Tunnels and earthquakes

Seattle is prone to earthquakes, so it’s not surprising that one of the most common questions we receive is this: Will the tunnel be safe in an earthquake? 

All modern structures in the area – including bridges and highways – must be designed with earthquake safety in mind. But geotechnical and structural engineers agree that tunnels are among the safest places to be during an earthquake. 

Some folks find this counterintuitive. How is it safe to be underground when the earth starts shaking? Turns out underground structures are safe because they move with the soil, while structures above ground sway back and forth. 

Imagine a plate of fruit-filled gelatin dessert. Tunnels are like the pieces of fruit at the base of the gelatin, while above-ground structures are like the fruit toward the top. If you shake the plate, the movement becomes more exaggerated as it flows up from the base of the gelatin. In an earthquake, this translates to tunnel movement measured in inches, while the movement above ground might be measured in feet. 

SR 99 tunnel design
Beyond these generally earthquake-resistant traits, the SR 99 tunnel has been specifically designed to withstand a 9.0 magnitude earthquake. This magnitude earthquake only occurs every 2,500 years on average. 
The design team had to ensure the tunnel could remain watertight after a seismic event to allow for the safe evacuation of people. Flexible joints between all component parts can accommodate an earthquake’s movements, and every piece of equipment to be installed in the tunnel has undergone seismic analysis. 
All in all, the tunnel and operations buildings were designed to meet 18 different seismic codes, standards and specifications from around the country. So while you might not be able to plan where you are when an earthquake hits Seattle, you may take comfort in knowing this: The tunnel was designed with safety in mind, not only for day-to-day operations, but for the day “the big one” hits.
Additional safety information
The ability to withstand earthquakes isn’t the only safety factor considered during design of the SR 99 tunnel. Learn more by reading our tunnel safety fact sheet (pdf 1.6 Mb).