subduction zone advances in scientificfig3.cdr knowledge and technology
Early 1960s – The revolution in earth sciences
spreading transform ridge fault earthquake subduction ("teeth" on zone overriding volcanoes zone plate)
Eurasia Plate North America Plate Japan g o f F i n i r R e
Pacific Plate
South America Plate Nazca Plate Indo-Australia Plate
New Zealand
spreading Andes Pacific ridge volcanoes Ocean
Nazca Plate South America Pacific Plate Plate mantle subduction zone
fig3.cdr
1963 - Drummond Mathewes and his PhD student Frederick Vine document magnetic striping on the seafloor
Earth’s lithospheric plates
spreading transform ridge fault earthquake subduction ("teeth" on zone overriding volcanoes zone plate)
Eurasia Plate North America Plate Japan g o f F i n i r R e
Pacific Plate
South America Plate Nazca Plate Indo-Australia Plate
New Zealand
spreading Andes Pacific ridge volcanoes Ocean
Nazca Plate South America Pacific Plate Plate mantle subduction zone
fig3.cdr 1970s – Recognition that there is a subduction zone off the coast of the Pacific Northwest …
spreading transform ridge fault earthquake subduction ("teeth" on zone overriding volcanoes zone plate)
Eurasia Plate North America Plate Japan g o f F i n i r R e
Pacific Plate
South America Plate Nazca Plate Indo-Australia Plate
New Zealand
spreading Andes Pacific ridge volcanoes Ocean
Nazca Plate South America Pacific Plate Plate mantle subduction zone
fig3.cdr But is there continuous slip or stick-slip at the boundary between the two plates? 1984 – Heaton and Kanamori ‘rock the boat’ Epicentre of 1964 earthquake
0 m Anchorage -1 m The story from
0 m other giant -2 m +1 m +2 m earthquakes
+6 m +3 m
region of uplift 0 100
region of subsidence km
Alaska Yukon N.W.T.
B.C.
Pacific Ocean George Plafker
fig30.cdr Between Uplift earthquakes Vancouver Island Pacific Ocean Strait of Georgia crust shortens (compression) locked
During Uplift earthquakes Subsidence
crust pulls apart rupture (extension)
fig51b.cdr 1987 - The “aha” moment
Brian Atwater
Niawiakum River, WA Brian Atwater Copalis River, WA tidal marsh
AD 1700
~AD 700 ~AD 300
~600 BC
~1000 BC
Niawiakum River, WA
fig60.cdr Recurrence Vikings Columbus Great Pyramids Greek State The Roman Empire settle NE "discovers" of Egypt built flourishes America America
Great earthquakes in the Pacific Northwest
2000 BC 1000 BC 1 AD 1000 AD Today Time 1990s - Support from the geophysicists Natural Resources Canada Roy Hyndman
Kelin Wang Between 1990s to present – Uplift earthquakes Vancouver Island Pacific Ocean Strait of Georgia Documentingcrust shortens the expected secondary effects (compression) locked
During Uplift earthquakes Subsidence crust pulls apart rupture (extension) Tsunamis
fig51b.cdr PhiPhi Island, Thailand Tofino, BC Chronologies
Twenty great earthquakes in the past 10,000 years The orphan tsunami They occur in clusters Are we in a cluster or in the interval between two clusters? The future –
• Refined map of the locked zone
• Further refinements in the earthquake
chronology
• Tackling the issue of segmentation
• Search for precursor signals The future –
• Refined map of the locked zone
• Further refinements in the earthquake
chronology
• Tackling the issue of segmentation
• Search for precursor signals Clague et al., 2006 "silent slip period of event" slow buildup of strain
short period of intense period tremor of no activity tremors
Herb Dragert Garry Rogers
fig54.cdr Natural Resources Canada Herb Dragert, National Resources Canada The End