A New Origin Story for Seattle’s Deadly Rift

The Seattle Fault Zone is a network of shallow faults running through the Puget Sound Lowlands, threatening to create devastating earthquakes for the more than four million people who live there.

A new origin story, proposed in a new study, could explain the beginnings of the fault system and help scientists improve hazard modeling for this densely populated region. The study is published in the journal Tectonic.

The Seattle Fault is active today due to forces exerted on the region by continuing tectonic deformation to the west and south, but this was not always the case. Washington in the Eocene was different from today, with a coastline well east of where Seattle is today and a chain of volcanic islands dotting the horizon offshore.

The study suggests that around 55 million years ago, this island chain was pulled toward the mainland. As it penetrated the North American plate, some of it rose up onto the crust while the rest was sucked below. The crust would have been heavily stressed and torn between these two parts. This ancient lachrymal zone paved the geological pathway for the modern Seattle Fault, the study authors say.

“It was a complete surprise,” said Megan Anderson, a geophysicist at the Washington Geological Survey and lead author of the study. “This wasn’t something we were originally looking for, but our results predict a major ancient fault where the Seattle Fault is today.”

A huge mystery

The Pacific Northwest lies just inland from the Cascadia subduction zone, where dense oceanic crust is pulled beneath the continent. In 1700, a rupture of about 1,000 kilometers (620 miles) of the subduction zone created a massive earthquake with a magnitude of 8.7 to 9.2; smaller earthquakes shook the region throughout the 1900s and, most recently, during the 2001 Nisqually earthquake.

The Seattle Fault notably ruptured in 923-924 AD, based on local indigenous oral traditions and geological evidence along the Puget Sound coastline.

Despite the region’s seismic activity, scientists only began seriously studying the Seattle Fault Zone in the 1990s.

“There’s a lot more uncertainty about the Seattle Fault than, say, the San Andreas Fault,” Anderson said. “The Seattle Fault could generate something like a 7.2 magnitude earthquake, and we want to prepare for that. There is still a lot to learn so that geological engineers can do better earthquake simulations and understand potential risks to our communities.”

Previous work to determine the geometry of the Seattle Fault at depth relied primarily on seismic data, which are sound waves passing through and reflected from underground rock layers. The data revealed faults and geological structures that seismologists and geologists interpreted differently. They knew the area was home to a major fault zone, but scientists had proposed different ways the parts of the fault are connected, depending on how deep it extends and how steeply it cuts through the bedrock.

Anderson and his co-authors set out to test existing hypotheses about fault zone geometry by mapping miles-deep bedrock in western Washington and building a more complete picture of the geological structure of the region. Gravity and magnetic fields vary on Earth’s surface depending on the density and composition of rocks. So Anderson compiled this data for western Washington and combined it with seismic data.

The researchers also collected rock samples from geological formations corresponding to different parts of the ancient fault and mountain system.

The researchers used computer models to see which assumptions, if any, matched the gravitational, magnetic and seismic data. The gravitational data didn’t show a complex pattern, but the magnetic data revealed a key missed secret seismic datum: Deep in the crust, the bedrock constantly alternates between being more and less magnetic, suggesting tilted-type layers. of changing rock.

In the map view, features on either side of the Seattle Fault Zone are moving away from each other; To the north of the Seattle Fault Zone, structures are oriented north-northwest, while to the south, they are oriented north-northeast.

These shaky directions gave Anderson pause; they hinted at an ancient mountain range, but to verify this, Anderson had to match the map data with deeper rocks. To connect the map view to the known, deeper geology of the bedrock, Anderson modeled a vertical profile of subsurface rocks and discovered that some of these structures also dip in different orientations underground.

“They’re all very different directions,” Anderson said. “It’s very difficult to do unless there is a place where the structures disconnect from each other and then restart.”

Anderson had stumbled upon a new possible explanation for the early history of the Seattle Fault Zone and why it is reactivated today.

A tear in the crustal continuum

Data suggests that about 55 million years ago, when the subduction zone resulted in a chain of oceanic islands, the northern half of the island chain was subducted, but the southern half was added to the top of the crust, or obducted. Over a few million years, as the islands were removed, they collapsed into a belt of folded and thrust mountains with a topography similar to the Blue Ridge Mountains of the Appalachians today.

The area in which the islands transitioned from subduction to accretion would have been under incredible stress and torn apart.

“It would have been a slow, continuous tear, almost like the crust was breaking down on its own,” Anderson said. “As it progressed, the tear gap got longer and longer.”

And this “torn” region overlaps perfectly with the modern Seattle Fault Zone.

The intense tearing should have stopped once the islands were crushed into the mainland, but the damage was done. The intense tearing zone created a fragmented and weakened crust, paving the way for the modern Seattle Fault Zone.

Beyond explaining why the fault zone exists, the study’s results on the geometry of Washington state’s oldest faults and geologic structures provide valuable details about the bedrock beneath and within the basin from Seattle. This basin is filled with miles of looser sedimentary rock, which makes earthquake shaking stronger, and the new data can help scientists develop more accurate models of future ground shaking in the region.

Anderson is excited to use her findings to next study active faults in Western Washington.

“This buried tectonic history was a lot of fun to uncover, and it will now provide a great basis for returning to our original questions about active fault geometry for the Seattle Fault and other faults in western Washington “Anderson said.