[2] After the Loma Prieta earthquake in 1989, EBMUD began studying the potential effect of a large seismic event along the Hayward Fault.

In the worst-case scenario, slip along the fault would sever the tunnel, disrupting the water supply to approximately 50 to 70% (800,000) of EBMUD's customers for up to six months.

[2] The original tunnel connects the EBMUD treatment plant in Orinda with the distribution system west of the Oakland Hills.

[6] The maximum credible earthquake (magnitude 7.0) predicted in studies would cause an offset of 7.5 feet (2.3 m).

[3][8] For the seismic upgrade, a reinforced bypass tunnel was constructed across the Hayward Fault starting in 2004.

[2] The seismic upgrade also injected grout to improve contact between the 1929 concrete tunnel and the surrounding earth.

The reinforced section is fitted with a steel carrier pipe 3 inches (76 mm) thick with a 6 feet (1.8 m) inner diameter, capable of delivering 130,000,000 US gallons per day (340,000 L/min).

[7][10] The bypass tunnel was designed by Jacobs Associates for EBMUD, and cost $38 million.