Caltech and the City of Pasadena are teaming up to create a citywide fiber optic earthquake detector capable of mapping how temblors are shaking the city at millimeter-scale resolution.
The work will take advantage of two currently unused—or "dark"—strands of Pasadena's fiber optic cable that stretch in a large loop around the city. Using a couple strands of fiber to measure seismic activity will gather data equivalent to more than 30,000 seismometers, while only 11 traditional seismometers exist within the city limits today. Zhongwen Zhan, assistant professor of geophysics, will tap into the fiber network at the Seeley G. Mudd Building of Geophysics and Planetary Science on Caltech's campus on California Boulevard.
There, Zhan will station two laser emitters that 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, which is typically buried just below ground level. 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 seismometers that allow seismologists to observe the motion of seismic waves.
"Engineers try to minimize the imperfections in the cable because it adds noise when transmitting information from one point to another. For us, however, the imperfections are the point. They turn the cable into a big chain of virtual seismometers," Zhan says.
The laser light launched into the fiber is emitted by distributed acoustic sensing (DAS) interrogators, devices designed for use in oil exploration. One, designed in the lab of Miguel Gonzalez-Herraez of the University of Alcalá in Spain and constructed by manufacturer Aragon Photonics, will track a 37-kilometer section of cable clockwise from Caltech's campus. The other, borrowed from manufacturer OptaSense, was deployed to monitor a section of fiber along the 395 Freeway, tracking aftershocks from this summer's Ridgecrest earthquake sequence. It recently returned to Pasadena to take a more detailed look at a 10-kilometer section along the same cable path, but counter-clock-wise from the Caltech campus.
The unbroken loop of cable allows for light to be shot in both directions through the cable, generating a clearer signal. Ultimately, the DAS devices should capture 20 terabytes of data every month. Because of the high resolution of data that the fiber can capture, the network could one day provide city officials real-time information during an earthquake about how severe the shaking is throughout the city on a block-by-block basis.
The city maintains a network of fiber optic cable running beneath Pasadena for municipal operations and commercial services, not all of which is currently in use. City officials granted access to that dark fiber to Zhan under a five-year agreement.
"The City of Pasadena's fiber optics paired with Caltech's research will produce a tremendous amount of data that will help our efforts to prepare, educate, and communicate the impacts of earthquakes in our community," says Phillip Leclair, chief information officer for the City of Pasadena. "Measuring seismic activity with fiber will give officials impact and damage predictions by neighborhood—a huge benefit for public safety and disaster recovery."
Zhan hopes that this project can serve as a model for other cities and seismologists, with municipalities throughout Southern California taking advantage of their fiber optic networks for seismic monitoring.
"These fiber optic networks already exist in many municipalities, creating the opportunity for this project to expand throughout the region and perhaps even beyond," Zhan says.
"The Pasadena project is an important step forward in lighting up dark fiber throughout Southern California and achieving our vision of a seismic monitoring system equivalent to having a million seismometers placed throughout the region," says Mike Gurnis, director of Caltech's Seismological Laboratory and John E. and Hazel S. Smits Professor of Geophysics. "This will be a leap forward in our ability to monitor the subsurface in much greater detail. We are particularly grateful for the support shown by the City of Pasadena. This advancement would never have happened without them."
The work is made possible by a National Science Foundation CAREER Award, funding from Caltech trustee Li Lu, and a partnership with the City of Pasadena, and Caltech’s Division of Geological and Planetary Sciences.