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Earthquake threat to major US city likely underestimated, scientists say

Seattle is no stranger to earthquakes. However, the potential magnitude of these tremors, and the resulting threat to the city, may be underestimated, according to a study published September 27 in the journal Scientists progress.

The Washington State Department of Natural Resources estimates that the state experiences earthquakes almost every day, although there have only been about 15 large earthquakes since the 1870s.

The state sits atop dozens of active faults, many of which pass beneath its major cities. The closest fault to Seattle is the Seattle Fault Zone, which runs east-west through the middle of the city. The Saddle Mountains, Tacoma, and Olympia fault zones are also all nearby.

“These faults can interact with each other and produce larger earthquakes in which their energies are combined, or in very rapid succession in a two-hit scenario,” Bryan Black, associate professor at the Tree Ring Research Laboratory trees at the University. of Arizona, who led the study, said News week.

This is called a multi-fault earthquake.

“Multi-fault earthquakes occur when two seemingly independent faults link together and produce larger earthquakes than either fault could produce individually,” Black said.

These events are not common, but previous studies have shown that at some point between 780 and 1070 CE, these faults produced a series of powerful tremors. However, the geological records studied so far do not provide sufficient precision to clearly determine whether two events occurred at the same time or several decades or even centuries apart.

“That’s where tree rings come in handy,” Black said. “Trees can give us that temporal resolution.”

To determine whether any of these events occurred simultaneously, Black and his team analyzed a selection of fir fossils collected from six sites in the region, using radiocarbon and tree-ring dating to determine exactly when these trees were dead.

“What we do is match the patterns to the curls,” Black said. “As climate varies from year to year, it induces synchronous growth patterns among local trees. So, for example, in a dry year, all the trees form a tight ring; in a wet year, they form a wide ring. And we can match these weather-specific synchronous growth patterns, like barcodes among 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.”

Image of the Seattle skyline. Seattle is no stranger to earthquakes, but the potential dangers posed by these events may have been underestimated.
George Dodd/Getty

By comparing the dead tree fossils to other living specimens from that era, Black and his team discovered that the fossils had died during the same 6-month period, between 923 and 924 CE, and that they had probably been hit by a stronger, multi-fault earthquake. than anything the Seattle or Saddle Mountain fault zones could produce on their own.

An earthquake of this magnitude could have reached a magnitude of up to 7.8, comparable to the 7.8 magnitude quake that devastated Turkey and Syria in February.

So, is Seattle ready? A 2005 estimate found that even a magnitude 6.7 earthquake along the Seattle Fault could cause 1,600 deaths and destroy about 10,000 buildings, resulting in a total economic loss of about $50 billion. A single earthquake linked to Seattle and the Saddle Mountains would release about 38 times more energy than that.

However, multi-fault earthquakes of this magnitude remain extremely rare. But what this study shows is that they are possible.

“We showed that, in the past, these defects were linked,” Black said. “It’s not like it happens regularly, but it’s also possible that it could happen again in the future, so that’s something to be aware of. Does this really change the risk (for Seattle) ?We’ll see what the engineers think.” “


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