Back-Projecting Volcano and Geyser Seismic Signals to Sources

Abstract:

Volcanic and hydrothermal systems are generally characterized by persistent, low-amplitude seismic “noise” with no clear onset or end. Outside of active eruptions and earthquakes, which tend to occur only a small fraction of the time, seismic records and spectrograms from these systems are dominated by long-duration “noise” (typically around 1-5Hz) generated by ongoing processes in the systems’ subsurface. Although it has been shown that these low-amplitude signals can represent a series of overlapping low-magnitude displacements related to fluid and volatile movement at depth, because of their “noisy” properties compared to typical active or earthquake sources they are difficult to image using traditional seismic techniques (i.e. phase-picking). In this study we present results from applying a new ambient noise back-projection technique to improve seismic source imaging of diffuse signals found in volcanic and hydrothermal systems. Using this new method we show how the distribution of all seismic sources – particularly sources associated with volcanic tremor – evolves during a proposed intrusion in early June 2010 at Sierra Negra Volcano on the Galápagos Archipelago off the coast of Ecuador. We use a known velocity model for the region (Tepp et al., 2014) to correlate and back-project seismic signals from all available receiver-pairs to potential subsurface source locations assuming bending raypaths and accounting for topography. We generate 4D time-lapsed images of the source field around Sierra Negra before, during and after the proposed intrusion and compare the consistency of our observations with previously identified seismic event locations and tomography results from the same time period. Preliminary results from applying the technique to a dense grid of geophones surrounding a periodically erupting geyser at El Tatio Geyser Field in northern Chile (>2000 eruptions recorded) will also be presented.