The origin of harmonic tremor at Old Faithful geyser

Abstract

VOLCANIC eruptions are sometimes accompanied by a characteristic type of seismicity known as harmonic tremor, in which the signal is dominated by discrete vibration frequencies^1–4. This harmonic structure could reflect resonance behaviour in the excitation source^4–6 or filtering of the seismic waves as they propagate through the surrounding rocks^7–10; but complexity and variability in the properties of volcanic systems make it difficult to discriminate between such mechanisms. To address this question, we have analysed the source and propagation characteristics of seismicity at Old Faithful geyser (Yellowstone National Park), the cyclic behaviour and accessibility of which make it an ideal natural laboratory for studying harmonic tremor associated with near-surface sources. We find that sharp pressure pulses inside the water column trigger distinct seismic events that give rise to a harmonic ground response whose frequency varies spatially but not temporally. A superposition of these seismic events creates the appearance of continuous harmonic tremor. The absence of resonance within the water column suggests that the harmonic motion must arise from the interaction of the seismic waves with heterogeneities in the surrounding elastic medium—most probably a near-surface soft layer.