Enigmatic Harmonic Tremors at the Oman Drilling Project Multi-Borehole Observatory: Cultural Noise or Subsurface Gas Flow?

Harmonic tremors are elusive seismic signals whose origins are often debated because they can be generated by both natural and anthropogenic processes. Harmonic tremors attributed to natural origins are often observed in systems hosting active subsurface fluid flow. Recent studies suggest that in peridotite formations, the migration of free hydrogen and methane gas produced by serpentinization likely drives active fluid flow through the fracture network. We report a set of harmonic tremors that lasted the entire nine months of observation at the Oman Drilling Project Multi-Borehole Observatory, a site experiencing active serpentinization. The tremors contain 12 harmonic modes at frequencies up to ~191 Hz, which exhibit a robust diurnal variation of ~0.1%. We carefully compare the various properties of the tremors with documented harmonic cultural noise and conclude that they are inconsistent with signals from known anthropogenic sources. Therefore, the tremors either result from an unknown anthropogenic process or are generated by sustained, vigorous multiphase flow in the formation.