Enigmatic Harmonic Tremor in the Samail Ophiolite, Oman: Evidence for Subsurface Gas Flow

Harmonic tremors are a unique class of seismic signals often attributed to subsurface fluid flow, but can also be generated by anthropogenic processes. We present observations of a tremor with a harmonic interval of ∼12.75 Hz up to the 15th mode at ∼191 Hz during a seismic‐borehole‐acoustic experiment at the Oman Drilling Project Multi‐Borehole Observatory, located on an ultramafic terrain of the Samail Ophiolite with minimal human activities. The tremor persisted for the entire 9‐month observation interval, with mode frequencies varying by ∼0.1% diurnally and by ∼1% overall. We find that the tremor characteristics are inconsistent with signals from documented anthropogenic sources and that sustained gas flow through a fluid‐filled, subsurface fracture is the most plausible source mechanism. Our results have potentially important implications for free gas processes during low‐temperature serpentinization and motivate further research to investigate the seismic signals generated in these environments.