Hazards in Snohomish County

People living in Snohomish County are exposed to a suite of human-caused and natural hazards. To lessen potential impacts, the County and its partners maintain and regularly update a Hazard Mitigation Plan. The last update was completed in 2020. Among other things, the work identified and ranked the community’s biggest hazards using criteria that included expected severity, magnitude, frequency, the ability to prepare for onset and expected durations for worst-case and most-likely scenarios.

The plan identifies strategies to reduce or alleviate loss of life, injuries and property damage from disasters. Keeping the plan updated increases the likelihood there will be well-considered strategies to respond to threats. It also maintains local government’s eligibility for recovery grant assistance to repair public infrastructure under the federal Disaster Management Act of 2000.

Earthquakes are ranked as the most-serious hazard in Snohomish County. They occur during sudden stress changes underground due to the slip of a fault, or volcanic activity. The resulting shaking and radiated seismic energy can cause widespread damage to buildings, roads and infrastructure and unleash other calamities, including landslides and tsunami waves. Earthquakes can last from a few seconds to over five minutes. Aftershocks can continue for days. Several major faults contribute to the risk here, including the Cascadia Subduction Zone off the Washington Coast, and the South Whidbey Island Fault, which runs under much of southwest Snohomish County. The community typically experiences a few minor earthquakes each year but needs to be prepared for major shaking.

The ground beneath our feet conceals dozens of faults and folds that contribute to making Snohomish County one of the more seismically active places in North America. Hundreds of earthquakes occur in the Puget Sound region every year, most so small that they only can be detected using sensitive instruments. However, at least 20 strong quakes have shaken the area during the past 125 years, resulting in 16 deaths and $2 billion in damage.

A major quake threat rests in the Cascadia Subduction Zone off the Washington Coast. If it lets loose, experts predict widespread damage to bridges and overpasses, likely shutting down the region’s transportation system for weeks. Closer to home, emergency managers worry about the Devil’s Mountain Fault in the county’s northeast corner, and the South Whidbey Island Fault. The latter runs below much of south county.

More information can be found on the Washington DNR  Earthquakes and Faults  webpage.

The South Whidbey Island Fault has been studied extensively in recent years. This map represents the potential shaking intensity from a shallow Magnitude 7.4 event with an epicenter two miles northeast of Langley. Areas in Dark Red have the potential for the highest levels of shaking. Areas in Green will likely experience less shaking.

Such a quake likely would cause an estimated $12.8 billion damage in Snohomish County and leave more than 5,000 people in need of short-term shelter.

Additional information related to the South Whidbey Fault can be found  here .

Liquefaction is a secondary effect of an earthquake. Hard shaking can cause some soil types to lose shear strength and behave as liquids. Structures built atop liquefaction zones – including buildings and road foundations – can lose their support and sink into what was previously solid ground. Water will sometimes come to the surface, bringing with it liquefied sand and creating sand boils, sometimes called “sand volcanoes.”

Water-saturated sands, silts or gravelly soils are especially prone to liquefaction. They can be found here along the Snohomish and Stillaguamish rivers, within the Quilceda Creek drainage, and bordering Puget Sound.

During liquefaction, these type of soils are shaken so violently that the individual grains lose contact with one another and float freely in the water, turning the ground into a pudding-like liquid.

Soil liquefaction maps are useful tools to help assess what potential damage from earthquakes may look like after the ground shakes.

Additional information related to liquefaction hazards can be found on the  Pacific Northwest Seismic Network  webpage.

After a large earthquake, hazards such as liquefaction, fallen bridges, roadway debris, and abandoned vehicles may cause barriers to travel. Data modeling suggests that the compromised transportation grid likely would isolate people into what planners are calling 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.

Quake 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 analysis suggests the fractured transportation system would make it extremely difficult to travel between population islands.

The map shows how the community may break apart into population islands. You can learn more and interact with the data by visiting the  Megaquake Population Islands in Snohomish County  story map. 

Do you know where your food, water and medicine will come from if the stores aren’t open, roads are impassable and cargo isn’t moving? The experts say you and your family may be on your own for up to two weeks after a major earthquake. 

Consider completing this   checklist  . It will help you prepare individually and also help the community become more resilient because you will be in a better position to help others. The steps you take to prepare for earthquakes also can help get you ready other hazards

The map below highlights a few consequences of earthquake activity from the past few decades in Snohomish county. Scroll on to learn about key events.

Everyone who lives in Snohomish County gets familiar with severe weather. High winds, torrential rains, ice and snow are regular visitors. They often cause millions of dollars in property damage and sometimes take lives. Experts say the community averages at least one high-wind event annually, and calculate a 58% probability of a severe winter storm any given year.

Storms not only knock out electricity, they often leave live power lines across roadways and topple trees into homes. The most vulnerable populations include the elderly, people living with life-threatening medical conditions, low-income families and those who are isolated either because they live in remote areas or do not speak English.

