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Magnetic data indicate that the dangerous Seattle fault zone may have resulted from the splitting of the continental margin more than 50 million years ago, offering a new
Magnetic data indicate that the dangerous Seattle fault zone may have resulted from the splitting of the continental margin more than 50 million years ago, offering a new potential explanation for the fault’s formation.
The Seattle Fault Zone consists of a series of shallow faults that cross the Puget Sound lowlands, posing a risk of devastating earthquakes for the region’s more than four million residents. A recent study offers a new look at the initial formation of the fault system, aiming to improve the prediction and understanding of potential hazards for this densely populated region. The study was published on: TectonicAn AGU journal dedicated to the study of evolution, structure and changes in the crust and upper mantle.
The Seattle Fault is active today due to forces acting on the region from ongoing tectonic deformation both to the west and south, but this was not always the case. Eocene Washington looked different than it does today, with a coastline far east of where Seattle is today and a chain of volcanic islands dotting the nearshore horizon.
The study shows that this island chain was pulled into the continent about 55 million years ago. When it collided with the North American plate, some of it went above the crust, while the rest was pulled under. Between these two parts, the shell is greatly stretched and torn. The authors of the study believe that this ancient fault zone formed the geological basis of the modern-day Seattle Fault.
“This was a complete surprise,” said Megan Anderson, a geophysicist at the Washington Geological Survey and lead author of the study. “We didn’t resort to this at first, but our results show that there is a large ancient fault at the current location of the Seattle fault.”
The Pacific Northwest lies slightly inland of the Cascadia subduction zone, where dense oceanic crust is being pulled beneath the continent. In 1700, a subduction zone rupture approximately 1,000 kilometers (620 mi) across produced a massive earthquake of magnitude 8.7 to 9.2; Smaller earthquakes shook the region in the 1900s and most recently the 2001 Nisqually earthquake. Based on oral histories of local indigenous peoples and geological evidence from the Puget Sound coastline, the Seattle Fault distinctly occurred in 923-924 AD.
Despite seismic activity in the area, scientists did not begin seriously studying the Seattle Fault Zone until the 1990s.
“There is much more uncertainty about the Seattle fault than, for example, the San Andreas fault,” Anderson said. “The fault in Seattle could cause a magnitude 7.2 earthquake, and we want to be prepared. There is much more to learn so engineering geologists can better model earthquakes and understand the 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 that travel through and are reflected by underground rock layers. The data revealed faults and geological structures that seismologists and geologists interpreted differently. They knew there was a major fault zone in the area, but scientists suggested different ways parts of the fault could connect together, how deep it was, and how steeply it cut through the bedrock.
Anderson and his co-authors set out to test existing hypotheses about fault zone geometry by mapping miles of bedrock in western Washington and creating a more complete picture of the region’s geologic structure. Gravitational and magnetic fields vary across the Earth’s surface depending on the density and composition of rocks; so Anderson collected these data for western Washington and combined them 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 of the hypotheses matched the gravitational, magnetic and seismic data. The gravity data showed no complex pattern, but the magnetic data revealed an important mystery of the seismic data: Deep in the Earth’s crust, bedrock constantly alternates between high and low magnetism, suggesting inclined layers of varying rock types. And on the map, objects on either side of the Seattle fault zone are placed at angles to each other; In the north of the Seattle fault zone, the structures are oriented north-northwest, and in the south they are oriented north-northeast.
These wavering tendencies got Anderson thinking; They pointed to an ancient mountain range, but to confirm this, Anderson had to match the map data with deeper rocks. To reconcile the map’s appearance with the known deeper geology of the bedrock, Anderson modeled the vertical profile of the subsurface rock and found that some of these structures also plunged underground in different directions.
“These are all very different orientations,” Anderson said. “That’s very difficult to do unless there’s a place where the structures are pulled apart and restarted.”
Anderson has stumbled upon a new possible explanation for the early history of the Seattle Fault Zone and why it is recovering today.
Evidence suggests that about 55 million years ago, when a subduction zone engulfed a series of oceanic islands, the northern half of the island chain sank, but the southern half was added to, or sealed off from, the upper crust. Over several million years, as the islands were swallowed, they collapsed into a folded and thrust mountain belt with a topography similar to today’s Blue Ridge Mountains in the Appalachians.
The region, where the islands transitioned from sinking to accretion, experienced incredible tension and disintegrated.
“This would be a slow, steady rupture, almost like a shell opening on its own,” Anderson said. “As we progressed, the gap got longer and longer.”
And this “broken” area matches up perfectly with today’s Seattle fault zone.
The intense rifting would end once the islands hit the continent, but the damage was done. A zone of intense rifting created a fragmented, weakened crust, forming the geological foundation of today’s Seattle Fault Zone.
In addition to possibly explaining why the fault zone exists, the study’s findings about the geometries and geological structure of Washington’s ancient faults provide valuable detail about the bedrock beneath and within the Seattle Basin. This basin is filled with miles of looser sedimentary rocks that increase seismic ground vibrations, and the new data could help scientists create more accurate models of future ground vibrations in the region.
Anderson is excited to use his discoveries to study the active faults of Western Washington.
“Uncovering this buried tectonic history has been very exciting, and will now be a great foundation to go back to answering our original questions about active fault geometry for the Seattle Fault and other faults in Western Washington,” Anderson said.
Source: Port Altele
As an experienced journalist and author, Mary has been reporting on the latest news and trends for over 5 years. With a passion for uncovering the stories behind the headlines, Mary has earned a reputation as a trusted voice in the world of journalism. Her writing style is insightful, engaging and thought-provoking, as she takes a deep dive into the most pressing issues of our time.
