Long-term seismic background level variations are consistent with volcanic activity at Kusatsu-Shirane volcano, Japan
This study demonstrates that analyzing long-term seismic background levels over 20 years at Kusatsu-Shirane volcano effectively reveals volcanic activity, including fluid supply and unrest episodes, even when traditional seismic or deformation signals are weak or absent.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Volcanoes are not merely mountains that erupt; they are complex, breathing systems where heat, gas, and water interact deep underground long before the surface shows any sign of trouble. For scientists trying to predict when a volcano might wake up, the challenge is often finding a signal in the noise. The ground is never truly still; it hums with the constant, low-level vibrations of wind, ocean waves, and human activity. This background hum usually masks the faint, subtle tremors that might indicate a volcano is preparing to erupt. To see the future of a volcano, researchers must learn to listen to these quiet whispers hidden within the roar of the earth.
At Kusatsu-Shirane, an active volcano in central Japan, a team of researchers has spent two decades listening to these whispers. This volcano is known for sudden, steam-driven explosions called phreatic eruptions, which happen when groundwater flashes into steam upon contact with hot rock. These events can occur with little warning, making the study of its quiet periods crucial. The researchers focused on a specific measurement called the seismic background level. Think of this as the baseline volume of the earth's hum. By statistically analyzing over twenty years of continuous recordings from sensors buried near the volcano's main crater, they were able to filter out the random noise and isolate the steady, underlying vibration of the ground itself. They found that this background hum does not stay constant; it rises and falls in ways that tell a story about what is happening deep beneath the crater.
The study revealed that this background vibration is a sensitive barometer for volcanic activity. During two major periods of unrest in 2014 and 2018, the background hum grew significantly louder. However, the timing of this increase offered a surprising twist. In 2014, the hum began to rise only after the volcano had already started shaking with more frequent earthquakes. The vibration seemed to be a reaction to the activity that had already begun. In contrast, during the 2018 unrest, the background hum started to increase months before the earthquakes became frequent. This early warning suggests that the volcano's internal system was changing in a way that the standard earthquake monitors had not yet detected. The researchers also noticed that the "sound" of the vibration changed depending on which sensors were listening. In 2014, the vibrations seemed to come from one location, while in 2018, the source appeared to have shifted to a different spot closer to the crater. This difference implies that the volcano does not behave the same way every time it wakes up; the path the heat and gas take to the surface can vary.
Even when the volcano was not in a state of high alert, the background hum was still telling a story. The researchers found that the vibration levels rose and fell in sync with small, visible changes on the surface, such as the water in the crater lake changing color or new steam vents appearing. These subtle shifts, which might be missed by other monitoring methods, were clearly reflected in the seismic data. The team concluded that these vibrations are likely caused by the movement of volcanic fluids—hot water and gas—interacting with the shallow groundwater system beneath the crater. As these fluids move and bubble, they generate a continuous, low-level tremor that the sensors can pick up.
The significance of this work lies in its ability to see the invisible. By tracking the background hum, scientists can detect the presence of volcanic fluids even when there are no strong earthquakes or ground deformation to signal danger. The study suggests that this method provides a new, reliable way to monitor the health of a volcano over long periods. It shows that the earth's quiet hum is not just noise to be ignored, but a vital signal that can reveal the hidden movements of magma and gas long before a disaster strikes. For the people living near Kusatsu-Shirane, and for volcanologists everywhere, learning to read this subtle language of the ground could mean the difference between a surprise eruption and a timely warning.
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