Researchers turned the fiber cables at the bottom of the Saratoga Passage, between Whidbey and Camano, into a science lab to compare different earthquake measurement methods.

For most of seismology’s history, earthquakes have been measured from land. Although about 70% of the Earth’s surface is covered by water, there are only a few ocean-bottom seismic stations around the globe, according to a research article featured in Science.

Because placing seismometers underwater is difficult, scientists have been trying to come up with practical and cost-effective ways to better measure submarine earthquakes. One of them is known as “ultrastable laser interferometry.”

This technique is explained in the scientific article “Ultrastable laser interferometry for earthquake detection with terrestrial and submarine cables,” which was published in 2018. It consists of shooting a laser beam into one end of the optical fiber and checking the light that comes out of the other end. When an earthquake shakes the cable, the laser traveling through gets distorted.

When comparing the original laser signal with the light that exits the fiber optic cable, researchers can determine the strength of the seismic wave by checking how much the beam was distorted. Data from multiple cables can also be combined to find the hypocenter.

Upon reading the paper, Dale Winebrenner, a University of Washington professor that specializes in applied physics, was inspired to test the new method himself.

“These ordinary fibers that are used for practical things, like phone calls and television, also have value as parts of a scientific observatory that captures seismic events worldwide,” he said.

He didn’t have to look far from Seattle to find the ideal setting. Whidbey Telecom, the telecommunications company headquartered on South Whidbey, happened to own a 4.25-mile optical fiber cable connecting Hidden Beach, in Greenbank, to the Saratoga Shores on Camano Island.

When Winebrenner approached Whidbey Telecom with his proposal in 2018, he recalled, the company was happy to support scientific research and allowed his team to set up their equipment in an enclosure in Greenbank.

The study began in 2019 and was funded by the National Science Foundation and the Defense Advanced Research Projects Agency.

To assess the efficacy of ultrastable laser interferometry and observe areas in need of improvement, Winebrenner and his team of researchers compared it to another method, known as “distributed acoustic sensing.”

According to Winebrenner, the findings show that distributed acoustic sensing provides data that shows where the disturbance occurs along the cable, but it can only be used when the disturbance is no farther than 62 miles from where the equipment is placed on land.

On the other hand, he said, ultrastable laser interferometry can detect disturbances on cables that stretch for thousands of miles, meaning this method could be implemented using cables that cross entire oceans. However, it is more difficult to pinpoint the location of the disturbance, a flaw scientists are trying to address, he said.

Weather conditions can also pose a challenge. For example, a big windstorm that hit the islands a few years back caused the laser to act “strangely” and needed a hands-on fix, Winebrenner recalled.

While analyzing the data, the research team also learned a few things about the topography of the Saratoga Passage.

Chih-Chieh Chien, a Ph.D. student at the Scripps Institution of Oceanography in San Diego who participated in the research, said he was surprised to learn that some of the signals detected did not originate from seismic activity, but from the tides and currents. This is likely because the cable, which lays at the bottom of the approximately 295-foot-deep channel, is in some points suspended over steep cliffs, which allows for the water to rush around it, he explained.

Chien, who joined Winebrenner in 2022, is also the lead author of the paper detailing the research process and findings. The paper, which was published last month, is titled “Calibrating Strain Measurements: A Comparative Study of DAS, Strainmeter, and Seismic Data” and is available for free online.

Winebrenner believes this study is an important piece to add to a large puzzle that scientists are still working to figure out.

“I would say, generally, this whole area of research is vibrant and changing rapidly,” he said.

Researchers also compared the data collected with the two underwater methods with land-based seismometer data.

Though on-land seismometers are more sensitive and can measure more types of ground motion than methods using optical fibers are currently able to, Winebrenner explained, underwater cables are often located closer to the submarine earthquake’s epicenter, therefore they can provide stronger signals and even detect quakes that on-land seismometers could be missing.

By improving ultrastable laser interferometry, Winebrenner and Chien believe seismologists can get a better picture of many processes within the earth that lead to seismic hazards and lead to better disaster preparedness.