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The seismic crown

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When 700,000 spectators line the streets of Pasadena, Calif., to watch the Rose Parade every January, they may notice which float has the best flowers or which marching band has the most astounding musical arrangements. But now, audiences can pay attention to one more contest: which group of ground-shaking musicians could best pass as an earthquake.

Seismic records show vibrations caused by motorcycles, floats, and bands during the 2020 Rose Parade. Courtesy of Seismological Research Letters.

Researchers at Caltech can actually make this determination through the Pasadena Array project, which transforms an old fiber optic telecommunications cable lying beneath the city into an earthquake detector. So, who took the seismic crown for the 2020 Rose Parade? That would be Southern University and A&M College in Baton Rouge, La.

Through the use of acoustic sensing, researchers turned a 2.5-km stretch of cable beneath the parade route into a dense seismic array. Two laser emitters shot beams of light through the cables, hitting tiny internal flaws that acted as seismic sensors. A portion of the light was reflected back to the source, where it was tracked and recorded. Researchers examined changes in the size, frequency, and phase of the reflected pulses to learn more about the resulting waves.

The Rose Parade is a particularly ideal setting for calibration, the researchers said, given its well-controlled unidirectional traffic at almost constant speeds. Disturbance to the cables was caused by the compression and flexure of the roads as parade participants marched over them.

The ultimate goal of the project, created in collaboration with the city of Pasadena, is to detect small earthquakes and image the geological structure underneath the city. So far, no major earthquakes have occurred for the cable to detect, so its data remains limited to marching bands and street traffic for now. It’s safe to say that the residents of Pasadena would like it to stay that way.

Photonics Spectra
Jul 2020
Lighter Side

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