The M9 Project (Part 1): A History of Large Earthquakes in Cascadia
The M9 Project (Part 1): A History of Large Earthquakes in Cascadia
December 13, 2017
by Elizabeth Urban
What is “The Big One” going to look like? How soon will we know it’s coming? How are our cities and communities going to fare?
Here at the University of Washington, a group of scientists, engineers and researchers are seeking to answer these questions. The M9 Project aims to reduce the devastating effects of large Cascadia subduction zone quakes that threaten the Pacific Northwest region. The M9 project is working to increase our understanding of earthquake-related hazards (tsunami inundation, liquefaction, seismically-induced landslides, aftershocks, etc.) and apply this knowledge to warning systems and community planning.
The M9 Project will be an ongoing topic here on the Seismo Blog. Over the next few months, some of the things we will be covering are; the Pacific Northwest’s seismic past, why we are preparing for the "Big One,” megathrust quakes, what the "Big One” will look like, how the Pacific Northwest will fare in the event of a M9 earthquake, and how our communities are planning for the aftermath of a large earthquake.
How do we know the “Big One” is coming?
In January of 1700, Japan experienced a tsunami with no warning. Waves 1 to 5 meters higher than normal hit the coast with no shaking to serve as a signal for what was to come. In recent years, scientists established that this “Orphan
Tsunami” originated from a “parent” earthquake on the Cascadia subduction zone. As scientists dug deeper into Cascadia’s past, they found that when Cascadia goes, it goes with gusto.
The Cascadia subduction zone currently produces fewer earthquakes compared to other subduction zones worldwide, so there are very few surviving accounts of major seismic activity in the Pacific Northwest. Native American stories along the coast told of a battle between the Thunderbird and the Whale, with elements of earth shaking and tsunamis as a result. Our PNSN website has a section on Native American Oral History here.
Over the past few decades, scientists started to look closer for evidence of past earthquakes in the Pacific Northwest. In 1964, a M9.2 earthquake in Alaska caused the coastline to drop 1.5 meters, creating “ghost forests” as tree stumps became preserved in the coastal sediments that could now reach their base. When searched for similar evidence, they provided examples of the phenomena all along the Pacific Northwest coast.
How do we know that Cascadia has recurring megathrust quakes?
There is evidence of recurring coastal subsidence off the coast of Washington. From samples at the Columbia, Copalis, and Waatch Rivers, as well as Willapa Bay and Grays Harbor, USGS geologist Brian Atwater determined that the Cascadia subduction zone has a cycle of violent earthquakes and rapid subsidence. After a major subduction earthquake, the land along the coast drops then accumulates sediment and vegetation until the next major earthquake.
Underwater slope failures can signify major ground shaking, such as a megathrust earthquake. The preserved landslides serve as a submarine record of historic seismic events. Among other things, a study by Oregon State marine geologist Chris Goldfinger found that the mass of turbidites give a crude estimate of an earthquake’s magnitude and duration. The data from these turbidites shows a consistent thickness of landslide deposits, and coincides with the most recent Cascadia megathrust earthquake in 1700. Our PNSN website gives a thorough description of turbidites here
So, how “Big” are we talking?
With physical effects like these, an earthquake has to be powerful. In a study led by University of Tokyo seismologist Kenji Satake on the 1700 Cascadia earthquake, tsunami reports from Japan were used to estimate the seismic moment of the Cascadia quake. Seismic moment is a measure of an earthquake’s size, which incorporates the area of fault rupture, how much displacement there is on the fault surface, and the amount of force needed to override the friction keeping the rocks “stuck” together. Satake et al. determined that the seismic moment for the 1700 Cascadia earthquake would be in the range of 1.2–9.2 × 1022 N*m, which corresponds to moment magnitudes from around 8.7-9.2.
How will The M9 Project help us understand and prepare for the next big earthquake?
The knowledge that the Cascadia subduction zone has previously generated powerful and potentially destructive earthquakes has lead to the advent of the M9 Project.
Keep an eye out as our blog is updated with more information and research!
Special Thanks To
Dr. Erin Wirth, UW Affiliate Assistant Professor
References
A Major Earthquake in the Pacific Northwest Looks Even Likelier - Robinson Meyer, The Atlantic
Ghosts of Tsunamis Past - American Museum of Natural History
Further Reading
Evidence for Great Holocene Earthquakes Along the Outer Coast of Washington State - Brian F. Atwater