I just got the kind of one-word text you never want to get from your grown daughter working from home in Tacoma, Washington, with your new baby grandson on the floor playing next to her. “Earthquake!”
A magnitude 4.3 earthquake emanating from somewhere close to home in Mendocino County on the San Andreas Fault system is usually not more than an uncomfortable reminder of the ongoing volatility and fragility of our seismic landscape, but a 4.3 in Puget Sound can signify something deeper and more dangerous still to arise from the ominous Cascadia subduction zone complex.
The powerful convergent tectonic boundary of the Juan de Fuca and the North American plates forming the Cascadia subduction zone stretches 620 miles from Cape Mendocino off the coast of Humboldt County to the northern tip of Vancouver Island in southwest Canada. And it is a dangerous tectonic margin of direct collisional and compressional forces to be taken seriously.
With large urban population centers within harm’s way of the subduction zone, and clear emerging evidence of repeated interval occurrences of massive earthquakes along the Cascadia subduction zone throughout pre-history (for which the Pacific Northwest may be overdue), heightened scientific awareness and the practice of tracking this region’s seismicity and tsunami-generating potential has accelerated over recent decades.
Humboldt, Del Norte, and the surrounding counties of California’s far northwest are no stranger to the persistent and damaging tectonic forces of the Cascadia subduction zone as the deteriorating southern chunk of the Juan de Fuca plate, known as the Gorda microplate, subducts beneath the North American continent at a shallow angle, generating a crumbled network of shattered faults extending eastward under the landscape.
Mendocino too is within range of the far-reaching seismic effects of this volatile subduction zone. Vulnerably situated directly south of Cascadia, and abutting the chaotic multi-component and multi-dimensional tectonic Mendocino Triple Junction, Mendocino County is within the northernmost region of the San Andreas Fault system, once thought to be a completely separate system from Cascadia. As it turns out, an overnight navigational error aboard a research ship accidentally led scientists offshore of the Mendocino coast and to the discovery of repeated seismic episodes linking the two massive and opposing tectonic systems
In 1999, researchers from Oregon State University were conducting intensive at-sea surveying of the Cascadia Subduction Zone by collecting ocean sediment core samples at deep sea canyons along its extent from Vancouver Island to Cape Mendocino. These stratified sediment samples taken from the steep walls of submarine canyons can be up to 90 feet in length and preserve multiple deep sea sedimentary layers spanning many thousands of years, as well as turbidite deposits, which are the remains of sudden undersea landslides.
The sedimentary layers of the cores are aged using carbon dating and sedimentation rates. Turbidite deposits of the same age and at the same intervals were found at multiple core sample locations along the extent of the Cascadia Subduction Zone: these were simultaneous deep sea landslides that could only be interpreted to have been caused by large megathrust subduction earthquakes.
On watch one night at the southern end of a research cruise in 1999, the fateful navigational error occurred and instead of ending up off of the Cascadia Subduction Zone at Cape Mendocino, the ship veered approximately 100 miles to the southeast. The crew found themselves waking up a few miles off of Fort Bragg and over Noyo Canyon.
Noyo Canyon is a prominent submarine geomorphic feature of Mendocino County’s bathymetric landscape, lying six to ten miles northwest of Fort Bragg, and the San Andreas Fault happens to slice straight through it.
In the spirit of random data collection and to take advantage of their position, the researchers decided to extract sediment cores from the walls of Noyo Canyon, in hopes of analyzing earthquake occurrences along the northern extent of the San Andreas Fault. Turbidite deposits at intervals spanning thousands of years were found within these Noyo Canyon sediment cores, and it was discovered that the ages and intervals closely followed those of the Cascadia subduction zone turbidite deposits.
In addition, a perplexing “upside-down” sedimentation grain pattern of the Noyo Canyon turbidites turned out to be two turbidity landslide events occurring in rapid succession of each other, indicating one earthquake quickly following another, as in a triggering event.
The deep sea geologic evidence found off the Mendocino Coast at Noyo Canyon correlated in age and interval occurrence with those of the Cascadia Subduction Zone, leading to the conclusion that large subduction earthquakes on the convergent Cascadia subduction zone regularly trigger large earthquakes throughout the transverse San Andreas Fault system as well.
As lead researcher, Chris Goldfinger, states, the subsequent triggered earthquake could happen, “as short as within half an hour (of the initial quake). It could be maybe a few days, possibly as much as a week, but most likely the timing is within hours.”
For further reading on this exciting study, look up the work of lead researcher Chris Goldfinger, a marine geologist and paleoseismologist at Oregon State University.
Rowena Forest is a physical geographer and science writer at Cal Geographic. Follow her investigations of California’s living landscapes and upcoming field trips at CalGeographic.