Simultaneous rupture of faults triggered massive earthquake in Seattle area 1,100 years ago

The skyline of Seattle during sunset — The realm surrounding what’s now Seattle was rocked by a large earthquake round 1,100 years in the past. (Picture credit score: Getty Photos)

The Seattle area could have been hit with a large, magnitude 7.8 earthquake 1,100 years in the past after a collection of hidden faults ruptured concurrently, a brand new examine suggests. That’s way more highly effective than present warnings counsel is feasible from the identical fault zones.

Researchers have identified for a very long time that Puget Sound, an space that accommodates Seattle, Tacoma and Olympia, was rocked by huge quakes prior to now. Geological proof suggests these tremblers rumbled someday between 900 and 930 A.D. and had magnitudes within the low 7s. The present greatest guess is that there have been at the very least two main quakes throughout this time, one ensuing from the Seattle Fault Zone (SFZ), positioned beneath and across the metropolis, and one other from the Saddle Mountain Fault Zone (SMFZ) in southwest Washington.

However in a brand new examine printed Sept. 27 within the journal Science Advances, researchers analyzed the fossilized stays of bushes killed by the quakes and located that every fault zone doubtless ruptured concurrently, or at the very least inside a really brief area of time of each other. If the zones did break aside on the identical time, the ensuing quake would have been extra highly effective than some other previous simulation has predicted.

This might have huge implications for the 4 million individuals at present residing in Puget Sound, researchers wrote. Present warning fashions needs to be up to date to account for a equally highly effective earthquake hitting sooner or later, they added.

Associated: How huge is the most important attainable earthquake?

Scuba divers pull up a fossilized tree stump from a lake bed

Researchers pulled up sections of fossilized tree stumps from the ground of Value Lake close to Seattle. (Picture credit score: Bryan Black)

Within the examine, the researchers analyzed fossilized Douglas fir (Pseudotsuga menziesii) bushes from six websites throughout Puget Sound. The crew estimated when the bushes died by combining tree ring counts and radiocarbon courting. The bushes within the SFZ and SMFZ doubtless died inside a six-month interval, someday between 923 and 924 A.D., the evaluation discovered.

This timeframe means that the quakes have been brought on by the identical tectonic plate actions, growing the probability that they occurred concurrently. If they didn’t, then the researchers suspect that every particular person earthquake would have had a magnitude someplace between 7.3 and seven.5.

This will likely not appear an excessive amount of weaker than the potential 7.8 magnitude quake — however earthquakes are measured on a logarithmic scale, which means {that a} magnitude 8 is 10 occasions extra highly effective than a magnitude 7, and so forth.

The SFZ and SMFZ are each very shallow, which implies they are often very damaging, much like the fault traces liable for the devastating earthquakes in Turkey and Syria in February.

Associated: The 20 largest recorded earthquakes in historical past

A simultaneous rupture would doubtless have prompted essentially the most highly effective quake the area’s faults are able to producing, the researchers wrote. If each zones have been to concurrently rupture once more then there may be each probability that it might be repeated, they added.

This isn’t the one discovery concerning the Puget Sound fault zones to emerge this yr. In March, researchers from the disaster modeling firm Temblor found proof on Bainbridge Island close to Seattle that implies a significant quake may have hit Puget Sound as lately as round 200 years in the past, though it is unclear how highly effective it was.

Nevertheless, regardless of the brand new findings, essentially the most harmful seismic risk to Seattle is the Cascadia Subduction Zone (CSZ), a large fault line that stretches roughly 620 miles (1,000 kilometers) from Vancouver Island, Canada, to Northern California. The CSZ is able to producing earthquakes with a magnitude of round 9, comparable to “the massive one” that rocked the Pacific Northwest within the yr 1700.

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