THE ORIGIN OF HARMONIC TREMOR AT OLD-FAITHFUL GEYSER

VOLCANIC eruptions are sometimes accompanied by a characteristic type of seismicity known as harmonic tremor, in which the signal is dominat ed by discrete vibration frequencies(1-4). This harmonic structure cou ld reflect resonance behaviour in the excitation source(4-6) or filter ing of the seismic waves as they propagate through the surrounding roc ks(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 ch aracteristics of seismicity at Old Faithful geyser (Yellowstone Nation al Park), the cyclic behaviour and accessibility of which make it an i deal natural laboratory for studying harmonic tremor associated with n ear-surface sources, We find that sharp pressure pulses inside the wat er 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 cont inuous harmonic tremor. The absence of resonance within the water colu mn suggests that the harmonic motion must arise from the interaction o f the seismic waves with heterogeneities in the surrounding elastic me dium-most probably a near-surface soft layer.