Snohomish County PUD provides up-to-date information related to power outages on its 24/7 outage map. The PUD recently launched outage text alerts so customers can receive updated information when they lose power. To receive text alerts, report an outage through the PUD’s outage map and opt-in to receive outages texts. For access or to report an outage, visit the  PUD Outage Center .

Here are some quick tips from the Snohomish County PUD on preparing for severe weather

Before storm season:

1. Assemble an emergency kit with food, water, flashlights, batteries, first aid and other essential items.
2. Make a plan for your family, including an out-of-town emergency contact.
3. If you have special medical needs – such as needing use of a respirator – consider a portable generator and a backup site with electricity.  Click here for tips on how to operate a generator safely.  
4. For other tips, visit  the Snohomish County PUD storm preparation site. 

When storms approach:

1. Stay informed through the PUD Outage Center, follow Facebook and Twitter posts and listen for radio news updates.
2. Report outages via the PUD website or call them in to 425-783-1001.

In the middle of the storm:

1. Designate a smaller room with few windows as a primary family living area. Keep window drapes closed, and dress warmly with several layers.
2. Use portable generators safely outside in a well-ventilated area. Carbon monoxide kills.
3. Stay away from fallen power lines – and report them to the PUD.
4. Visit  the Snohomish County PUD outage map .

The Snohomish County PUD has hard-won experience preparing for storms and restoring electrical service in their aftermath. Here is just a partial sample from its list of memorable storms:

November 4, 2022 – most outages to date

Strong winds out of the west blasted Snohomish County and Camano Island on Nov. 4, knocking out power to more than 220,00 homes and businesses, or about 60% of PUD customers. Damage was widespread across the PUD’s service area and restoration efforts took a week. More than a dozen utilities from across the state and Pacific Northwest sent mutual aid crews to help in restoration efforts. Crews worked more than 700 incidents and replaced more than 60 utility poles during restoration efforts. 

November 17, 2015 – mid-November storm

A series of wind gusts throughout the day saw PUD crews chase one set of outages after another, with the number of impacted customers bouncing up and down. By late afternoon, gusts reached nearly 70 mph near Puget Sound. Nine transmission lines were taken out of service. During the storm, about 205,000 customers lost power. The PUD recruited mutual aid from more than 12 utilities. At the height of the restoration, the utility had nearly 90 crews in the field – a total of about 500 workers. Within 48 hours, the outages were down to about 24,000. However, given a large number of smaller jobs, restoration took nearly five days. For seven years, the 2015 storm held the record for causing the most outages here.

Dec. 18, 2008 – pre-holiday snow storm

Early in the day, a winter storm hit the region with very cold temperatures and one of the heaviest snowfalls in several years. Arlington received 30 inches in the first 48 hours. Winds picked up during the weekend in the foothills of the Cascade Range, in areas such as Gold Bar and Sultan, with gusts up to 40 mph. Access to many areas was extremely limited. Several snow storms hit the region over a week-long period through Christmas Day. More than 44,000 customers had a sustained outage at one point during the 10-day event, which finally wrapped up on Dec. 27.

January 20, 1993 – Inauguration Day storm

The Inauguration Day storm is considered by many the most severe to hit the Puget Sound region. Winds hitting 66 mph struck the area in the late morning, causing 175,000 of the PUD’s 219,000 customers to lose power. All five circuits from Bonneville Power Administration’s Sno-King Substation near Bothell were taken out at one point, causing a loss of power to everything from the King County line to south Everett. Total restoration cost about $3.9 million and took about 10 days.

December 17, 1990 – pre-holiday storm

A strong wind storm with gusts of about 70 mph initially struck the Puget Sound area, followed immediately by heavy snow and, finally, by an extreme cold snap with temperatures of less than 10 degrees. The triple whammy caused 110,000 of the PUD’s 200,000 customers to lose power and caused $1.6 million in damages. Restoration, hampered by the bitter cold and snow, took about a week to complete.

More than 75,000 people in Snohomish County live and work in places where potentially devastating floods occur. Historically, the major river basins – the Snohomish and Stillaguamish – have flooded every three to five years, often at the same time. Big creeks draining through south Everett, Mill Creek, Lynnwood and Bothell also bear watching. Since 1962, the county has weathered 18 floods large enough to be Presidentially declared disasters. The worst arrived during winter 1975, causing $42 million damage and the deaths of 3,500 head of livestock.

Flood season here typically begins in October and lasts into spring. The threat is greatest between late November and early February. That’s when storms heading inland from the Pacific Ocean bring heavy rains and mild temperatures, often melting mountain snows and spurring rivers to jump over their banks.

While heavy downpours can produce flash flooding, particularly in some urban neighborhoods, there usually is some warning that high water is coming. That means there is time to prepare.

