Seismologists from the California Institute of Technology (Caltech) are using fiber optic network cable to monitor and record aftershocks from the July 4 and 5 Ridgecrest, CA, earthquakes. By using optical fiber, the scientists can gather, track, and analyze data in much greater depth from the thousands of daily aftershocks.
To do this, the scientists send a beam of light down optical fiber in an unused or “dark” fiber optic cable. When the light reaches tiny blemishes in the optical fiber, a small portion of the light is reflected back and recorded. In this way, each fiber imperfection acts as a trackable location along the buried fiber optic cable. When seismic waves move through the ground, the cable expands and contracts slightly. This change affects the travel time of light to and from the locations. By monitoring these changes, seismologists can monitor the motion of seismic waves.
According to Caltech, the miniscule fiber imperfections occur often enough so that every few meters of optical fiber act as an individual seismometer. In fact, monitoring 50 kilometers of fiber optic cable in three different locations is roughly equal to deploying more than 6,000 seismometers in the area.
Caltech launched the project within days of the two large earthquakes and began contacting groups in a search for unused fiber optic cable that would be close enough and long enough to be useful. The scientists finally contacted the California Broadband Cooperative’s Digital 395 project. The goal of the Digital 395 project is to build a new 583-mile fiber optic network that will run north to south, along the eastern Sierra Nevada, passing near Ridgecrest. Digital 395 offered three segments of its fiber cable to which Caltech connected sensing instruments.
Information gathered from the Ridgecrest fiber network will help seismologists learn more about the way that earthquakes move through the earth, and specifically how seismic waves move through the area around Ridgecrest.