Insights Into Permafrost and Seasonal Active‐Layer Dynamics From Ambient Seismic Noise Monitoring

Widespread permafrost thaw in response to changing climate conditions has the potential to dramatically impact ecosystems, infrastructure, and the global carbon budget. Ambient seismic noise techniques allow passive subsurface monitoring that could provide new insights into permafrost vulnerability and active‐layer processes. Using nearly 2 years of continuous seismic data recorded near Fairbanks, Alaska, we measured relative velocity variations that showed a clear seasonal cycle reflecting active‐layer freeze and thaw. Relative to January 2014, velocities increased up to 3% through late spring, decreased to −8% by late August, and then gradually returned to the initial values by the following winter. Velocities responded rapidly (over ~2 to 7 days) to discrete hydrologic events and temperature forcing and indicated that spring snowmelt and infiltration events from summer rainfall were particularly influential in propagating thaw across the site. Velocity increases during the fall zero‐curtain captured the refreezing process and incremental ice formation. Looking across multiple frequency bands (3–30 Hz), negative relative velocities began at higher frequencies earlier in the summer and then shifted lower when active‐layer thaw deepened, suggesting a potential relationship between frequency and thaw depth; however, this response was dependent on interstation distance. Bayesian tomography returned 2‐D time‐lapse images identifying zones of greatest velocity reduction concentrated in the western side of the array, providing insight into the spatial variability of thaw progression, soil moisture, and drainage. This study demonstrates the potential of passive seismic monitoring as a new tool for studying site‐scale active‐layer and permafrost thaw processes at high temporal and spatial resolution. Plain Language Summary Seismic vibrations in the ground generated by background sources of noise (vehicle traffic, wind, ocean waves, etc.) occur continuously and provide a way to monitor environmental changes with time. We used 2 years of noise data to study belowground changes in Alaska, where the upper layer of soil freezes and thaws seasonally and deeper soil (permafrost) remains frozen year‐round. Warming temperatures may alter the depth of thaw each summer and degrade permafrost, which could significantly impact ecosystems and infrastructure. Our results show a clear seasonal pattern corresponding with the timing of soil freeze/thaw. Vibrations traveled at the