Seattle Public Utilities

Seismic Planning

  • 30-inch water maiNew earthquake-resistant 30-inch watermain on S Spokane St.
  • Cylinder on the base of Myrtle Tank Cylinder on the base of Myrtle Tank was installed to reduce earthquake vibrations.

What & Why

Over the past several decades, Seattle Public Utilities (SPU) has invested hundreds of millions of dollars in water system infrastructure improvements. These improvements include:

  • Federal- and state-mandated water projects
  • Rehabilitation and replacement of aging infrastructure
  • Seismic upgrades to selected facilities

All new projects meet or exceed current seismic standards. To date, SPU has invested $60 million in seismic upgrades to existing water infrastructure. Completed and future seismic upgrades will provide more protection of the water system during an earthquake and decrease system disruptions after an earthquake.

Determining future seismic improvements

Scientific knowledge about the impact of earthquakes on water systems has increased dramatically over the last 20 years. Scientists also have a better understanding about the frequency and magnitude of earthquake activity in the Puget Sound region. In particular, more information about the Seattle Fault and the Cascadia Subduction Zone is known.

In a study scheduled to finish in 2017, we are evaluating how our water system might respond to two earthquake scenarios:

  • A magnitude 9.0 Cascadia subduction earthquake off the Washington coast
  • A magnitude 7.0 Seattle Fault earthquake centered in Seattle

Retrofitting all existing facilities to meet current seismic codes would be too expensive. Therefore, SPU is concentrating on the most critical facilities that affect water system performance and post-earthquake recovery.

By upgrading the most critical facilities and focusing on emergency preparedness and response planning efforts, SPU will be able to significantly increase the seismic resiliency of the water system over the next several decades. Also, as aging water system facilities and pipelines are replaced by infrastructure that meets modern seismic codes, SPU will be able to further minimize water system disruption after an earthquake.

Soon SPU will be asking residents that represent Seattle’s demographics to provide their opinions about different seismic resiliency levels and cost. The goal is to learn what people are prepared to spend on seismic upgrades.

Once SPU determines the desired balance between seismic resiliency and system improvement costs, SPU will develop a detailed seismic improvement plan and program. One set of upgrades and improvements will likely be developed with the goal of increasing seismic resiliency over a shorter period, such as the next 20 years. Further minimizing earthquake effects on the water system will likely occur over a longer period, such as the next 50 to 100 years. These improvements are expected to cost hundreds of millions of dollars.

Regional water system resiliency

SPU has also been a regional leader in helping to develop water system resiliency. SPU is a charter member of The Water Supply Forum, which recently completed a regional earthquake vulnerability assessment (pdf). This assessment will help water utilities in King, Pierce and Snohomish counties:

  • Evaluate and increase our region's existing water supply systems resiliency
  • Plan for potential water supply disruptions

Additionally, SPU is working with other water utilities around the country through the Water Research Foundation and the American Society of Civil Engineers to share water system seismic resiliency improvement methodologies and approaches, and to develop seismic standards for new water mains.