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SYFY WIRE Earthquake

Seattle Was Rocked By a Devastating Double Earthquake 1,100 Years Ago

Scientists pinpoint tree death to an ancient double earthquake in the Pacific Northwest. 

By Cassidy Ward

It was an otherwise ordinary day in Los Angeles when a massive sinkhole opened up a portal to a lost world. As we walk through the world day to day, we take for granted that the ground will remain stable beneath our feet. We forget that our planet is geologically alive, and the ground could decide it’s time for an adjustment at any moment.

You’re unlikely to get swallowed by a time hole like in NBC's La Brea (streaming now on Peacock) but an earthquake is a distinct possibility. And if you’re in the Seattle, Washington area you might even get a one-two punch from a double earthquake, according to new research published in the journal Science Advances.

A Double Earthquake in the Pacific Northwest Killed Thousands of Trees 1,100 Years Ago

Previous research in and around the Puget Sound region uncovered evidence of ground movement resulting in massive tree die off roughly 1,100 years ago. However, the precise dates weren’t known, so scientists went in search of preserved trees they could investigate.

RELATED: Predicting Earthquakes Anywhere on Earth with GPS

They gathered wood samples from six sites scattered around the region, many of which are now underwater. When the quake kicked off, parts of the ground rose up to 24 feet from their previous position, blocking drainage routes and creating new lakes. All the trees living in those lakes were drowned as they filled with water. Many of them are still down there, drowned ghosts swaying in the deep.

In particular, researchers looked for trees with intact outer rings with the bark still attached. The high level of preservation allowed scientists to precisely date the death of each tree down to the exact year and season. Using tree ring data and radiocarbon dating, the team found that the trees from all six sites died within a 6-month window between the fall of 923 and the spring of 924. The thing is, those sites are spread across two distinct fault lines: the Seattle Fault and the Saddle Mountain Fault.

Pacific Northwest Fault Lines Might Be Connected by Master Fault

Washington State shake map

Researchers present two possible scenarios, both of which are consistent with the evidence. The first is that one fault kicked off at some point in that 6-month window and the other fault popped sometime later. The gap between quakes, if any, is unclear and could have been a matter of hours or months. In this scenario, researchers estimate the quakes’ magnitude at 7.5 and 7.3, one after another.

RELATED: Could 'Knock at the Cabin's Pacific Northwest tsunami really happen?

Alternatively, both quakes might have let loose simultaneously in an even more powerful compound earthquake with an estimated magnitude of 7.8. In either case, the faults may be more closely related than we previously realized and could interact with one another in complex ways. It’s possible that both faults are connected by an as yet undiscovered master fault beneath the region. It’s also possible that the two faults are simply close enough that a rupture of one can transfer stress to the other, tipping it over the edge.

Figuring out exactly what happened, and what might still be happening, between the Seattle Fault and the Saddle Mountain Fault will require additional research. In the meantime, it presents a new (to us) potential danger for residents in the area, raising the bar for earthquake preparation in the region. Fortunately, the double quake 1,100 years ago was the last major quake in the area since the last ice age. The likelihood that Seattle gets hit with another double quake in our lifetime is exceedingly small… but not zero.

Catch the first two seasons of La Brea streaming now on Peacock!