'A total surprise' – Geologists have discovered a new origin story for the deadly Seattle fault

The Seattle Fault Zone, a network of shallow faults that threaten the Puget Sound region, has a new origin story based on a study published in Tectonics that suggests it formed about 55 million years ago due to tectonic deformation. This study, which reveals ancient volcanic island interactions with the North American plate, aims to enhance risk models for the densely populated region.

Magnetic evidence suggests that the dangerous Seattle Rift Zone may have arisen from the continent's edge splitting in half more than 50 million years ago, offering a possible new explanation for the formation of the rift.

The Seattle Fault Zone consists of a series of shallow faults that cut through the lowlands of Puget Sound, posing a risk of devastating earthquakes to more than four million people in the region. A recent study provides a new perspective on the initial formation of a fault system, with the aim of enhancing prediction and understanding of the potential hazards of this densely populated region. The study was published in Tectonicsthe Arabian Gulf University's journal of research exploring the evolution, structure and change of the Earth's crust and upper mantle.

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

The study indicates that about 55 million years ago, this island chain was pulled towards the continent. When it collided with the North American plate, part of it rose above the crust while the rest was sucked underneath. Between these two parts, the crust was under great pressure and torn. The study's authors hypothesize that this ancient tear zone set the geological stage for the modern Seattle Fault.

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“It was a complete surprise,” said Megan Anderson, a geophysicist with the Washington Geological Survey and lead author of the study. “This was not something we were originally seeking, but our results predict an ancient fault as large as the Seattle Fault exists today.”

Seattle's fault

The modern Seattle Fault Zone passes directly through the densely populated Puget Lowlands, including Seattle and its metro area. Fifty million years ago, the continent split in two here, paving the geologic way for modern rifts, according to a new tectonic study. Credit: Washington Geological Survey.

A huge mystery

The northwestern Pacific Ocean lies just inland from the Cascadia subduction zone, where dense oceanic crust is being pulled beneath the continent. In 1700, a subduction zone roughly 1,000 km (620 mi) long ruptured a massive earthquake of magnitude 8.7 to 9.2; Smaller earthquakes shook the region throughout the 20th century, most recently during the 2001 Nisqually earthquake. Seattle's fault It was noticeably torn apart in 923-924 ADbased on local Indigenous oral traditions and geological evidence along the Puget Sound coast.

Despite seismic activity in the area, scientists did not begin to study the Seattle Fault Zone in earnest until the 1990s.

“There is a lot more uncertainty about the Seattle Fault than, say, the San Andreas Fault,” Anderson said. “The Seattle Fault could generate a 7.2-magnitude earthquake, and we want to be prepared for that. There is still a lot to learn so that engineering geologists can perform better earthquake simulations and understand the potential risks to our communities.”

Previous work to determine the geometry of the Seattle Fault at depth has relied primarily on seismic data, which is sound waves transmitted through and reflected by underground rock layers. The data revealed faults and geological structures that seismologists and geologists interpreted differently. They knew the area contained a major fault zone, but scientists proposed different ways to connect parts of the fault, how deep it ran, and how steep it ran through the rock.

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Anderson and her colleagues set out to test current hypotheses for fault zone geometry by mapping kilometers-deep rocks across western Washington and building a more complete picture of the region's geological structure. Gravity and magnetic fields vary across the Earth's surface based on the density and composition of rocks, so Anderson compiled that data for western Washington and linked it to seismic data. The researchers also collected rock samples from geological formations that correspond to different parts of the ancient fault and mountain system.

Magnetic anomalies in Puget Sound

Aerially collected magnetic data (background colors) for western Washington reveal that faults (black lines) on either side of the modern Seattle Fault are oriented in different directions, indicating a significant north-south separation. The authors of a new tectonic study hypothesize that a massive rupture between subducted and subducted (sinking and accreted) material could form from stress. Credit: Modified from Anderson et al./Tectonics

The researchers used computer models to see which, if any, of the hypotheses fit the gravity, magnetic and seismic data. The gravity data did not show a complex pattern, but the magnetic data revealed key secret seismic data that had been lost: deep in the Earth's crust, the rock constantly alternates between being more or less magnetic, indicating tilted layers of a variable rock type. In map view, features on either side of the corner of the Seattle Fault Zone are far apart from each other; North of the Seattle Fault Zone, the structures are oriented north-northwest, while to the south, they are oriented north-northeast.

Those wonky trends gave Anderson pause; They hinted at an ancient mountain range, but to verify this, Anderson needed to match map view data with deeper rocks. To correlate the map view with known, deeper basement geology, Anderson designed a vertical model of the underground rocks and found that some of those structures dip in different directions underground as well.

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“These are all very different trends,” Anderson said. “This is very difficult to do unless there is a place where the structures can be taken apart and then put back together.”

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

A rupture in the continuity of the Earth's crust

The data suggest that about 55 million years ago, when the subduction zone was dragging down 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 Earth's crust, or was obscured. Over a few million years, as the islands were obscured, they collapsed into a folded and thrust mountain belt with topography similar to the Blue Ridge Mountains of Appalachia today.

The area where the islands went from submergence to accretion would have been subjected to incredible pressure and torn apart.

“It would have been this slow, steady rupture, almost like a shell coming apart on itself,” Anderson said. “As this progressed, the tear fault became longer and longer.”

This “ruptured” zone completely overlaps the modern Seattle fault zone.

The severe rupture could have stopped after the islands collided with the continent, but the damage had been done. The zone of intense rupture created fragmented and weak crust, setting the geological stage for the modern Seattle Fault Zone.

In addition to providing a possible explanation for why the fault zone exists, the results of the study on ancient fault geometry and geological structures in Washington provide valuable details about the bedrock beneath and within the Seattle Basin. This basin is filled with kilometers of loose sedimentary rock that increases the strength of seismic ground shaking, and the new data can help scientists make more accurate models of future ground vibrations in the region.

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

“Uncovering this buried tectonic story has been a lot of fun, and now it will provide a great foundation for returning to answering our original questions about the geometry of active faults of the Seattle Fault and other faults in western Washington,” Anderson said.

Reference: “Deep structure of Silicetsia in the Puget lowlands: imaging the obscured plateau and accretionary thrust belt with potential fields” by M. L. Anderson, R. J. Blakely, R. Wells, and J. D. Dragovic, 06 February 2024, Tectonics.
doi: 10.1029/2022TC007720

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