Located along the infamous Ring of Fire, Washington state experiences an earthquake about once every 8 hours. As an underwriter, chances are you have a number of risks on or near a fault line — over 100 known faults crisscross Western Washington and Oregon.
The city of Seattle is located directly over a fault — a hazard first recognized as a serious problem in 1992. The South Whidbey Fault cuts through Seattle’s northern suburbs and as far as North Bend and is capable of producing an earthquake as severe as magnitude 7.5.
While it’s important to know the distance to a fault line, it’s not the only factor in determining a risk’s potential damage from an earthquake. Much of it depends on the land on which a building resides, and this is where soil liquefaction comes into play.
When soil becomes saturated with water, it enters a state known as liquefaction where it stops acting like a solid and starts behaving like a liquid. If you’ve ever spent time at a beach, you’ve seen liquefaction in action.
Liquefaction bubbles rising to surface after an earthquake.
Normally, soil consists of densely-packed particles, like the firm ground far from shore. The particles exert force on one another, and this force holds them in place and creates strength. During an earthquake, the shaking increases groundwater pressure on soil particles, forcing them apart. As water saturates the packed soil, particles are forced farther apart and the soil softens and shifts, like beach sand close to the water.
Like sandcastles at high tide, structures on liquefied soil can buckle, break, and even sink. As you might guess, some soils are more susceptible than others. It takes less water to liquefy soil that’s already loose and sandy. And soils located at or below sea level are subjected to greater groundwater pressure during an earthquake.
If a structure is new construction, you should check liquefaction susceptibility before you build. However, if a structure already exists, there are measures you can take to reduce the damage caused by earthquake-related liquefaction. Structures can be retrofitted and reinforced to reduce the impact of violent shaking, and the soil under and around them can also be densified, solidified, reinforced, drained and/or dewatered.
All buildings in earthquake-prone areas can be strengthened through bracing, reinforcing masonry, sheer plating (such as adding plates of plywood to stud walls), and bolting walls to foundations. In the interior, it’s always a good idea to strap water heaters to the wall and secure heavy objects like bookshelves and mirrors to prevent them from falling when the building shakes.
Under the new RCW (Revised Code of Washington), HB 2322, signed by Gov. Jay Inslee on March 22, 2018, property insurance companies may be allowed to assist their clients with goods and/or services related to earthquake retrofitting in order to mitigate risk. The specific rules are under review by the Department of Insurance.
How do you determine your liquefaction risk? Soil liquefaction maps can help you to pinpoint it, especially when combined with MMI scores and fault line locations. These are all available as part of the hazards data in PropertyEDGE for address- specific risks. WSRB business analysts can also do a full book review for a number of perils to help the book perform better. If you are interest in a full book review, contact us.