The technology used to measure and monitor earthquakes has remained largely unchanged over the past century, but a breakthrough from Lawrence Livermore National Laboratory utilizing telecommunication lines as earthquake sensors has given researchers a magnified window into the Earth’s crust.
The instrument that makes it possible, known as an interrogator, enables seismic researchers to effectively hijack fiber optic cables for use as thousands of seismometers, providing a reading of quakes that is 1,000 times more detailed than current seismometers in place across the Bay Area — and which may help prepare for future earthquakes and identify hidden faultlines.
“Traditionally, seismologists put sensors in the ground. You have to dig a hole or build a structure for it to couple it with the earth,” Lawrence Livermore National Laboratory researcher Gene Ichinose said. “(But) you have limited resources, so you can’t buy too many of them, and it’s hard to get access into populated areas.”
Originally used by the oil and gas industry for drilling and fracking operations, interrogators send pulses of light along fiber optic cables to record small vibrations along the fiber using a method called distributed acoustic sensing. In February, Ichinose and his team plugged the interrogator – an instrument about the size of a microwave – into an 80-mile network of fiber optic cables between downtown San Francisco and Sunnyvale for a month-long experiment. The corridor contains eight seismometers, but with the interrogator in place, each 10-meter segment of fiber optic cable captured seismic waves – the equivalent of 8,000 seismometers, Ichinose said.
Researchers who were initially skeptical of using an interrogator for earthquakes were quickly proven wrong, Ichinose said. The technology was so sensitive that researchers immediately captured the subtlety of train cars rattling on rail lines after connecting it to the cable network.
“We saw Caltrain traveling along, and then within about a day, we had a small earthquake near Gilroy,” Ichinose said. “It was beautiful. The quality was amazing.”
Earthquakes happen along several fault lines in the Bay Area. The most infamous of these, the San Andreas fault line, runs through the San Francisco peninsula and was responsible for an estimated 7.7 magnitude earthquake that leveled San Francisco in 1906. The San Andreas fault line struck again during the 6.9 magnitude Loma Prieta earthquake in 1989, killing 63 people and causing an estimated $6 billion in damage. In the next 30 years, there is a 72% probability of a 6.7 magnitude earthquake striking the Bay Area, according to U.S. Geological Survey official Sarah Minson.
A 3.9 magnitude earthquake strikes in the East Bay hills near Dublin. The red line is the approximate path of the fiber used in the distributed acoustic sensing experiment performed by the Lawrence Livermore National Laboratory.On May 17, Ichinose and LNLL struck “seismological paydirt,” according to an LLNL press release. Ichinose was at his home in Richmond when he began receiving alerts from colleagues about a 3.9 magnitude earthquake centered in western Dublin. He logged into the interrogator and watched as the data showed the quake roll with “unprecedented” detail across the Bay Area.
Amy Williamson, a research scientist focusing on early detection at the UC Berkeley Seismology Lab, said the breakthrough could have broad applications for areas with seismic activity. Because the technology can be easily deployed, she suggested interrogators could be employed off California’s coast, where there are relatively few seismometers to monitor the seismically active areas between the North American tectonic plate and the Pacific Plate.
“It can be used for landslide monitoring, volcanic monitoring, and that data is available quickly, so having enough cables in the right spot provides that timeliness,” Williamson said.
The interrogator’s level of detail could also help seismologists identify smaller, “hidden” fault lines, Ichinose said. These likely exist under cities like Oakland and San Jose, where it has been difficult to place seismometers because of existing developments and the prohibitive cost of installing equipment. He said DAS technology may even be used to identify fractures in buildings and bridges that might be at risk during a large quake.
Still, earthquakes remain one of the most unpredictable natural disasters. There are few indicators of an upcoming quake, Ichinose said, and even those are not reliable based on shifting volcanic gases inside the Earth’s interior. The only way to know of an upcoming earthquake is to increase the detection of current seismic activity for “foreshocks,” small shifts along fault lines, that increase the probability of a larger earthquake.
“Having a data set that’s close to the source of the earthquake can really help with some of those very timely characterizations of the earthquake for uses like early warning,” Williamson said. “That would be a huge supplement to our onshore network.”
The highly sensitive, high-resolution technology of the interrogator may prove to be vital as seismologists seek hidden faults through urban cores, identify unknown fractures in urban cores and keep tallies of foreshocks ahead of the next big one.
“There’s a lot of excitement,” Ichinose said about prospects for the novel breakthrough. “Not everybody has heard of it yet. But for those of us like myself, we’re really excited, because I think this is going to revolutionize how we record seismic data.”
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