Deciphering water from molten rock: Insights into Uturuncu volcano, Bolivia, using seismic velocity and attenuation tomography

Abstract:

Uturuncu volcano, Bolivia, sits above the Altiplano-Puna Magma Body, which hosts the world’s largest zone of silicic partial melt. Although Uturuncu has not erupted in over 250,000 years, it recently underwent significant uplift and is seismically active. A key question is the origin of the deformation: is it related to magmatic mush destabilization, formation of a diapir, and/or activity of a shallow hydrothermal system? We use seismicity occurring over a three-year period to image the shallow crustal stress state and interrogate any fluids present. To do this, we present a new catalogue of seismicity and calculate moment magnitudes for the events. B-values, which represent a particular crustal stress state, are found to be significantly higher than previously reported (greater than 1). This agrees with observations at other volcanoes. Our moment magnitude results also provide us with estimates of seismic attenuation, Q. This emphasises the benefits of moment magnitude over other magnitude scales. We therefore suggest that if possible, moment magnitude should be used instead of local magnitude. We then use this catalogue of events to undertake velocity and attenuation tomography. Observed decreases in shear-wave velocity at shallow depths indicate the presence of fluids, in agreement with other geophysical observations. Furthermore, low vp/vs ratios suggest the presence of gases. Mapping attenuation characteristics then helps discriminate between water, melt and gas. Our findings elucidate the structure of a volcano situated above a massive melt storage region that has not erupted in thousands of years, yet is still seismically active. Although we do not expect this volcano to erupt in the near future, ways of monitoring and identifying melt intrusions at such volcanoes is important for a basic scientific understanding of transcrustal magmatic systems and how ore bodies form.