Review the  Snohomish County Flood Safety Guide  and  Flood Information Center  for locally tested advice on how best to protect your family before, during and after the water arrives. Visit the county's  Real-Time Flood Information Website  for up-to-the-minute river level and rainfall data. Confirm your route is open using the county’s  Road Closure Information page .

The county uses SnoCoAlerts to warn people of emergencies. When threatened areas have been identified, Snohomish County notifies residents of potential hazard conditions.  Sign-up Now! 

There are more than 60 dams in and around Snohomish County where planners say a mishap or failure could affect people and property. Dams here are well-maintained, but nearly one in five is classified as presenting the potential for high downstream risks. Dam failures in the U.S. usually occur as the result of other hazards, such as earthquakes, landslides, storms, excessive runoff from snowmelt, or sabotage. Although it is considered unlikely, dam failure is ranked as the county's sixth-biggest hazard, in part because of the potential for widespread disruption. The community of Sultan has designated evacuation routes to help residents reach safety in the unlikely event of an emergency involving Culmback Dam at Spada Lake, the county’s largest reservoir.

The wildland fire season in Snohomish County usually begins in early July and ends with the rains of fall. However, fires have occurred in every month of the year, influenced by drought, limited snowpack, and local weather conditions.

For people, the potential danger zones lie in the places where trees, shrubs and homes are mixed together. These can include lone houses surrounded by woods or suburban neighborhoods bordering forest lands. Emergency experts call these areas the wildland urban interface. In Snohomish County, they are home to an estimated 130,000 people with property valued at more than $9.7 billion. Use the map below using data from the Washington State Department of Natural Resources to view the wildland urban interface in Washington State and Snohomish County.

There are steps you can take to reduce the risk of wildfire damage to your home and to help keep you safe when flames are on the move. Visit the  Snohomish County Wildfire Preparedness Webpage . 

Snohomish County uses SnoCoAlerts to warn the public of emergencies. When threatened areas have been identified, Snohomish County notifies residents of hazard conditions.  Sign-up Now! 

That was the morning a wall of mud swept away the Steelhead Haven neighborhood and buried Highway 530 between Oso and Darrington. A total of 43 people died and more were injured in what is now recognized as the deadliest landslide in U.S. history.

People rose to help each other after the hill fell. They continue to do so today. The tragedy brought into focus just how much harm slides can cause, and how widespread the risks can be in a damp, hilly place such as Snohomish County.

Planners estimate that up to 60,000 people live in areas of the county where steep slopes, soil types and lots of rain combine to create risk from landslides. In all, more than 12,400 structures have been identified as being located on, above or below slopes with the potential for slides. Analysis shows that 95% of those buildings are somebody’s home.

Landslides here typically occur from January into spring after the water table has risen during the wet months of November and December. Human influences, including development activity and removing vegetation, factor into more than 80 percent of reported slides, experts say.

Information about  slide-prone areas  is readily available, as are tips for helping to recognize the risk and  take appropriate steps .

In general, if you see cracks forming in the ground, changes in vegetation from ground movement, sudden changes in creek levels or the sound of cracking wood, get clear of slide prone areas.

Here are some resources to learn more about landslides:

 http://www.dnr.wa.gov/geology 

 http://landslides.usgs.gov/ 

 http://www.ready.gov/landslides-debris-flow 

 https://www.insurance.wa.gov/landslide-insurance 

The Glacier Peak volcano may be Snohomish County’s least-understood natural hazard.

Although it stands roughly 10,525-feet and historically has produced some of the most-explosive eruptions of any volcano in the Pacific Northwest, the mountain is tucked so far back into the wild spaces of the Cascade Range as to be invisible to many people living around Puget Sound. The land bears scars from Glacier Peak's fury, geologists say. Among the region’s volcanoes, it is second only to Mt. St. Helens in the frequency and intensity of its eruptions.

Like St. Helens, Glacier Peak has the nasty habit of occasionally belching clouds of choking volcanic ash. It’s most-serious threat to people, however, comes from periodically unleashing major debris flows known as lahars.

A lahar is a mixture of volcanic debris and water, often from melting glaciers. Consistency can range from that of muddy dishwater to something approaching wet cement. Lahars race downstream with tremendous destructive force, reshaping the land and rerouting rivers. Indeed, a lahar from Glacier Peak turned the Sauk River to its present course north toward the Skagit River instead of its original path west down what is now the Stillaguamish River valley.

Scientists consider a lahar from Glacier Peak statistically unlikely, but still a risk to anyone who lives in the Stillaguamish River basin, from Darrington to Stanwood.

Do you spend time in a Glacier Peak lahar zone? What route would you take to reach high ground or travel out of harm’s way? Get prepared by reviewing the  Snohomish County Glacier Peak Volcano Preparedness Webpage. 

