Lava lake sloshing modes during the 2018 Kīlauea Volcano eruption probe magma reservoir storativity

Abstract

Basaltic volcanoes like Kīlauea, Hawai'i, pose hazards through lava flows and other eruptive activity. Quantifying hazard requires constraining the volume of magma stored in shallow reservoirs, as well as the capacity of those reservoirs to accumulate and discharge magma in response to pressure changes (i.e., storativity). Shallow reservoirs are often connected through magmatic conduits to lava lakes, which are frequently perturbed by rockfalls, convection, and outgassing. Here we present a novel method to quantify reservoir storativity at Kilauea that exploits the seismically observable oscillatory response of the lake-conduit-reservoir system to perturbations during the May 2018 eruption. Seismic events recorded by the summit broadband seismometer network during the eruption feature multiple oscillatory modes in the very long period (VLP) band. The longest period mode (∼30-40 s, here termed the conduit-reservoir mode) persisted during drainage of the lava lake, indicating its primary association with the underlying conduit-reservoir system. Additional modes, with periods of 10-20 s, vanished during lake drainage and are attributed to lava lake sloshing. The surface displacements of the two longest period sloshing modes are explained as the superimposed response to a near-surface horizontal force along the axes of the crater (from the pressure imbalance during sloshing on the crater walls) and a volume change in a reservoir located at the conduit-reservoir mode source centroid. Both sloshing modes are in the “deep-water” surface gravity wave limit where disturbances are largest near the lake surface, though pressure changes on the top of the conduit at the lake bottom provide the observed excitation of the underlying conduit-reservoir system. Combining magma sloshing dynamics in the crater and a conduit-reservoir oscillation model, we bound the storativity of the reservoir to be greater than 0.4 m3/Pa. This implies a storage volume of a few km3 for an approximately equidimensional (e.g., spherical) reservoir. A smaller volume is possible but requires the presence of other shapes of magma bodies, such as sills or dikes, and/or significant finite source effect. Our work demonstrates the signature of lava lake sloshing modes in the seismic data and the potential of these modes in constraining the structure of the shallow magmatic system.