Back‐Projection Imaging of Extended, Diffuse Sources of Volcano‐Tectonic Seismicity During June 2010 at Sierra Negra Volcano, Galápagos

Abstract

AbstractSeismic signals generated by subsurface volcano‐tectonic processes commonly have low signal‐to‐noise ratios that make it difficult to resolve both 3‐D subsurface structures and seismic source locations using established seismic imaging methods (e.g., phase picking, template matching). High‐amplitude bursts within the noise (i.e., eruptions) are sometimes suitable for traditional seismic imaging but occur infrequently relative to the duration of a systems eruption cycle and offer minimal insight into mechanisms that occur during more common intereruption periods. Prevalent noise recorded near volcanic systems is often nearly continuously generated by many overlapping low‐magnitude displacements at depth that can be directly linked to magma, fluid, and volatile transport. This noise contains valuable information about subsurface processes that occur between volcanic eruptions and may provide critical information for mitigating risks from volcanic hazards. We present a new computationally efficient method to comprehensively study temporal and spatial variations in extended, diffuse, low‐magnitude volcano‐tectonic seismicity by applying a back‐projection search algorithm to analyze coherent seismic energy arriving from potential subsurface source locations to available pairs of broadband seismometers. We apply this method to Sierra Negra Volcano for a 2.5‐week study period coinciding with a dike intrusion in June 2010. Through averaging (stacking), the back‐projection algorithm illuminates the most likely source locations of low‐magnitude, high‐frequency volcano‐tectonic seismicity before, during, and after previously identified volcano‐tectonic earthquakes. Our results corroborate known volcano‐tectonic earthquake locations and support the interpretation of a small‐volume dike intrusion at ~5–8 km below sea level along the south‐southeastern flanks of the Sierra Negra caldera in June 2010.