In August 2014, after traveling for weeks through subterranean channels, molten rock surged through cracks in the 84-square-kilometer Holuhraun lava field north of Iceland’s most voluminous glacier. It was the largest volcanic eruption in Europe in 230 years and was recorded in detail by a dense network of seismic sensors deployed by the University of Cambridge to monitor volcanic activity in the area. Now, a new study shows that scientists can accurately track underground lava flow with far fewer sensors, a boon in regions with scarce seismic data.
In the 2-week period leading up to the eruption, magma snaked its way through fissures in the rock, forming a moving structure called a dike. From the tip of one dike rippled migrating swarms of more than 30,000 earthquakes. The dense network of more than 70 traditional seismic monitoring stations recorded the earthquakes, providing a convenient baseline for testing alternative measurement systems.
Traditional earthquake sensors rely on two types of seismic waves, called P waves and S waves, to determine where an earthquake occurred. The farther away the earthquake is, the greater the distance is between the arrival of these two wave types, which travel at different speeds. Here Caudron et al. used a novel method called seismic amplitude ratio analysis (SARA) to track the melting rock’s underground path.
In contrast to traditional methods, SARA requires only the background amplitude of seismic waves to locate where an earthquake originates. Although previous studies have shown that the method can complement the traditional approach and reveal the path of traveling magma, it has never been directly compared with such a high-resolution set of traditional sensors. Using just 15 stations, the team was able to track the magma’s trajectory with remarkable accuracy, they report. The SARA technique also revealed new dynamics in the magma’s flow. The technique could help scientists track volcanic activity in settings where there aren’t many monitoring stations, the team says. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1002/2017JB014660, 2018)
—Emily Underwood, Freelance Writer