Fiber Optic Seismic Array Tracked Pasadena’s Rose Parade

This article has been updated from its original 2020 publication. Read the full article at this link.

Pasadena City College Tournament of Roses Honor Band (Pasadena, CA) (Photo – Ezekiel Isbell)

In 2019, Caltech and the City of Pasadena unveiled a citywide fiber optic earthquake detector capable of mapping how temblors are shaking the city at millimeter-scale resolution.

By Robert Perkins, Caltech

While millions of spectators watched floats and bands parade down Colorado Boulevard, Caltech researchers watched the seismic signal of those floats and bands, captured in fine detail by the array.

The Pasadena Array takes advantage of two currently unused, or “dark,” strands of the City of Pasadena’s fiber optic cable network, which is typically buried just below ground and stretches in a large loop around the city. Two laser emitters shoot beams of light through the cables. The cables have tiny imperfections every few meters that reflect back a minuscule portion of the light to the source, where it is tracked and recorded. In this manner, each imperfection acts as a trackable waypoint along the fiber optic cable. Seismic waves moving through the ground cause the cable to expand and contract slightly, which changes the travel time of light to and from these waypoints. Thus, the imperfections act like individual “virtual” seismometers that allow seismologists to observe the motion of seismic waves. The technique is known as Distributed Acoustic Sensing, or DAS.

The Pasadena Array has more than 5,000 of these virtual sensors; about 400 were positioned along the Rose Parade route. Zhongwen Zhan, professor of geophysics and Director of Seismological Laboratory, found that heavy floats—those weighing about 16,000 to 18,000 kilograms—produced distinct long-period signals, while marching bands produced weaker long-period signals but strong signals in the 1.0–10 Hz frequency band.

This research was supported by a five-year agreement with the City of Pasadena to use the dark fiber; a National Science Foundation CAREER Award; and the Caltech-JPL President’s and Director’s Research and Development Fund (PDRDF).