Wednesday, March 23, 2022

It's Only A Test: Tsunami Warning System Drill

Posted By on Wed, Mar 23, 2022 at 8:38 AM

click to enlarge Tsunami warning sign. - FILE
  • File
  • Tsunami warning sign.
The annual Tsunami Warning Communications Test takes place tomorrow (Wednesday), giving local emergency officials the chance to tryout the local warning system and residents an opportunity to learn more about the ways those same officials will be getting out the word if a distance-source tsunami is heading to Humboldt's shores.

The drill is scheduled to start at 9 a.m. with the county Office of Emergency Services testing the local warning system: Humboldt Alert. Residents who are signed up will receive notifications on their cellphone, landline and/or email. That will be followed at 11 a.m. by the National Weather Service running a test of the Emergency Alert System until noon, with notifications being sent out via radio and TV broadcasts, NOAA weather radio and reverse 911 calls in Humboldt, Del Norte and Mendocino counties. In some areas, tsunami sirens may be activated and people on the coast may hear test broadcasts from planes.

During last years's test, nearly half of the Humboldt's 12 sirens remained silent after being "corroded to oblivion" by years of exposure to salt air and the North Coast's notoriously wet weather. The Journal's March 10 cover story "Icons of Preparedness," looks at the cost vs. the benefit of replacing the sirens, as well as whether the devices are as effective as cellphone alerts and other warning systems, as local emergency officials weigh the options moving forward.

But the warning systems being tested are really meant for use in a distant-source tsunami, like the one generated back in March of 2011 by a devastating earthquake off the coast of Japan, when Humboldt County was under the highest threat level, a tsunami warning, and the local sirens were last sounded as an alert. In that scenario, officials are likely to have hours to get out the word. (The sirens are not activated if the region is under a lower-level tsunami advisory, which was the case back in January.) Find the county's tsunami hazard areas here.

click to enlarge tsunami_alert_levels.jpg
A far bigger threat lurks in our own backyard, a rupture along the Cascadia subduction zone, which could send surges beyond beaches and the harbor in minutes.

In that case, local emergency officials stress, the only warning will be the ones Mother Nature has to offer: prolonged, intense shaking, a loud ocean roar and the sudden receding of the water to show the sea floor. Any of these means time is of the essence and to head for higher ground immediately.

Nearly 30 years have passed since a small corner of the Cascadia subduction zone ruptured near Petrolia on April 26, 1992, shaking the region with such intensity that seismic sensors in the area were overwhelmed and a 15-mile section of coastline was thrust several feet into the air.

Within minutes, for the first time ever recorded on the West Coast, a locally generated tsunami arrived on shore, with a small wave arriving at the North Spit less than a half-hour after the magnitude-7.2 earthquake struck just before 11 a.m. Southern Humboldt beaches were hit even sooner.

In the quake’s aftermath, landslides shut down roads, water mains burst, windows shattered, a wide swath of the North Coast was left without power and fires destroyed the Petrolia post office and a shopping center near Scotia. Hundreds of people were injured and homes damaged.

But for all the ferocity released by the earth that day, the Cape Mendocino Earthquake was just a sampling of what the Casacadia subduction zone has the power of unleashing — a magnitude-9.0 or greater megathrust quake, which last occurred in 1700.

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About The Author

Kimberly Wear

Kimberly Wear is the assistant editor of the North Coast Journal.

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