Using dark optical cables on the seabed to build a global earthquake monitoring network

Celeste Labedz heard a loud bang, like rolling thunder coming from the ice. She was in Alaska's Taku Glacier, a snow-covered area surrounded by towering mountains when the quake struck.

The earthquake was triggered by the sudden movement of the glacier. She quickly wrote down the time in her notebook. Labez, a graduate student at Caltech, is laying a set of fiber-optic cables that could be used to study earthquakes—a promising new method that is profoundly changing geology and related fields.

When information is transmitted in the form of pulsed light in a fiber optic cable, most of the light travels smoothly along the fiber, which is thinner than a human hair. However, if there is a defect in the fiber, some of the light will be scattered back toward the light source.

Scattering also behaves differently when the cable is stretched or bent due to earthquakes, vibrations caused by passing trucks, etc. Therefore, scientists can quantify the intensity of the vibrations by detecting changes in the intensity of the scattered light. The technology, called distributed acoustic sensing (DAS), was pioneered by the oil industry more than a decade ago.

At present, this technology is also beginning to be used in academic research. "DAS technology has grown in popularity over the past few years," says Jonathan Ajo-Franklin, a geophysicist at Lawrence Berkeley National Laboratory.

In December 2019, many scientists who had used the technology attended a workshop hosted by the American Geophysical Union, where they used it to map glaciers, detect thunderstorms, and study the deep ocean.

The first advantage of DAS technology is that the fiber optic cables laid with this technology can be up to several kilometers long, and each fiber optic cable is equivalent to a network of thousands of sensors, which can record data within a few meters of the surrounding area. In contrast, conventional seismometers can only record surface movement as a single point (one of the main problems when mapping the Earth's interior). In 1980, for example, Mount St. Helens kept roaring before it erupted violently. With only one seismometer nearby, scientists couldn't even tell if the shaking was caused by a waking volcano.

"It's like street lights on the street," said Nathaniel Lindsey, a graduate student in Earth and Planetary Sciences at Lawrence Berkeley National Laboratory. "If you don't have enough lights, you can't illuminate the entire volcano."

The second advantage of this technology is that it has spread all over the world. Although new cables need to be laid in areas such as the Taku Glacier, most areas such as cities and the seabed have already been laid. Some optical cables are not yet in use, and some can be used after modification.

All thanks to the boom of the Internet in the 1990s. At that time, communication companies laid a large number of optical cables, and the part that was not used was called dark optical cable. So scientists only need to connect an "interrogator" (interrogator, which sends a laser beam to the other end of the cable and detects the change in light intensity after scattering) at one end of these cables, and a new seismic wave detection network is set up. .

Tieyuan Zhu, a geophysicist at Penn State University, modified the school's existing fiber-optic network last year to try to measure the faint vibrations beneath the campus grounds. On a night of thunderstorms, he was pleasantly surprised to find multiple fluctuations in the data. Although scientists have known for a long time that the vibrations of gas molecules can also cause the ground to vibrate when there is a loud bang in the air, no one knew whether the new technology would be able to detect such "thundershocks".

When Zhu Tieyuan synchronized his monitoring results with NASA's data, they got a very clear answer, "Lei Zhen" can indeed be monitored. Zhu Tieyuan said: "I think this technology has great potential to give cities all-round early warning. It can monitor not only earthquakes, but also geological hazards such as landslides, tsunamis, and weather changes."

There are also scientists testing the system in more remote locations. In November 2019, Lindsay published a paper in the journal Science as the first author. The researchers attached an interrogator to a 20-kilometer fiber-optic cable. The fiber optic cable connects scientific instruments on the seabed off Monterey Bay and was originally used to transmit instrument data. The facilities were under maintenance at the time, so scientists just had the opportunity to use the cables to detect vibrations along the way.

In just four days, they mapped multiple underwater fault zones and detected seabed shaking caused by waves. More detailed mapping of the seabed could help scientists better predict earthquakes and undersea volcanoes -- phenomena that have the potential to trigger deadly tsunamis.

At Taku Glacier, Labez and his colleagues retrofitted 3,000 seismic sensors with a single fiber optic cable. Early results show that the system, operating continuously for five hours, detected 100 ice shocks, most of which were likely caused by meltwater swelling through cracks in the glacier.

Zhongwen Zhan is Labez's mentor and a seismologist at Caltech. He hopes to one day lay permanent fiber-optic cables in Greenland or Antarctica to help researchers gather information to better understand the impact of climate change-induced glacial melt on sea-level rise.

Not only that, Zhan Wenwen also wants to use nearly 1,000 kilometers of dark optical cables to build a monitoring network equivalent to millions of sensors in California. In Pasadena, he has converted 37 kilometers of dark fiber optic cables into a permanent seismic monitoring network. After that, he plans to do the same in other California cities.

The data collected by the network can reflect the strength of the city's infrastructure and alert citizens immediately when an earthquake begins. Scientists can't yet predict earthquakes, but it would be valuable to gain a better understanding of the precursor earthquakes that could trigger major quakes.

Robert Mellors, a seismologist at Lawrence Livermore National Laboratory, who was not involved in the study, said: "Any data that helps to understand exactly how earthquakes start and form is likely to be Completely change the status quo of earthquake prediction."

It's worth noting that this method collects a huge amount of data. A single fiber optic cable can generate 10 terabytes of data in a day, which means it only takes 100 days to grow to 1 petabyte. However, the International Seismic Database, which collects data on all earthquakes in the world, is also less than 1 petabyte in capacity.

Before scientists can deploy fiber-optic cables to more distant regions, they may have to find a suitable solution to store and share such vast amounts of data.