EVERETT — Two reservoir sites — one on Rucker Hill and another off of Evergreen Way — provide water to over 640,000 people.

Before 2021, a major earthquake would have likely caused those reservoirs to fail. Studies show the Rucker Hill reservoir would most likely “crush down” into Pigeon Creek, said Souheil Nasr, utilities engineering manager for Everett Public Works. The Evergreen Way reservoir would also likely collapse.

Now, the city is spending over $23 million to replace both reservoirs, so the structures are more seismically sound in the event of a catastrophic earthquake along the Cascadia Subduction Zone off the coast of Western Washington, or on the southern Whidbey Island fault running through Snohomish County.

Both reservoirs are over 100 years old, Nasr said.

“Usually those reservoirs are designed for 75 to 100 years,” he said.

In place of the original Rucker Hill reservoir near Edwards Avenue, construction workers built two new reservoirs, with 2½ million gallons of capacity each, on a more secure area right next to the original reservoir. The new Rucker Hill reservoirs have operated since April. The city is scheduled to finish construction by the end of this month.

Last month, the Everett City Council approved a change order for the Rucker Hill project, allowing workers to finish reinforcing the bottom of the reservoir and relocating an underground power line on the site, Nasr said. The charge order also increased the cost of the project by roughly $2 million.

Having two small reservoirs instead of one will make maintenance easier for city workers, Nasr said. One reservoir can be taken offline and cleaned while the other stays in operation.

There is still a chance the lines feeding water into the reservoirs might fail in the event of a major earthquake, but workers installed an earthquake valve onto one of the reservoirs during the replacement process to help, Nasr said.

“If there’s an earthquake, that valve will shut off,” he said, “and we’ll preserve water in the reservoir for people to be able to access temporarily until we can figure out what to do and how to fix the system.”

Nasr said the new Evergreen Way reservoirs will have the same feature. Originally, the city planned to replace the old Evergreen Way reservoir — with a capacity of 20 million gallons — by 2030. But one corner of its concrete cover was deteriorating, so Nasr said the city changed the timeline, with the hope of completing construction in 2026.

The city is currently in the design phase of the Evergreen Way project, with plans to start building in 2024.

Unlike the Rucker Hill site, the city can’t do a complete replacement of the Evergreen Way reservoir because there isn’t enough space, Nasr said.

Officials plan to divide construction for the Evergreen Way reservoirs into two phases, since they can’t take the existing reservoir offline. In phase one, workers will build a new small reservoir that can hold 8 million gallons of water, while the larger, older reservoir remains in service. In the second phase, workers will demolish the older reservoir and replace it with one that can hold at least 12 million gallons of water.

Scientists don’t know when the next earthquake will occur along the southern Whidbey Island fault. They estimate the last one happened about 2,700 years ago.

Everett’s reservoirs provide water to residents in Mukilteo, Snohomish, Marysville and on the Tulalip Reservation. With nearly three-quarters of Snohomish County depending on Everett’s water supply systems, the reservoirs have to be replaced, Nasr said.

“We are hardening our water system to withstand an earthquake of sizable magnitude,” he said. “And hopefully it will be good for the next 100 years.”

Ta’Leah Van Sistine: 425-339-3460; taleah.vansistine@heraldnet.com; Twitter: @TaLeahRoseV.