Winter-to-spring changes in borehole seismic noise beneath permafrost in the northeastern Tibetan Plateau
This study analyzes borehole seismic noise beneath permafrost in the northeastern Tibetan Plateau during the winter-to-spring transition, revealing significant frequency-selective increases in noise power and spectral entropy driven by seasonal changes rather than thawing, which underscores the need to evaluate both absolute power and spectral distribution when characterizing station recording backgrounds.
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
Deep beneath the frozen ground of the Tibetan Plateau, where the earth remains locked in ice for most of the year, scientists have been listening to the planet's constant, low-level hum. This hum, known as ambient seismic noise, is not silence but a continuous vibration caused by wind, ocean waves, and human activity. In cold regions, the ground itself changes as it freezes and thaws, altering how these vibrations travel. By placing sensitive instruments deep underground, researchers can track these subtle shifts, much like a doctor listening to a heartbeat to understand a patient's health. The goal is to understand how the environment shapes the background noise that these instruments record, ensuring that when a real earthquake occurs, the signal is clear and the background static is understood.
In a specific mining area in the northeastern part of the plateau, a team of researchers installed a highly sensitive seismometer nearly 300 meters down a borehole, placing it well below the local layer of permafrost. They set out to watch how the noise at this depth changed as the region moved from the dead of winter into the early days of spring. They analyzed continuous recordings from late January through mid-April 2019, looking for patterns in the strength and the "shape" of the vibrations. The team used a method that breaks the noise down into different frequencies, or pitches, to see if certain tones became louder or if the overall mix of sounds changed as the seasons turned.
What they found was a distinct and selective shift in the earth's background noise. As the season progressed, the vibrations at a specific higher pitch, around 3.4 hertz, grew significantly louder, increasing by as much as 21 decibels compared to the quiet of late January. However, this increase did not happen everywhere in the spectrum. The vibrations at a much lower pitch, around 0.2 hertz, remained relatively steady, changing by only a tiny amount. This told the researchers that the change was not a general increase in all noise, but a specific amplification of certain frequencies. Furthermore, the "shape" of the noise changed. In the early weeks, the energy was concentrated at the very low end of the range they were listening to. By spring, that energy had spread out, forming a broader hump of activity centered around 2 to 3 hertz.
To understand if this change was simply a matter of the ground getting louder overall, the researchers also measured the "entropy" of the noise, a way of describing how spread out or concentrated the energy is across different frequencies. They found that the noise became more spread out, or less concentrated, as the season advanced. Interestingly, they discovered that the spread of the noise and the total loudness did not always move in lockstep. During a specific comparison between mid-March and early April, the spread of the noise remained almost identical, yet the total power of the vibrations was noticeably higher in April. This means that looking at just the loudness or just the spread of the noise would give an incomplete picture; both must be considered together to truly understand the recording environment.
The team also checked the shallow ground temperature, which rose from deep winter cold toward the freezing point but stayed below zero degrees Celsius throughout the study period. While the ground was warming, the data did not show that the permafrost had begun to thaw at the depth of the sensor. Because the temperature data came from just half a meter below the surface, it could not confirm what was happening 300 meters down. Consequently, the researchers could not definitively say that the changing noise was caused by the thawing of the frozen ground. The changes might have been driven by shifts in the wind, changes in the mining activity nearby, or subtle alterations in the rock layers themselves. The study successfully mapped the changing background noise, showing that it is not a static environment, but it stopped short of pinpointing the exact cause of the shift, leaving the door open for future investigation into how the deep earth responds to the turning of the seasons.
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