Enigmatic Harmonic Tremors in the Samail Ophiolite: 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 tremors with a harmonic interval of ~12.75 Hz up to the 15th mode at ~191 Hz during a seismic-acoustic experiment at the Oman Drilling Project Multi-Borehole Observatory, located on an ultramafic terrain of the Samail ophiolite with minimal human activities. The tremors persist for the entire nine-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.