Snohomish County uses SnoCoAlerts to warn the public of emergencies. When threatened areas have been identified, Snohomish County notifies residents of hazard conditions.  Sign-up Now! 

Tsunamis are waves caused by earthquakes, volcanic eruptions, or landslides under the sea. As waves travel toward shore, they build higher as the depth of the ocean decreases. Waves can reach heights over 100 feet and can travel at the speed of a commercial jet plane. Tsunamis can injure people in the water and destroy boats and infrastructure. Low-lying areas such as beaches, bays, lagoons, harbors, river mouths, and areas along rivers and streams leading to the ocean are most vulnerable. Snohomish County’s greatest tsunami risk is associated with earthquakes involving Puget Sound. The Tulalip Tribes tell of a great wave in the 1820s kicked up by a landslide on Camano Island. It drowned people on the beach at nearby Hat Island.

Evidence of other past tsunamis have been found in sediments along the lower Snohomish River and other waterfront locations along Puget Sound. Still, not enough is known about the rare hazard to rank it compared to other known threats. 

Experts have used increasingly sophisticated data modeling to estimate tsunami timing and inundation after earthquakes. A 2019 analysis by the National Oceanic and Atmospheric Administration determined that within 1.5 hours of a 9.0 megaquake along the Cascadian Subduction Zone off the Washington coast the Everett waterfront likely could see a series tsunami waves and inundation approaching six feet. In 2022, a  study by the state Department of Natural Resources  found that a quake along the Seattle Fault could spawn waves of up to 40 feet along the Seattle waterfront, with smaller, but still destructive, waves hitting Edmonds and Everett in under a half hour. 

DNR maintains a comprehensive webpage filled with the results of  tsunami research , including inundation maps and videos. 

The state’s Emergency Management Division maintains a network of 122 state-of-the-art tsunami sirens along Washington’s inner and outer coasts. Three sirens are in Snohomish County, one in Edmonds and two along the Everett waterfront. The state conducts routine  testing of the tsunami siren system  on the first Monday of each month. Listen for Westminster Chimes at noon and the siren’s wail once a year in October. 

When visiting a waterfront area, do you know how to quickly get out of harm’s way? Signs a tsunami may be imminent include water rapidly receding from the shoreline. The only option is to head for higher ground or evacuate inland. If you don’t have time to get to high ground, but are in a multi-story building, go to an upper level.

The state’s Emergency Management Division offers more  tips for tsunami safety .

The risk of sickened people, shuttered schools and a sputtering economy combine to make public health epidemics the second-greatest hazard in Snohomish County. As we have experienced with COVID-19, and before that H1N1 flu, people countywide are at risk for an epidemic or outbreak, particularly those living with underlying health conditions or unequal access to healthcare. Epidemics involving infectious diseases are occurring more often, spreading faster and further throughout the world. Newly discovered and re-emerging diseases magnify vulnerability. Viruses, bacteria, parasites, fungi, or toxins can be spread by people, contaminated food or water, healthcare procedures, animals, insects or the environment.

An active assailant can be an armed and dangerous individual or a group of people intent on causing harm in a social setting. For this risk think: crowds. If caught up in one of these emergencies, do whatever it takes to keep yourself safe (run, hide, fight).  Keep your hands up, open and in clear view when approaching first responders. Once away from the immediate threat do not congregate in a large group. Encountering an active assailant is ranked as the eleventh-biggest hazard in Snohomish County.

Snohomish County is a hub for aerospace manufacturing and other activity. The skies host commercial and cargo flights, as well as military and private aircraft. The largest and busiest local airport is Snohomish County-owned Paine Field, but there is considerable activity at airports in Arlington and Snohomish. Aircraft headed to and from bustling Seattle-Tacoma International Airport also make heavy use of airspace here. The risk of a major aircraft accident is ranked as the community’s twelfth-biggest hazard.

Cyberattacks are malicious attempts to access or damage digital systems using computers, mobile phones, gaming systems, and other devices. From an emergency management perspective, concern in Snohomish County focuses most on potential disruptions of computer systems that support the distribution of energy, water, food, sanitation, communications, government services, economic activity, health care and emergency response. Bad actors elsewhere have amplified damage from cyberattacks by timing their actions to coincide with other emergencies, hampering the response. Cybersecurity threats are ranked as the eighth-biggest hazard for Snohomish County.

Hazardous materials are substances with a chemical, physical, or biological nature which can pose a potential risk to human health, property, or the environment. Think: flammable gas and fuels, caustic substances and materials that can poison the air and water. Hazardous materials mishaps can occur during the manufacture, transportation, storage, and use of these substances. Some 20 billion gallons of oil, gasoline and other hazardous chemicals are transported through Snohomish County each year by road, rail and pipeline. Because of the associated risks, hazardous materials incidents are ranked the community’s third most-concerning.