cascadia

I've survived five quakes, starting with Sylmar on Mom's birthday in 1971

San Fernando {Sylmar}, February 9, 1971
Coalinga, May 2, 1983
Whittier, October 1, 1987
Landers, June 28, 1992 {first-born had their second hip surgery and were discharged post-haste from Valley Presbyterian  }
Northridge, January 17, 1994 {in a rental two miles from the epicenter}


First learned of the Cascadia subduction zone from  John Nance's engaging thriller,  Saving Cascadia.



Guilt-ridden engineer and whistleblower Diane Lacombe, daughter of California State Senator Ralph Lacombe, has a CD that proves her company, Chadwick and Noble, is at fault in green-lighting Mick Walker’s big, new $100-million casino, hotel, and convention center on Cascadia Island off the Washington coast. Hey, it’s being built on a seismic fault absolutely certain to collapse under the weight of the new structures now about to open.  

Kirkus Reviews


Cascadia Subduction Zone

The Cascadia Subduction Zone is a 600-mile fault that runs from northern California up to British Columbia and is about 70-100 miles off the Pacific coast shoreline. There have been 43 earthquakes in the last 10,000 years within this fault. The last earthquake that occurred in this fault was on January 26, 1700, with an estimated 9.0 magnitude. This earthquake caused the coastline to drop several feet and a tsunami to form and crash into the land. Most surprising is that evidence for this great earthquake also came from Japan. Japanese historical records indicate that a destructive distantly-produced tsunami struck their coast on January 26, 1700. By studying the geological records and the flow of the Pacific Ocean, scientists have been able to link the tsunami in Japan with the great earthquake from the Pacific Northwest. Native American legends also support the timing of this last event.

 

Oregon has the potential for a 9.0+ magnitude earthquake caused by the Cascadia Subduction Zone and a resulting tsunami of up to 100 feet in height that will impact the coastal area. There is an estimated 5-7 minutes of shaking or rolling that will be felt along the coastline with the strength and intensity decreasing the further inland you are.

 

The Cascadia Subduction Zone has not produced an earthquake since 1700 and is building up pressure where the Juan de Fuca Plate is subsiding underneath the North American plate. Currently, scientists are predicting that there is about a 37 percent chance that a megathrust earthquake of 7.1+ magnitude in this fault zone will occur in the next 50 years. This event will be felt throughout the Pacific Northwest.

 


https://www.washington.edu/news/2023/04/10/warm-liquid-spewing-from-oregon-seafloor-comes-from-cascadia-fault-could-offer-clues-to-earthquake-hazards/

This article about Pythia's oasis  discusses mechanism.


This sonar image of the Pythias Oasis site shows bubbles rising from the seafloor about two-thirds of a mile deep and 50 miles off Newport, Oregon. These bubbles are a byproduct of a unique site where warm, chemically distinct fluid gushes from the seafloor. Researchers believe this fluid comes directly from the Cascadia megathrust zone, or plate boundary, and helps control stress buildup between the two plates.Philip et al./Science Advances

[Updated 4/18/2023 for clarification:

The field of plate tectonics is not that old, and scientists continue to learn the details of earthquake-producing geologic faults. The Cascadia Subduction Zone — the eerily quiet offshore fault that threatens to unleash a magnitude-9 earthquake in the Pacific Northwest — still holds many mysteries.

A study led by the University of Washington discovered seeps of warm, chemically distinct liquid shooting up from the seafloor about 50 miles off Newport, Oregon. The paper, published Jan. 25 in Science Advances, describes the unique underwater spring the researchers named Pythia’s Oasis. Observations suggest the spring is sourced from water 2.5 miles beneath the seafloor at the plate boundary, regulating stress on the offshore fault.

The new seeps aren’t related to geologic activity at the nearby seafloor observatory that the cruise was heading toward, Solomon said. Instead, they occur near vertical faults that crosshatch the massive Cascadia Subduction Zone. These strike-slip faults, where sections of ocean crust and sediment slide past each other, exist because the ocean plate hits the continental plate at an angle, placing stress on the overlying continental plate.

Loss of fluid from the offshore megathrust interface through these strike-slip faults is important because it lowers the fluid pressure between the sediment particles and hence increases the friction between the oceanic and continental plates.

“The megathrust fault zone is like an air hockey table,” Solomon said. “If the fluid pressure is high, it’s like the air is turned on, meaning there’s less friction and the two plates can slip. If the fluid pressure is lower, the two plates will lock – that’s when stress can build up.”

This is the first known site of its kind, Solomon said. Similar fluid seep sites may exist nearby, he added, though they are hard to detect from the ocean’s surface. A significant fluid leak off central Oregon could explain why the northern portion of the Cascadia Subduction Zone, off the coast of Washington, is believed to be more strongly locked, or coupled, than the southern section off the coast of Oregon.

“Pythia's Oasis provides a rare window into processes acting deep in the seafloor, and its chemistry suggests this fluid comes from near the plate boundary,” said co-author Deborah Kelley, a UW professor of oceanography. “This suggests that the nearby faults regulate fluid pressure and megathrust slip behavior along the central Cascadia Subduction Zone.”