Seattle is a lovely place, but it come with risks. Our region ranks number one in the country for the number of hazards we face- earthquakes, winter storms, landslides, and flooding. Earthquakes in particular are a concern, as our area sits on top of several faults and is in the Cascadia Subduction zone.
The map above shows Seattle color-coded by earthquake risk. Most of Seattle is at mid to high-risk. The dark red areas are of most concern, as they sit on top of liquefaction zones. Seattle's waterfront sits on a tidal flat overtopped by loose fill (our infamous Seattle underground), and is ripe for major damage in the next major earthquake.
What is a Liquefaction Zone?
Liquefaction is a special type of ground settlement that occurs in water saturated sands, silts and gravels, like our waterfront. In an earthquake, loose soils compact, displacing and pressurizing the water. The “solid ground” then liquefies. Whole buildings have overturned when the underlying soils lose enough tensile strength to support the structure. Once liquefaction has occurred, the muddy soil will often flow laterally (laterally spreading) and cause severe structural damage.
Where are the highest risk areas?
Here in Seattle, the downtown and areas in red are highly vulnerable to liquefaction. Even during the modest magnitude 6.8 Nisqually quake in 2001, areas around the tideflats/Port of Seattle turned to jelly and spouted fountains of water burbling out of the ground. That's what happens during liquefaction. Building walls crumbled, bridge supports cracked, and the cost ultimately tallied at $4 billion. 400 people were injured, and more than 300,000 buildings were damaged.
The concern is that the Nisqually quake wasn't nearly as big as we can get in the Seattle area. Seismologists predict that our area can see 9.0 earthquakes- that would release 1,000 times more energy than the Nisqually quake.
What does this mean for me?
The city's Emergency Management department urges people to understand the risks earthquake pose, know where you are located, and how liquefaction increases our risk. Most importantly, emergency agencies ask that people have supplies covered at least for the first few days following an earthquake, as well as a plan. Seattle is an amazing place to live, but being responsible and prepared for disasters is a part of life here.
If you don't have an Earthquake Bag and a plan in place, get it done today. You need to have food and water for at least a few days, communication and light sources that don't rely on the electric grid (like a hand-crank radio/flashlight/phone-charger), shelter and warmth, first aid, hygiene, and tools and equipment needed in the aftermath of an earthquake. If your house or building has gas lines, get a gas shutoff tool and ziptie it to the turn-off valve.
Additionally, make sure you and your family have a plan. Know where you'll meet after an earthquake. Plan and practice an evacuation route from your home. Designate an out-of-town contact that everyone can connect with, as phone service may be down. Write down and keep copies of the emergency contacts for your area, and your out-of-town contact.
Preparing for a major earthquake really doesn't have to be daunting. Pick up an Earthquake Bag now, take an evening to create a plan for your family or house, and feel better knowing you've prepared!