Blanco Fracture or Cascadia Subduction? It’s important to understand the difference between offshore earthquakes

By DANA TIMS/Lincoln Chronicle

When things shake hard enough off the Oregon coast to be felt miles inland, it’s more than fair to wonder if a tsunami might be right behind.

A recent swarm of undersea earthquakes felt as far inland as Eugene served only to spike those concerns.

But scientists who study coastal seismology say that, in this instance, there was no great cause for worry. That’s because these quakes were generated in an area known as the Blanco Fracture Zone, which is far enough offshore and located in deep enough waters not to present any significant threats to humans on land.

The fracture zone starts about 95 miles off Cape Blanco on Oregon’s southern coast and extends northwest to about 310 miles off Newport. It separates two tectonic plates — the Juan de Fuca place and the Pacific Plate — and basically lets them slide in opposite directions past each other with relatively little resistance.

It’s very different from the better known and far more dangerous Cascadia Subduction Zone, which runs the length of the coastal Northwest and pinpoints the place where the westward-moving Juan de Fuca Plate “subducts” beneath North America.

Recent modeling and historical records show that Cascadia Subduction Zone earthquakes can trigger massive tsunamis and result in extensive destruction for miles inland. A major quake along this 600-mile-long fault is what the fuss about coastal tsunamis is all about.

The Blanco Fracture Zone, however, for all of its recent activity, poses almost no such danger, said Harold Tobin, director of the Pacific Northwest Seismic Network and the  Washington state seismologist.

Explaining the BFZ

“The Blanco Fracture Zone is a distinct fault and separate from Cascadia,” Tobin told the Lincoln Chronicle. “The chances of it triggering a Cascadia quake are almost impossible.”

The Blanco zone, like the Mendocino Fracture Zone, its counterpart to the south, is known as a “transform” fault system. It marks an area where two sections of the Earth’s crust slide horizontally past one another, rather than colliding or pulling apart.

Quakes in the zone are usually no higher than 6.0 on the Richter scale, Tobin said. A quake measuring 6.3 was included in the recent swarm of disturbances there, matching a 6.3 quake in 2019.

In addition to registering only lower-intensity quakes, the Blanco zone has a couple of other things going for it when it comes to human safety, he said.

For one, unlike California’s potentially deadly San Andreas Fault, which is also a transform fault system, Blanco quakes occur at least 200 miles offshore, rendering them relatively harmless in terms of safety. They also occur in deep enough waters than any resulting tsunami is so small it won’t even show up on measuring instruments.

“At the bottom of the sea floor, they simply don’t displace much water,” Tobin said. “It’s the ones like Cascadia that dramatically lift up and drop down that displace enough water to create dangerous tsunamis.”

In terms of topography, the Blanco Fracture Zone is a series of dramatic wrinkles and rumples that are the results of eons of geologic pressures and movement. It is lined with steep walls that tower over deep basins and channels.

One of these channels has been found to contain sediments from the ancient Missoula floods, which occurred at the end of the last Ice Age when ice dams near modern-day Missoula, Mont. repeatedly broke, unleashing torrents of water that rushed through eastern Washington, northern Idaho and into Oregon.

The floods were still so massive when they reached the Pacific Ocean at Astoria that, like some massive, super-charged firehose, they carved a channel south and west as far as the Blanco Fracture Zone.

The zone itself continues to be one of the most seismically active areas off Oregon’s coast, said Lydia Staisch, a United States Geological Survey research geologist.

She agreed with Tobin that the lateral movement of the two involved plates creates low danger for humans living along the coast.

“Tsunamis are created where you get a lot of uplift on one side or another of a fault,” Staisch said. “This particular area is not something we’ve typically worried about.”

Anyone feeling a quake from the zone, however, can help scientists by going to the USGS website and finding the earthquake-reporting button to click that will let them enter what they felt, when, and how significant it felt.

“We actually create maps out of that information showing the intensity of the quakes people were feeling,” she said. “Even minor events help people contribute a lot of information into what we know about these earthquakes.”

• Dana Tims is an Oregon freelance writer who contributes regularly to Lincoln Chronicle and can be reached at [email protected]