Imaging Volcanic Systems Using The Ambient Seismic Field PDF Download

Seismic signals generated by volcano-tectonic and hydrothermal processes typically have low signal-to-noise ratios (snrs) that make it difficult to locate their subsurface sources using established seismic imaging methods (e.g., phase-picking, waveform or spectral template-matching). High-amplitude bursts easily identifiable on otherwise "noisy" seismograms from these systems (i.e., eruptions) can be suitable for traditional seismic imaging, but these events tend to occur infrequently relative to the duration of a systems total eruption cycle. Studies that utilize only these types of observations are therefore inherently limited to analyses of individual eruptions or high-magnitude seismic events and offer less insight into subsurface processes that occur as a system builds up to and recovers from an eruption. Improving our understanding of significantly more common inter-eruption periods is critical to (1) answer fundamental geologic questions about volcano-tectonic and geyser system processes, (2) make accurate assessments of volcanic and geothermal hazards, and (3) improve our understanding of how magmatic and geothermal reservoirs evolve. Fortunately, the apparent "noise" recorded by seismic stations in volcanic and hydrothermal systems in reality represents a compilation of many overlapping, low-magnitude pressure perturbations at depth. These "micro-events" have also been shown to be directly linked to fluid (magma or brine) and volatile movement in the subsurface. The low snr signals these events generate therefore likely contain valuable information about subsurface processes that occur between volcanic and geyser eruptions. In this thesis I present a new computationally efficient back-projection imaging technique to comprehensively study temporal and spatial variations in extended, diffuse, low-magnitude volcanic and hydrothermal seismicity using low snr seismic array data. I use observations of coherent seismic energy arriving at pairs of receivers in the array to back-project low snr seismic signals to their most likely subsurface source locations. Applying this technique to active volcanic and geothermal systems can help to provide new insights into the fundamental physical processes that occur before, during and after potentially hazardous eruptions.