Seattle is no stranger to earthquakes. However, the potential extent of these tremors, and the resulting threat to the city, might be underestimated, according to research published on September 27 in the journal Science Advances.
The Washington State Department of Natural Resources has estimated that the state experiences earthquakes nearly every day, although it has only had around 15 large earthquakes since the 1870s.
The state lies atop dozens of active faults, many of which run underneath its major cities. The closest fault to Seattle is the Seattle Fault Zone, which extends east-west through the middle of the city. The Saddle Mountains, Tacoma, and Olympia fault zones also all run within close proximity.
“These faults can interact with one another and produce larger earthquakes in which their energies are combined, or in very rapid succession in a one-two-punch scenario,” Bryan Black, an associate professor at the Laboratory of Tree Ring Research at the University of Arizona, who led the study, told Newsweek.
This is what is known as a multi-fault earthquake.
“Multi-fault earthquakes happen when two seemingly independent faults link up and produce earthquakes that are larger than what either of the faults could produce individually,” Black said.
These events are not common, but previous studies have shown that, at some point between the years 780 and 1070 CE, these faults produced a series of powerful tremors. However, the geological records studied so far do not provide enough resolution to clearly pinpoint whether two events took place at the same time or several decades, or even centuries, apart.
“That is where tree rings come in handy,” Black said. “Trees can give us that temporal resolution.”
To determine whether any of these events took place simultaneously, Black and his team analyzed a selection of fir tree fossils collected from six sites across the region, using radiocarbon and tree-ring dating to determine exactly when these trees had died.
“What we do is pattern match the ringlets,” Black said. “As the climate varies from year to year, it induces these synchronous growth patterns among local trees. So, for example, in a dry year, all the trees form a narrow ring; in a wet year, they form a wide ring. And we can match those time-specific synchronous growth patterns like bar codes among the trees.
“And we can see here that the trees [along the Seattle and Saddle Mountain faults] all shared the same specific growth patterns—they lived together and died together in the same year.”
By comparing the dead tree fossils to other living specimens from that time, Black and his team found that the fossils had died within the same 6-month period, between 923 and 924 CE, and were likely hit by a multi-fault earthquake, stronger than anything the Seattle or Saddle Mountain fault zones could produce on their own.
A quake of this size may have reached a magnitude of up to 7.8, comparable to the 7.8 magnitude earthquake that devastated Turkey and Syria in February.
So, is Seattle prepared? An estimate in 2005 found that even a magnitude 6.7 earthquake along the Seattle fault today could cause 1,600 deaths and destroy roughly 10,000 buildings, resulting in a total economic loss of approximately $50 billion. A single linked earthquake across the Seattle and Saddle Mountains would release roughly 38 times more energy than that.
Even so, multi-fault quakes of this magnitude are still extremely rare. But what this study shows is that they are possible.
“We have shown that, in the past, these faults have linked up,” Black said. “It’s not like this happens regularly, but it’s also a possibility that it could happen again in the future, so it’s something to be aware of. Does that really change the risk [to Seattle]? We’ll see what the engineers think.”
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