Researchers detected low-frequency volcanic tremor using a fiber optic cable on the ice cap of an Icelandic subglacial volcano, implying that this technology could be useful in monitoring other ice-covered volcano systems.
Their findings, published in The Seismic Record, show that the floating ice cap, which is part of the Vatnajökull glacier, acted as a natural amplifier of the tremor signals generated by Iceland’s most dangerous volcano, Grmsvötn.
According to Andreas Fichtner, an ETH Zürich professor of seismology and wave physics, this appears to be the first observation of a floating ice sheet acting as an amplifier of tremor. “Oscillations of ice shelves in Antarctica or Greenland have been known for a long time, but they are mostly excited by ocean waves,” he explained.
Although the exact mechanisms underlying volcanic tremors very, Fichter believes it can be an indicator of deep volcanic or geothermal activity. “Tremor, in addition to providing information about the underlying processes, may also serve as a precursor of volcanic eruptions, which should be closely monitored.”
Grmsvötn is one of Iceland’s largest and most active volcanoes, with major eruptions occurring every ten years on average. Geothermal heating melts the ice cap, forming a subglacial lake on the volcano that occasionally erupts, flooding the coastal plains. Its explosive eruptions produce massive ash plumes that have an impact on agriculture, human health, and aviation. The last major eruption in May 2011 caused the closure of Iceland’s main airport and the cancellation of 900 flights.
Researchers want to learn more about the seismic environment of Grmsvötn, but installing a traditional seismic network in the remote and harsh conditions of the subglacial volcano is expensive and difficult. Fichtner and colleagues instead used Distributed Acoustic Sensing.
DAS, or Distributed Acoustic Sensing, employs thousands of seismic sensors in the form of tiny internal flaws in a long optical fiber. An interrogator at one end of the fiber sends laser pulses down the cable, which are reflected off the flaws in the fiber and bounced back to the instrument. When seismic activity disrupts the fiber, researchers can examine changes in the timing of the reflected pulses to learn more about the resulting seismic waves.
In May 2021, the researchers installed a 12.5-kilometer-long fiber-optic cable on Grmsvötn and collected data from the DAS system for three weeks.
“We wanted to know if a large DAS experiment in such a difficult and remote environment would be feasible at all, and if it might teach us something new,” Fichtner explained. “We now know, after thoroughly analyzing the data, that the discoveries we made would not have been possible with conventional stations. This includes not only the tremor-related ice sheet oscillations, but also the nearly 3000 local earthquakes detected during the experiment’s three weeks.”
After analyzing the densely sampled DAS data, the researchers discovered that the floating ice sheet was acting as a natural resonator of seismic signals, allowing them to detect volcanic tremors that would otherwise have been overwhelmed by other ambient or instrument “noise” in a traditional seismic network.
During the fiber optic deployment, the research team was fortunate to have unusually nice weather, as well as research huts equipped with a geothermally heated sauna on the highest point of the Grimsvötn caldera. A trenching sled developed by Icelandic Meteorological Office researchers, which ploughed and placed the cable at the same time, also aided.
“The real challenge was field splicing,” Fichtner recalled. “We needed to splice the fibers because we had three cable drums with four kilometres of cable on each of them. Because optical fiber is thinner than a human hair, it is difficult to handle on a glacier.”