This Caltech researcher turned to Pasadena’s broadband underground to detect earthquakes

Zhongwen Zhan, 36, can trace one watershed moment in his life to a literal earth-shaking event.

He was an undergrad student in China when a 9.1 magnitude earthquake ruptured the sea floor in the Indian Ocean, generating a massive tsunami that killed 230,000 people. It was Dec. 26, 2004.

“I knew I was interested in earth science, but when I saw how the entire community was mobilized to study that quake within a few days, how we were able to learn a lot, I was amazed by the scientists’ capability to gather so much data without actually going onsite, just by using sensors, to learn more and to help people,” Zhan said. “It was fascinating to see that.”

That fascination led Zhan to Caltech, where he earned his doctorate in geophysics in 2014, and where he now teaches geophysics.

This week, Zhan and a four-member team published a study in the journal Nature, reporting success in piggybacking off underground fiber-optic cables in Pasadena to detect temblors at a higher resolution, parsing intricate details of the earthquake.

Zhan and colleagues studied a section of the fiber-optic cable using DAS, or distributed acoustic sensing. The team used laser emitters to shoot light through the cables, which have tiny defects every few meters that reflect a part of the light to the source. This array acts as individual seismometers and gives earthquake scientists details on the motion of seismic waves.

It was a lucky catch that the team had their emitters in place when a magnitude-6 quake shook the Antelope Valley in 2021. They were able to examine the light signatures traveling through a 100-kilometer section of cable to precisely plot the quake, pinpointing a sequence of four smaller ruptures that could not be detected by a conventional seismic network.

The exciting possibility of repurposing the millions of miles of fiber-optic cables worldwide into a dense network of makeshift seismometers can deepen scientists’ understanding of earthquake physics and ultimately better earthquake early-warning systems.

“Using fiber optic cable as a series of seismometers reveals aspects of earthquake physics that have long been hypothesized but difficult to image,” Zhan said. “Better understanding can help a lot in our preparations for earthquakes, our knowledge about kinds of earthquakes and gives us a higher chance of capturing quakes.”

The Zhan Group spent about four years on this research. They include Jiaxuan Li, first author; Nadia Lapusta, professor of mechanical engineering and geophysics at Caltech; graduate student Teaho Kim and scientist Ettore Biondi. The National Science Foundation paid for the study.

The idea of using optical fibers to sense the environment came in the ‘70s and ‘80s, when fiber optics started, Zhan said.

“People quickly realized that environmental perturbations could cause problems in communication,” he added. “That also means sensing is possible. The new thing here is to use the technology to study earthquakes. Seismologists have always wanted better and more sensors to study earthquakes, and fiber sensing with telecom cable is just a perfect fit.”

Mike Gurnis, director of the Caltech Seismological Laboratory, called Zhan’s research a key pillar in the Seismo Lab’s priorities.

“We may be the only regional network trying to integrate that kind of a signal into our traditional dense broadband seismic network through different research projects with faculty,” Gurnis said in a campus article celebrating the lab’s centennial last November.

Zhan is mindful of the seismological footsteps he is following at Caltech, where modern earthquake seismology was born in the 1920s and ‘30s. That was the time Hugo Benioff invented instruments for detecting seismic waves, and Charles Richter and Ben Gutenberg collaborated on what became known as the Richter Scale for measuring an earthquake’s magnitude. Closer to today’s timeline, seismologist Lucy Jones continues to serve as a visiting associate in geophysics at Caltech.

Zhan said a collaborative “learn more, do better” attitude at Caltech can move mountains, or at least help scientists fine-tune their understanding of the Earth.

“It’s a privilege to be back here, knowing some of the most famous people worked here, but at the same time we’re always trying to redefine seismology, we keep looking for new technologies to make the network even better, from satellites to artificial intelligence,” Zhan said. “It’s a great time for earthquake studies because the data is absolutely exploding.”

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