The 101-year-old Devil’s Gate Dam is the subject of sundry studies these days, with Cal Tech, the Jet Propulsion Laboratory and the County of Los Angeles preparing to turn the structure into an earthquake detector. 

While the project is still in the early stages, there is support for the concept from the County Board of Supervisors, who, with a gentle push from construction groups behind the Rebuild SoCal Partnership (RSCP), is directing the L. A. County Flood Control District to get with the program.

It’s only fitting. The Devil’s Gate Dam was the first of the 14 dams erected by the district in its 106-year history. They haven’t built a new one since 1939, and that’s the rub. Old dams are dangerous, especially when located in earthquake country.

Strike Up the Band

This story starts with a discovery by the Cal Tech team that we have reported on before—it turns out that fiber optic cables can pick up the vibrations caused by ground movement—useful for people worried about subsidence and detecting the odd earthquake. 

As we reported last August, Pasadena contracted to build a network of buried fiber optic cables around the city to take advantage of the flaws in the product that allows for the measurement of temblors. As with all of man’s creations, there are imperfections in the glass fiber cable. Using a laser as a light source, a bit of the light bounces back when it hit an “imperfection.” The time it takes from the initial pulse to the time it returns is measurable in nanoseconds

The geniuses at Caltech, who designed the Pasadena system, spent their 2020 New Year’s holiday shooting lasers through two unused strands of the system in the Rose Parade route area. Seismic waves moving through the ground from the floats and bands caused the cable to expand and contract minutely, which changes the travel time of light to and from these waypoints creating “virtual” seismometers. It works the same way for the traffic monitoring system envisioned by Caltrans. 

Now the search is on beyond traffic monitoring for other applications, giving birth to the notion of using Devil’s Gate Dam as a seismometer. This project is in the planning stages and probably won’t see the bid/award stage until next year at the earliest. In addition to Cal Tech, JPL will be providing services and material support—it’s right in line for a flood if a quake strikes the Arroyo Seco gorge.

By Dave Sorem, P.E. ECA Government Affairs Chairman email: [email protected]