A Self-Consistent Model to Explain Shallow Volcanic Tremor

Abstract:

Volcanic tremor sourced at shallow depths (~ hundreds of meters) is one of the most characteristic seismic signals recorded at active volcanoes and yet to be fully understood. It typically appears preceding many impending volcanic eruptions. However, the origin of this seismic signal remains unclear, which limits our understanding of the sub-surface processes during volcanic unrest. In this study, we propose that the shallow volcanic tremor emerges from pressure oscillations occurring beneath volcanic craters as consequence of continuous degassing. To test this hypothesis, we have coupled a new three-dimensional two-phase flow agent-based model that simulates the bubble dynamics in a magma conduit, a lumped-parameter model for the evolution of the gas pressure beneath the crater, and a lumped-parameter model to simulate the permeable transfer of volcanic gases to the atmosphere. Our numerical experiments reveal that pressure oscillations beneath volcanic craters can explain the main features of shallow volcanic tremor, namely, the direct correlation with gas emissions, the increase of amplitude of the ground vibrations when an eruption approaches, the broadband character of the dominant frequencies (in the range ~ 0.5 - 5 Hz), the frequency gliding towards higher values when the volcanic activity increases, and the power law distribution in the very high-frequency range of the spectra (from the dominant peaks till ~ 10 - 15 Hz). Our model also predicts that the exponent of the power law of the frequency spectra changes systematically with the physical parameters that are thought to vary during the onset of an unrest episode.