Magma oscillations in a conduit‐reservoir system, application to very long period (VLP) seismicity at basaltic volcanoes–Part II: Data inversion and interpretation at Kīlauea Volcano

Very long period (VLP) seismic events (with dominant periods of 15 to 40 s), observed from 2007 to 2018 at the summit of Klauea Volcano, Hawai'i, arise from resonant oscillations in the shallow magma plumbing system. Utilizing an oscillation model developed in the companion paper (Liang et al., 2020), we perform Bayesian inversions on seismic data from four representative VLP events separately for the parameters of the shallow conduit-reservoir system, exploring both sphere and crack reservoir geometries. Both sphere and crack geometries are preferentially located similar to 1-2 km beneath the northeast edge of Halema'uma'u crater and produce similar fits to the data. Considering a reasonable range for reservoir storativity, magma density, and density contrast between the top and bottom of the conduit, we favor a spherical reservoir with a radius of 0.8 to 1.2 km and a short conduit of less than a few hundred meters. For this geometry, buoyancy from density stratification in the conduit provides the dominant restoring force for the VLP oscillation. Viscosity is constrained within an order of magnitude for each event (e.g., approximately 2 to 23 Pa s for one event versus 27 to 513 Pa s for another). Changes in VLP period T and quality factor Q can be explained by changes in viscosity, density stratification, and/or conduit/reservoir geometry. In particular, observed fluctuations in Q over short time intervals (e.g., hours) with minimal changes in T apparently require rapid changes of magma viscosity by over an order of magnitude, assuming geometry remains unchanged, possibly reflecting changes in volatile content, bubble concentration, or conduit flow regime.