Extremely Shallow Semi-Repeating Tremor Caused by Water Hammers in a Sewer Pipe in Social Circle, Georgia

Repeating earthquakes are mostly generated by small asperities that are loaded by continuous creep surrounding them, and their recurrence times are inversely proportional to the loading rates. However, sometimes anthropogenic activities can also produce repeated seismic shakings with shorter recurrence intervals, and their source mechanisms can vary. Here we investigated semi-repeating ground vibrations recorded in Social Circle, Georgia starting on August 14th 2025. Ten tremor-like events were captured by sixteen three-component SmartSolo sensors during one hour on August 15th 2025, followed by four days of continuous observation at three stations. The signals recur nearly every six minutes with similar but not truly repeating waveforms and clear diurnal amplitude variations that are stronger during daytime. Clustering analysis reveals several sub-event types with distinct coda durations. Their timing correlates with expected residential water-use patterns, suggesting a shallow anthropogenic source. Later excavations revealed that a forced main pipe connected to a local pump station vibrated nearly every six minutes, likely caused by the water hammer effect due to a faulty check valve. These findings demonstrate that human activities in the shallow subsurface can generate semi-repeating seismic waves, exhibiting recurrence behavior analogous to natural fault or volcanic systems. Correctly identifying their causes help to better distinguish them from naturally occurring events.