Lecture 18 // West Coast Tectonics

Question 1

Ring of Fire

Answer 1

The Ring of Fire is the name given to the nearly continuous chain of 452 volcanoes that surrounds the Pacific Ocean.

Including the:
– Andes Volcanic Arc (Nazca Plate subducts below the South American Plate),
– Volcanic arcs of Central America (Cocos Plate subducts beneath Caribbean Plate)
– Cascade Volcanic Arc (Juan de Fuca Plate subducts beneath the North American Plate)
– Aleutian Islands (island arc formed from the Pacific Plate subducting beneath the N.A. Plate)
– Japan (island arc formed from the Pacific Plate subducting beneath the Philippine Plate)
– New Zealand (island arc formed from the Pacific Plate subducting beneath the Indian-Australian Plate)

Question 2

The North American continental plate interacts with the oceanic plates:

Answer 2

Pacific Plate; Juan de Fuca Plate; Explorer Plate; and Gorda Plate.
– The oceanic plates are pushed by seafloor spreading at the: East Pacific Rise; Juan de Fuca Ridge; Explorer Ridge; and Gorda Ridge.

Question 3

Triple junction

Answer 3

A triple junction is the point where three plates meet.

Question 4

Cascadia subduction zone

Answer 4

Lacks a defined trench. Instead the area where plates meet is marked by a deformation zone on the Pacific Ocean seafloor. Why?

The small Explorer, Juan de Fuca, and Gorda plates are young and thus warm and buoyant. They subduct at a very shallow angle beneath the North American Plate.

Subduction of the oceanic plates at the Cascadia Subduction Zone brings water into the hot mantle. The steam that forms rises through the North American lithosphere, creating volcanoes of the Cascade Volcanic Arc.

Closest to us is Mt Baker
and Mt Garibaldi.

Question 5

A full-margin rupture of Cascadia

Answer 5

Earthquakes also occur along our active plate margin, including great Cascadia mega-thrust EQs.

A full-margin rupture of the roughly 1000km long Cascadia Subduction Zone would generate an EQ in thelikely range of MW 9.0 to 9.2. The duration of shaking would be at least 5 minutes.

Question 6

The last Cascadia megathrust EQ

Answer 6

Occurred on January 26, 1700 around 9pm.
– A full-margin rupture with an estimated Mw 9.0
– Generated a massive tsunami that impacted First Nations villages along the Pacific Northwest coast, including the complete destruction of one village on Vancouver Island.

Question 7

Evidence of the Great 1700 Cascadia EQ

Answer 7

1. Oral record passed down amongst First Nations coastal communities
2. Japan record of tsunami (the thensmall tsunami hit Japan’s coast about 10 hours later)

Radiocarbon dating of:

3. Drowned trees in subsided coastal areas in OR & WA
4. Turbidite deposits seaward of the continental shelf.

Question 8

Turbidity current

Answer 8

• A turbidity current is a sediment-gravity flow; essentially a dense currrent (carrying lots of sand & mud) that hugs the bottom (seafloor) as they travel downslope.
– Turbidity currents travel down submarine canyons and deposit turbidites at the base of those channels, as well as spilling out of channels forming submarine fans.

Question 9

Cascadia history

Answer 9

Looking at the offshore turbidite record over the past 10,000 years reveals:
• 19 full-margin ruptures
• 4 partial-margin ruptures
– 50 to 70% of the subduction zone (rupture that do not include the northern end)
• 18 partial-margin ruptures
– southern OR & northern CA only

Recurrence interval for:
– full-margin rupture = 500 to 530 years
– partial-margin rupture = ~240 years

Question 10

The Next Cascadia Mega-thrust EQ

Answer 10

Not a question of if, but a question of when and how big.
•The Big One: We are ‘overdue’ for a partial-margin EQ (expected magnitude ~ 8.0).
•The Very Big One: We are still well within the 500-year recurrence interval for a full margin rupture (estimated magnitude ~ 9.0), but that doesn’t mean the Very Big One couldn’t occur tomorrow.
– In fact, there is a 10% chance of one occurring in the next 50 years.

• Shaking hazards are significant and will likely result in casualties, but most deaths will come from the massive tsunami that will be generated by a MW 9.0 mega-thrust EQ.

– Fatality estimates total thousands or tens of thousands (drowning / being crushed by tsunami-swept debris).
– Displaced residents will tally in the millions, & damage in the billions.
– Estimates are highest for a fullmargin rupture with an epicentre off the coast of Oregon.