Megaquake Population Islands in Snohomish County

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A new analysis shows many roads and bridges in Snohomish County, and elsewhere around Puget Sound, likely will become impassable after a major earthquake due to collapse, fallen debris, stalled or abandoned vehicles. 

Taking these steps can help make you more resilient for such a disaster, as well as a host of more common hazards. 

• Spend some time with this map. Learn more about how “population islands” may form where you live, work and travel. Think about what that may mean for meeting basic needs. Familiarize yourself with early planning that has identified potential community points of distribution during a disaster. 
• Sign up for SnoCoAlerts:  https://bit.ly/3nRIMX9 . During a major quake, emergency managers and public safety partners plan to make heavy use of this tool to share critical information right to your phone. 
• Become two weeks ready. Maintain at least a two-week supply of food, water, and medication for you and your pets and animals, plus an extensive first aid kit, water purification tools, and other supplies. You never know when you will need it. And by preparing for the worst, you are better positioned for lesser challenges. 

Put simply, population islands are areas that will possibly become isolated after an earthquake. After a large earthquake, hazards such as liquefaction, fallen bridges, roadway debris, and abandoned vehicles may cause barriers to travel that may make it impossible to cross the boundaries of these islands by road.

Snohomish County has been identifying these islands for disaster recovery purposes. The County in 2019 was chosen by FEMA to administer a Regional Catastrophic Preparedness Grant. In the years since, it has quietly helped lead a ground-breaking disaster planning effort involving eight central Puget Sound counties, five cities and the Tulalip Tribes.     The work, which began before the COVID pandemic and continued despite the disruption, focused on better understanding impacts from the Big One – a magnitude 9.0 megaquake along the Cascadia Subduction Zone off the Pacific Northwest coast.

Scientists say the region is overdue for such a catastrophe. The fault typically cuts loose every couple centuries, most recently in 1700. When that next occurs, the ground is expected to shake for up to several minutes. The energy released under the Pacific Ocean almost certainly would spawn tsunami waves, inundating coastal areas, including some low-lying places here. A major focus of the planning effort has been identifying what likely would break within the region’s transportation system, as well as options for continuing to get food and water to the millions of people who call the region home. Modeling used federal, state and local data. It demonstrated that during a megaquake, most of the region’s roadways and bridges would be significantly damaged by shaking and in many cases rendered impassable.

The compromised transportation grid would isolate people into what planners are calling population islands. Timing and commute patterns likely would determine whether people wind up stranded in the island where they make their homes, or at work or school. Local governments would work with state and federal partners to repair and reopen key routes, but the analysis suggests the fractured transportation system would make it extremely difficult to travel between population islands. That certainly would be true for at least a couple of weeks immediately after the quake. In some instances, isolation may last months, or longer.

 SnoCoAlerts  is the county’s early warning and notification system for delivering urgent emergency information, right to your cellphone. This robust system is relied upon by emergency managers and other first responders as one of the key tools to keep you informed during a disaster. Signing up for SnoCoAlerts is quick and easy. The system is powered by Smart911. Your zip code and street address are used to tailor alerts for where you live. You can choose which hazards result in alerts. You also can add more alerts or update your contact methods at any time. Even if you don’t use a cellphone, you still should consider signing up. Through the Smart911 portal, you are given the option of securely providing critical information that can be shared with emergency responders. Among the data to consider providing:

• People living in your household.
• Phone numbers associated with your family.
• Pets, service animals, and livestock.
• Medical conditions and allergies.
• Medications and medical equipment.
• Property details, layout, and utility information.
• Access to vehicles, their descriptions.
• Your ability, or inability, to evacuate if necessary.
• Emergency contacts.

All of that information can assist emergency dispatchers in determining how best to get help to you and your family during an emergency.

Another resource is the  USGS ShakeAlert™ Earthquake Early Warning  system. The system is available on all mobile phones in Washington and may provide seconds of warning to protect yourself before earthquake shaking arrives. It relies on a network of sensors designed to detect and provide data about earthquakes in progress. Captured information includes estimates on earthquake size, precise location and the shaking it may produce. 

Because the alerts can travel faster than quake waves in the ground, ShakeAlert promises a few moments warning, allowing people to Drop, Cover and Hold On. Other automatic capabilities are potentially game changing for mitigating earthquake damage in the region. The system provides data that can help slow trains to prevent derailments, open fire station doors so they don’t jam shut, activate hospital generators, and close valves to protect water systems. 

If the Big One hits, you will be glad to be two weeks ready

We know that people becoming confined to population islands is a real-world consequence of disasters, and not just earthquakes. During the deadly mudslide near Oso in 2014, for example, Darrington was cut off from the rest of the County when debris buried Highway 530. September 2022 saw U.S. 2 closed for days between Index and Skykomish as the Bolt Creek Fire burned surrounding hillsides and created conditions that made the highway too hazardous for traffic.   More frequently, river flooding, downed trees, heavy snows and road washouts have isolated communities here, sometimes for extended periods.       That’s why we urge you to make sure your household maintains at least a two-week supply of food, water, and medication, plus an extensive first aid kit, water purification tools, and other supplies. You never know when you will need it. And by preparing for the worst, you are better positioned for lesser hazards. Your emergency supply of food and medicine is there to spare you from making a trip out during a severe snowstorm, for example.

Need help building an emergency kit? Click the button to view a checklist, or review the graphic below.