seattle skyline mt ranier
© Donald Miralle / Getty ImagesResearchers studied tree rings to determine that a single earthquake along two fault zones may have occured near Seattle around 1,100 years ago.
Using tree ring dating, scientists have revealed a massive earthquake — or two in succession — struck the Puget Sound region in the Pacific Northwest almost exactly 1,100 years ago.

The destructive event demonstrates that the area — which contains Seattle, Tacoma and Olympia, Washington — could be susceptible to more extreme earthquakes than previous research had indicated.

Scientists already knew that multiple quakes shook the region between 780 and 1070 C.E., but they could not precisely date each one — for some, they could pinpoint only a wide window of more than 100 years. The new study, published last week in the journal Science Advances, gets much more specific: Researchers say two fault zones near Seattle ruptured within a six-month period between 923 and 924 C.E. Whether these faults ruptured at the same time, or spread a couple of months apart, the resulting quakes would each have been more than magnitude 7.3, per the paper.

Current hazard models for the region, which inform building code standards, do not consider quakes of that strength. The results suggest hazard models should be updated to account for the possibility of more damaging earthquakes, the authors write.

"It's showing that these complex, large earthquakes are a real possibility," Bryan Black, lead author of the study and tree ring dating expert at the University of Arizona, tells Scientific American's Robin George Andrews.

"The chance in any given year is not high, and there's no reason to freak out because of this study," Harold Tobin, director of the Pacific Northwest Seismic Network at the University of Washington who did not contribute to the research, tells the Seattle Times' Sandi Doughton. "But it underscores that these are things that we need to be prepared for."

The Puget Sound region has multiple potential sources of earthquakes, including the Cascadia subduction zone off the coast and four nearby shallow fault zones: Seattle, Tacoma, Saddle Mountain and Olympia.
Cascadia subduction zone
Cascadia subduction zone
Earthquakes involving multiple fault zones at once are often the worst possible type of quake an area could have. However, they are rare and difficult to observe in the historical record.

"If faults rupture together, it raises the potential for larger earthquakes," Lydia Staisch, a geologist at the U.S. Geological Survey who did not contribute to the findings, tells Science's Michael Price.

To find out if such a calamitous event happened 1,100 years ago, the researchers turned to trees. They knew that earthquakes at that time caused mass tree death by sparking landslides and a tsunami, burying and drowning trees throughout the region. Precisely determining when the trees died would reveal when the earthquakes took place.
earthquake history pacific northwest tree rings
© Bryan A. Black http, Jessie K. Pearl, et alRegional setting spanning southwestern Canada to Washington and Oregon, USA. “MB” is the location of the “MacBlo” tree-ring chronology. Main map: Puget Sound region, principal fault locations (19), and study sites. SFZ, Seattle fault zone; SMFZ, Saddle Mountain fault zone; TFZ, Tacoma fault zone; OF, Olympia fault zone. “Crossdated trees” are sites where trees were sampled for this study: DL, Dry Bed Lake, PL, Price Lake, HH, Hamma Hamma, WP, West Point, LW, Lake Washington, LS, Lake Sammamish. “Radiocarbon only” are previously published radiocarbon dated earthquake sites with uncertainties that overlap 923 CE: LSI, Little Skookum Inlet, RCS, Red Salmon creek, TAC, Tacoma
The researchers analyzed Douglas Fir rings from six sites around Puget Sound. By comparing the trees' rings to each other and to those in a reference chronology, they determined the trees died between October 923 and March 924.

Several of the sites could have been affected by a Seattle fault rupture, but one site, called Price Lake, could only have been flooded by activity on the Saddle Mountain fault. The team concluded that the Saddle Mountain and Seattle fault ruptures occurred at or near the same time.

To bolster their confidence in the finding, the team scoured the tree rings for a telltale sign that aids in dating: a spike in stellar radiation called a Miyake event. This event, which could have been a solar flare or an exploding star, would have left a signature jump in carbon-14 concentration within the trees, writes the Seattle Times. They located such an event between 774 and 775 C.E. and used that as a reference point in dating, confirming the last tree ring across the sampled sites was 923 C.E.

"I am in awe of the power this method shows to precisely date earthquakes for which we have no direct historic records," Tobin tells Scientific American. "The result seems extraordinarily robust."

Based on the results, the researchers estimate that during this six-month window, either two closely spaced earthquakes of magnitude 7.5 and 7.3 or a single, multi-fault earthquake around magnitude 7.8 struck the area. The single earthquake scenario, however, is around three times as likely as the two separate quakes, the authors write.

A 2005 study predicted that if a 6.7-magnitude earthquake occurred at the Seattle fault, it could cause an estimated 1,600 deaths, destroy almost 10,000 buildings and cause $50 billion in economic losses. A 7.8-magnitude earthquake like the one described in the new paper would release 38 times as much energy — and it would likely cause a local tsunami.

Such an extreme event seems rare — the earthquake described in the new study was likely the region's most intense in the last 16,000 years, since the end of the last ice age. Still, modern hazard models need to consider the possibility of earthquakes of this magnitude, per the study.