PNW’s Shake Alert system passes with flying colors after Friday quakes
Jul 12, 2019, 4:34 PM
Washington's Shake Alert system passed a big test early Friday morning. (PNSN)
(PNSN)
Between incidents in California and Snohomish County, earthquakes are front-of-mind for many along the West Coast. But what sort of measures do we have in place to help warn us when a quake hits? For that, we have the Shake Alert system, which debuted in California in 2018, and is currently being tested in Washington.
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Promisingly, the Pacific Northwest’s own iteration of the system functioned just as it was designed to early Friday morning, giving roughly two to 10 seconds of warning before shaking began.
“It worked well — we at least got some warning out, enough to give people an alert to get in a protected spot before the shaking arrived,” the Pacific Northwest Seismic Network’s Bill Steele told KIRO Radio. “We’d prefer 10 to 20 seconds, but you don’t always get it. ”
The system works by detecting what are known as p-waves: Sound waves produced by an earthquake before shaking begins. S-waves follow p-waves and move slower, producing what we typically associate with ground motion from an earthquake.
Once four Shake Alert sensors detect those p-waves, it takes two-tenths of a second’s worth of data to locate the quake and estimate its magnitude. It then sends out an alert before the shaking from the s-waves arrive, giving people valuable seconds to react and get to cover.
“Before the s-wave even arrives, we can have the earthquake assessed and warn if its dangerous,” said Steele.
Oddly enough, between Los Angeles and Snohomish County’s separate quakes, we saw two different results from Shake Alert. While the Shake Alert system was triggered in Snohomish County, it wasn’t triggered in LA last weekend.
That said, it technically did work, just not in the way people expected.
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“LA’s did work — the problem with LA is that the (United States Geological Survey) is being kind of conservative,” said Steele. “They were concerned that if people were alerted for a lot of earthquakes, they might grow bored and ignore them.”
The USGS set the system to alert people for a shaking intensity of 4 or greater (a different number than magnitude). Because that number was around 2 or 3, an alert wasn’t sent out. If it had sent an alert, Steele estimates that there would have upwards of 30 seconds warning before shaking began.
The system started rolling out in Los Angeles in late-2018. Washington’s system is live with beta testers, but won’t be fully online until sometime in 2020.
As of late last year, just 277 of the 560 seismic stations needed statewide for ShakeAlert were unfunded, a stark contrast to the $25 million California has poured into its own sensors.
