Constraints on absolute magma chamber volume from geodetic measurements: Trapdoor faulting in the Galapagos

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As magma rises from the Earth's mantle it accumulates in magma chambers in the Earth's crust. As magma pressure in these chambers increases it causes the Earth's surface to bulge upward, a process termed "inflation". Inflation can be measured with a variety of instruments, both ground-based and space based, and is a useful predictor of impending eruptions. Standard analysis methods can be used to derive the change in volume during an inflation episode but are insensitive to the total chamber volume. It would be extremely useful to scientists and those monitoring volcanic hazards to know the volume of active magma chambers. The total volume provides an upper bound on the possible eruptive volume and is a constraint on models of magma chamber evolution. This project uses unique observations at Sierra Negra volcano in the Galapagos to place rigorous bounds on the total volume of Sierra Negra's magma chamber. The Broader Impacts of this study are particularly timely because the observations suggest that an eruption and/or earthquake at Sierra Negra may be imminent. The volcano has been uplifting since mid-2015 and is now higher than it was prior to the most recent (2005) eruption. The project also supports a graduate student to work on this problem.Between 1992 and the 2005 eruption Sierra Negra uplifted by nearly 5 meters, with maximum uplift in the center of the caldera. This data is well fit by a shallow sill-like magma chamber with its top at roughly 2 km depth. Inflation has been punctuated by three trapdoor faulting events, with slip along an intra-caldera fault system. The most recent trapdoor event occurred only 3 hours prior to the 2005 fissure eruption. The response of the magma chamber to the faulting depends on both the volume of the chamber and the compressibility of the magma contained within and is clearly expressed in observable surface displacements. By constraining the relative compressibility of the magma and the elastic chamber, which can be achieved with observations from the 2005 eruption, the researchers will be able to estimate the absolute chamber volume. This will permit estimation of the fraction of total magma that was erupted in 2005, a useful parameter for forecasts at other volcanoes.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.